ML20065G725
| ML20065G725 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 06/26/1975 |
| From: | Cairella J ARIZONA PUBLIC SERVICE CO. (FORMERLY ARIZONA NUCLEAR |
| To: | |
| Shared Package | |
| ML17310B177 | List: |
| References | |
| 13-EC-PA-210, 13-EC-PA-210-R05, 13-EC-PA-210-R5, NUDOCS 9404130164 | |
| Download: ML20065G725 (57) | |
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36
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CALCULATION SHEET CALC, NO. I1-Ff-PA-210
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F0 m CARL &
AMPACITIES 6
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ENCLOSURES APS INTRODUCTION AND BECHTEL MEMORANDUM IOM-E-13521 DATED FEBRUARY 18,1987 N
l The following is provided to clarify the calculation method used by Bechtel in memorandum IOM-E-13521, dated February 18,1987. 1 1 Pomose .I The calculations in Table 1 establishes that the Watts /ft method is conservative, resulting in heat dissipation values less than the resulting Watts /ft values calculated based on the U.L. test results. i The calculations are based on overfilled tray since U.L. loaded the test trays to approximately 61% where as the Bechtel design limit is 30%. Although Bechtel did not use the Watts /ft method for detemiining the derating of Thermolag (it is based on a 12.5% derating specified in T.P.O. E2.6.4),it is shown that the Watts /ft calculation of overfilled and uncovered tray, calculated in accordance with 13-EC-ZA-300,is more conservative than the calculated result derived from the data provided in the U.L. report for overfilled and uncovered tray. Similarily, the Watts /ft. calculation of overfilled and covered tray is more conser-vative than the U.L. based calculated result for Thermolag covered tray. In fact, even the result above for overfilled uncovered tray, based on 13-E-ZA-300, is more conservative than the results of the U.L. based calculation for Thermolag covered tray. 1 U.L. based thermolag, overfilled dissipation: 49.1 W/ft 13-EC-ZA-300 based overfilled, uncovered: 35.5 W/ft 13-EC-ZA-300 based overfilled, covered: 26 W/ft The above results establish that the 13-EC-ZA-300 Watts /ft method for covered tray provides conservative results, even for thermolag covered trays. It was then used as the basis for Tables 2,3, and 4 to compare the Watts /ft of thermolag coverd trays, calculated in 13-EC-ZA-300, to the maximum heat dissipation levels for covered tray. These maximum levels are established in 13-EC-ZA-300, especially Attachment E. Table 1 The following equation is used in Table 1 for determining the Watts /ft from the U.L. data. 21R W atts
- cables conductors
- of conductors conductor
, conductor cable total length of conductors total length of conductors Watts
a The depth of cable fill in the tray for the U.L. report is determined based on the following:
- of cables cross sectional area of cable (in)2
- cables - (cable diameter)23 4
1 ~ tray width tray width = depth of fillin tray i 9 't 4 1 9 h r I i f i 9 s -+-+m - - =
p a mWhi i Bechtel Western Power Corporation Interoffice Memorandum. To R. A. Schmitter File No. D.4.32.3 % ru~ary ?g( g -453836 E l3S Subject Bechtel Job 10407 Date Derating of Cables PIR LA 86-22 From C. M. Herbst Of Engineering Copies to J. Aguilar At BWPC Ext5150 V. Karrian J. E. Mahlmeister R. R. Stiens All w/ enclosures b [c,- 9 [f ),, c, The results of the formal test reports covering the recently conducted Ampacity Tests by Underwriters Laboratories using TSI 330 r (Thermo-lag) material were reviewed by project engineering, and as a res ult we are revising our " action taken" statement to PIR LA 86-22 as follows: ACTION TAKEN: The fireproofing material used at PVNGS is Thermo-lag 330-1. The cable derating calculation was based on a Thermo-lag derating factor of 12.5% which was given in TP0 Design Guide E2.6.4, Rev. 1. Since all power cables were sized to at least 125% of full load currents, there was sufficient margin to compensate for the 12.5% derating and no additional cable derating was taken. The PVNGS cable derating calculation was based on information at the time of issue and was approved by the Chief Engineer. Since no additional cable derating was taken for Thermolagged cable trays, the U.L. test results were compared to the cable derating calculation (13-EC-ZA-300) for trays overfilled, covered or passing through firestops. Table 1 shows the comparison between the U.L. test report and calc. 13-EC-ZA-300. The U.L. test configuration of 71-3/C#6 cables in a tray is equivalent to 2.323 inches depth fill or 60.8% fill which is an overfill tray condition (allowable is 1.15" depth or 30% fill of a 3" power tray). LAO-04 00 4le s 4
v Bechtel Western Power Corporation R. A. Schmitter Page 2 IOM-E-13511 MOC-453836 February 18, 1987 Based on the 2.323" depth fill, PVNGS derating calc.13-EC-ZA-300 has more conservative derating values as compared to the U.L. test results as shown on Table 1 (at 400C ambient temperature, 62.6% better for overfilled open trays and 47% better for Thetmo-lagged overfilled trays). Af ter the comparison of the U.L. test data and the 13-EC-ZA-300 calc., a review of Thermo-lagged cable trays in Units 1, 2, and 3 was done using EE580 reports [ Enclosure (2) outlines the procedures used to track Thermo-lagged raceways in the EE580 programl. Tables 2 through 4 in Enclosure (1) list the Thermo-lagged trays and their calculated watts / f t and allowable watts /f t as documented in cale. 13-EC-ZA-300. The calculated watts /ft values are_all below the_ allsLwable Fnr covered _ trays. No ~ overfilled tray conditions exist.
- r Based on the above study and investigation, the derating f actors of 28% for one-hour protective system and 31% for a three-hour protective system has no safety impact on the PVNGS project. The proj ect's current cable derating calculation 13-EC-ZA-300, which is by watts per foot method has more conservative values than the U.L. te st results.
b4.ddwtva C. M. Herbst CMH:JSF:eg i l i
Enclosure:
(1) Tables Listing the Thermo-lagged Trays ] (3 pages, 1 copy) (2) Procedures to Track Thermo-lagged Raceways (1 page, I copy) 1 Written Response Required: NO
MC G 8 4,/A d '/ PROJECT JOB NO. - /# 4# ~ / C 4 f f.E.b 4 4 4 7 W Y Q ' SUBJECT $2 b N <' ' ~E C ~ 5 & ' '3 # # Vf A'f'M Y' *U SHEET NO. / *E A REV ORIGIN A TOR D' ATE CHECKER DATE REV ORIGIN A TO R DATE CHECKER DATE A ,q c/ew' 2-7:s 7 A 6 O A ) [M $y Ga .fe! G t 7 t-h H6 ua 4 m is '*1 %. Mt g t N v s Q { ,i t of} '= N ( y s% bx g s is o k' ( 4,Dg)S 9 A g i, d ,: 7 : 't b ', i w?* s W wg' s S s x y N "x S s 't s g T 4' % % Q I g ij h }, t L. % i. t i A o h 25 N 9 0-h Y s b s P p (h d Mjn y ~. S U dl R 4-g m 0 N, x b ri Tk% 9 ow k,d (9 I [R h k T L h 25 1 D V as sg 4 q 4 k *u s '-8 2, ) x,.e ,9 u $L p, e D D< g 5 g3 '4 'w& 4 =a r 2 9 at y k 3 t 1,% v a' l u'o V y gyN 9h 0 V is g 3el 0 i La o-s e e s.: iss
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ENCLOSURE 2 PROCEDURE: Raceways wrapped with Thermo-lag to comply with Appendix "R"
- requirements are shown on Appendix "R" drawings and trackec in the EE580 program.
Raceways wrapped with Thermo-lag to comply with Reg. GuiJe 1.75 are determined in a case-by-case basis by the field engineer and ' documented via an FCR and in addition are tracked in the EE580 program. t EE580 tracking. of wrapped raceways is accomplished by assigning a unique secondary status (G7) control characteristic.value of "WP" (wrapped) to the raceway when the Appendix "R" drawing is issued and/or when review and approval of the FCR is completed by the home of fice engineering. y 1 4 M l
^ E e. ( ' UNDERWRITERS LABIRATORIES INC. se e en independent, not-/pr profit oryonsstion fastingfer pu6lk spety January 21, 1987 ['N'h Thermal Science, Inc. / I \\'N Mr. Rubin Feldman, President 2200 Cassens Drive yh(( St. Louis,Mo 63026 Our
Reference:
Project 86NK23826, File R6802
Subject:
Special Services Investigation Of Ampacity Ratings For Power Cables In Stee. Conduits And In open-Ladder Cable Trays With Field-Applied Enclosures Dear Mr. Feldman vc The following is a Letter Report summarizing the details and results of the ampacity investigation conducted at our Northbrook Testing Station. The sole purpose of this investigation was to develop information which you intend to use to determine if the ampacity derating caused by the field-applied enclosures meet the requirements of Bechtel Power Corporation for use at the South Texas Project Nuclear Power Plant. It is understood that the information developed as a result of the investigation described herein is to be submitted only to Bechtel Power Corporation. In no event shall Underwriters Laboratories be responsible to anyone for whatever use or nonuse is made of the information contained in this Letter Report and in no event shall Underwriters Laboratories, its employees, or its agents incur any obligation or liability for damages, including, but not limited to, consequential damages, arising out of or in connection with the use, or inability to use, the information contained in this Letter Report. The issuance of this Letter Report in no way implies Listing, Classification or other Recognition by UL and does not authorize the use of UL Listing or Classification Markings or any other reference to Underwriters Laboratories Inc. on or in connection with the product or system. THERMAL SCIDCE. INC. AND ITS EMPIDYEES AND AGENTS SHALL HAVE NO OBLIGATION OR LIABILITY IDR DAMAGES, INCLUDING BUT NOT LIMITED TO CONSEQUENTIAL DAMAGES ARISING OUT OF OR IN CONNECTION WITH THE USE, OR INABILITY TO USE, THE INFORMATION INCLUDED IN THIS REPORT. Look For The @ Listing or Cia funny pig FPMuut 810 thef au moamit emur WLAIC - Wetenueme menne =-
~- r fih }f f!&0$? Y e January 21, 1987 EEEEElPTigN MATERIALS: The followin'g is a description of the materials used in the test investigation. Cable Tray - The nominal 24 in wide open-ladder galvanized cable tray consisted of nominal 4 in deep siderail members with ribbed and vented rungs. The rungs were spaced 12 in. OC. The loading depth of the cable tray was 3-5/8 in. The cable tray, manufactured by MP Husky Corp., Greenville, South Carolina and designated Type S9J-24-144 VENTRAY, was supplied in a nominal 12 ft length. The cable tray was purchased by Houston Lighting and Power Company, Wadsworth, Texas under their Customer Order No. CF28294. Steel conduit - The nominal 4 in diameter rigid galvanized steel conduit had an outside diameter of 4.500 in., and an inside diameter of 4.026 in, and a wall thickness of 0.237 in. Two nominal 10 ft lengths of conduit were purchased locally and connected together using a threaded steel coupling. After assembly, one conduit was cut to provide an overall conduit length of 12 ft, O in. Each length of conduit bore the UL Listing Mark. Cables - The 3-conductor No. 6 AWG power cable was marked "THE OKONITE CO PLT $7 OKONITE VFR POWER CABLE 3CDR 6 AWG CU 2000V 90C RA-306 1979.* Each of the three stranded conductors consisted of seven 0.060 in, diameter tinned copper strands. The outside diameter of each insulated and jacketed conductor was 0.340 in. The outside diameter of the cable was 1.000 in. The reel of cable was shipped from the South Texas Project (Shipping Notice 5813 dated August 29, 1986). The reel of cable bore the following information imprinted on aluminum plates: 4W D
e. i R6802/86NK23826 f's, Page 3 January 21, 1987 - j's, i e y#$'L;. BECHTEL REEL j j&,Aj[,- RA306 503 '#d yh P.O. 35-1197-8046-POC2 1 B&R ITEN NO. 9 - VIOLET s1' NP SOUTH TEXAS PROJECT 's
- b.V C,% +y THE OKONITE CO.
CUSTOMER REEL RA-306503 2100 FT QC 23588B3 CLASS 1E j 3/C 6 7X CC-2000V .055 OKONITE.030 OKOLON .080 OKOLON F.O. 07-2597-1 SEQ FTG T.7010108 B.7008008 Enclosure - A total of six different enclosure materials were W supplied by Thermal Science, Inc. for inclusion in the ampacity investigation. The four enclosure materials used on the cable tray sample were each supplied in sheets and were identified by the manufacturer as being: 1. THERMO-LAG 330 Prefabricated Panels Regular Density - Nominal Thickness: 1/2 in. Color - Off White 2. THERMO-LAG 330 Pref abricated Panels Regular Density - Nominal Thickness: 1 in. Color - Off White 3. THERMO-LAG 330 Prefabricated Panels Low Density - Nominal Thickness: 1/2 in. 4. THERMO-LAG 330 Prefabricated Panels Low Density - Nominal Thickness: 1 in. Color - Charcoal Grey The two enclosure materials used on the conduit sample were preformed sections split in half, longitudinally, and were identified by the manufacturer as being: 1. THERMO-LAG 330 Preshaped Conduit Sections Regular Density - Nominal Thickness: 1/2 in. Color - Off White 2. THERMO-LAG -330 Preshaped Conduit Sections Regular Density - Nominal Thickness: 1/2 in. Color - Off White y
i yy ,3~ -1 ,3-- R6802/86NK23826 TO sr~q,b DJ T ~. 3 9 '
- l. 1
,K.) $.,j d i Page 4 January 21, 1987 ^ Small samples of each material were obtained by representatives of'Bechtel Power Corporation and Houston Lighting & Power Company. I Joint Sealan't Material - The material.used to cover the joint openings of the various enclosures on the cable tray and conduit samples was supplied by Thermal Science Inc. and was identified by the manufacturer as being "THERMO-LAG 330-1 Trowel Grade." The material was suppli,ed in.a 5 gal plastic pail. Banding Straps - The stainless steel banding. straps were 1/2 in. wide by 0.020 in. thick. The 13/16 in. long by 0.605 in, wide winged-sleeve cinch clips used in conjunction with the banding straps were formed of 0.028 in. thick stainless steel. The steel strapping and clips were manufactured by Childers Products Co., i Cleveland, OH. Corner Angles - The corner angles used in conjunction with the stainless steel banding straps on the cable tray sample consisted ?> of nominal 2 in. lengths of nominal 2 by 2 by 0.046 in, thick stainless steel angle. Tie Wire - The stainless steel tie wire used in conjunction with the 1/2 in. thick panels on the cable tray sample had a diameter of 0.030 in. J CONSTRUCTION OF TEST ASSEMBLIESr The cable tray and conduit samples, with cables, were assembled by members of the technical staff of Underwriters Laboratories j Inc. under the supervision of the engineering staff of Underwriters Laboratories Inc. The various enclosures were installed by workmen in the employ of the submittor under the ) supervision of representatives from Bechtel Power Corporation and Houston Lighting and Power Company. The installation was also witnessed by members of the engineering staff of Underwriters ) Laboratories Inc. The nominal 12 ft long cable tray was supported 18 in. from each of its ends by a nominal 5 ft long H-shaped Type P1001 steel i Unistrut channel. A nominal 3 by 25 by 1 in. thick piece of ceramic fiber blanket insulation was placed atop the Unistrut channel beneath the cable tray in order to provide a thermal break. 1 o N
R6802/86HK23826 Page 5 4 9 ; N [.' C ~ V ^ ~
- }., N. -
gl 8 v** January 21, 1987 The cables were installed in the nominal 12 f t long cable tray as shown in ILL. 1. The first (bottom) layer consisted of 24 runs of cable looped back-and-forth in the cable tray with each loop of cable extending approximately 8 to 12 in. beyond the cable-tray end. Ehch cable was secured to the cable tray rungs with No. 16 SWG steel wire ties spaced 12 to 24 in. OC. The second layer consisted of 23 runs of cable looped back-and-forth in the cable tray and secured to the cable tray rungs using No.16 SWG steel wire ties. The third (top) layer consisted of 24 runs of cable looped back-and-forth in the cable tray without attachment. The first and second layers of cable in the cable tray system were installed using a continuous lergth of cable. The third layer of cable was installed in one continuous length with the three conductors of the third layer length spliced to the three corresponding conductors of the second layer length using split-bolt connectors in conjunction with multiple wraps of Pyc electrical tape insulation. The three conductors of the spliced cable were then wired in series in such a manner as to repretent 7 a single No. 6 AWG conductor having a total length in the tray of l approximately 2980 ft. The measured resistance of the No. 6 AWG conductor was 1.287 ohms. The cables were installed in the nominal 12 f t long steel conduit adjacent to the cable tray as shown in ILL. 2. Seven cables, each 14 ft long, were tightly bundled together using nylon ties and were inserted in the steel conduit system such that 1 f t projected from each of its open ends. After installation in the steel conduit, the individual conductors were wired in series using split-bolt connectors in conjunction with PVC electrical tape which resulted in a single No. 6 AWG conductor having an overall length of approximately 294 ft. The four enclosure configurations for the cable tray sample were each installed in essentially the same manner. The general installation details for the four cable tray enclosures are shown in ILLS. 3, 4 and 5. The two enclosure configurations for the conduit sample were each installed in essentially the same manner. The two halves of the preformed panel sections were installed about the conduit with the longitudinal seams oriented at the 3 o' clock and 9 o' clock positions. Adjacent 3 ft. lengths of the preformed panel sections I were butted together. The pairs of preformed panel sections were I secured to the conduit sample with stainless steel banding straps j located at each end of esch 3 ft long section and maximum 12 in. OC along the length of the conduit sample. After completion of the banding installation, the longitudinal seams and end seams 4 were covered with the joint sealant material. 9
R6802/86NK23826 Page 6 \\ u - '.r .J fy
- y r v ~ n:3 p m :m
. l. 7". ' A 1 n 3.. January 21, 1987 ~ i A piece of glass fiber insulation was placed beneath the enclosed conduit sample at each support channel location to afford a themmal break. As a final step in the installation of the protective enclosure on each test sample, the ends of the cables projecting 8 to 12 in. from each end of each system were wrapped with glass fiber insulation covered with PVC duct tape. I g s. I ggCggg AMPACITY TESTS: SAMPLES The ampacity tests were conducted on the cable tray and conduit configurations described previously in this Letter Report under " Construction of Test Assemblies." METHOD For each test of the cable tray configuration, 53 fusion-welded No. 24 gauge chromel-alumel (Type K) thermocouples were used to measure temperatures. Thirty-six of the thermocouples were located on the copper conductors of the cables, as shown in ILL. 1. To obtain accurate conductor temperature readings, a slit was made in the cable jacket and insulation materials, and the thermocouple was inserted in the slit, in contact with the copper conductor. To ensure that the thermocouple remained in intimate contact with the copper conductor, the strands of the conductor were spread apart, the beaded tip of the thermocouple was inserted between the strands and the copper strands were released, thereby locking the beaded thermocouple tip in place. The slit in the cable jacket was then sealed with multiple wraps of PVC electrical tape. The remaining thermocouples were used to measure the ambient temperature of the test enclosure and the top surface temperature of the cable tray protective system, as shown in ILL. 1. 4
R6802/86NK23826-Page 7 ~- January 21, 1987 3.b d. d fl.m W. M i w ;d );iLY - i j l 8 5 - --.- -.-,_..... Thirty fusion-welded No. 24 gauge chromel-alueel- (Type R),. thermocouples were used during the conduit ampacity tests to l measure temperatures. Twelve of the thermocouples were located on the copper conductors of the cables, as shown in-ILL. 2. To - obtain accurate conductor temperature readings, a slit was made in the cable jacket and insulation materials and the thermocouple was inserted in the slit, in contact with the copper conductor. To ensure intimate contact with the copper: conductor, the strands of the copper conductor were spread apart, the beaded tip of the thermocouple was inserted between the strands and the strands were released, thereby locking the thermocouple tip in. place. The slit in the cable jacket material was then sealed with multiple wraps of PVC electrical tape. The remaining thermocouples were used to measure the ambient temperature of the test enclosure and the temperatures of the top and bottom surfaces of the conduit er conduit protective materials, as shown in ILL. 2. Testing was performed using house current in combination with a. variable load bank. For each configuration, one end of the series-wired No. 6 AWG cable conductor was connected to 115 V ac house current protected with a 110 A fuse. The return leg of the series-wired No. 6 AWG cable conductor passed through a 0.1-101 A variable load bank., The current was measured using an ammeter shunted from the load bank. The thermocouple wire used for each test configuration was purchased from Claud S. Gordon, Richmond, Illinois, and was designated K24-2-305, Type K. The data logger used to measure and record the temperature data for each test configuration was a Fluke 2285B Data Logger (UL Asset No. 85 1075, Serial No. 3910000). The analog ammeter used to measure the current for each test configuration was manufactured by Yokogawa Electric Works, Ltd., Tokyo, Japan (UL Instrument No. 97836M). The calibration records for the data logger and ammeter are on file at Underwriters Laboratories Inc. .r mw m.
~~ .i R6002/86NK33826 1 Page O_ ~ ~ ~ ' L *'l sS LE,~, "'7 January 21, 1987 i d, c L h s. w. u sn/.' l e sit a The ampacity. tests were each conducted in a d aft-free enclosure having inside dimensions of 7 ft, 6 in. wide by 15 ft, 6 in. long j by 5 ft, 10-1/2 in. high. The floor, ceiling, walls and door i were each in.sulatad. A 240 V, 2100.W heater was mounted on the inside surface of the insulated door, 4 ft above the floor of the-enclosure, to supplement heating' of the enclosure as required, j The hester was sounted at an angle such that its heat was directed upward with no direct radiation onto the test sample.- The. radiant heater was provided with a variac to allow manual i control of the heater output. A small exhaust fan was located'in the ceiling at the center of the enclosure to exhaust heat from i the room.as needed. In addition, a nominal '# 'by 12 in. shuttered i opening 'was provided in each corner of the ceiling to vent heat, as.necessary, through natural convection. To prevent movement of. air across the tect samples with the exhaust fan.in use,.a. nominal 4 by 8 ft sheet of plywood was suspended approximately 8 in. below the ceiling of the enclosure, centered under.the j exhaust fan outlet. -.) 7: The cable tray and conduit " baseline" ampacity tests and the ampacity tests on the various cable tray and conduit I configurations were all performed using the same procedure. For each test, the sample was installed in the draft-free enclosure and the cable circuit was electrically loaded with current at 110 V ac. The load.on the cable circuit was adjusted'to the value necessary to attain a steady-state temperature of 90*C i 0.4*C as measured on the hottest' cable conductor at the center section of thermocouples (Thermocouple Nos.13 through -24 on the cable tray sample and. Thermocouple Nos. 2, 5, 8 and 11,on the conduit samples). During each ampacityJtest, the ambient temperature within the enclosure, as determined from. .{ Thermocouple No. 0 (average of three thermocouples wired in j parallel) was maintained.at 40 1 0.3*C using the radiant heater, ceiling vents and/or exhaust fan, as necessary. For each ampacity test, approximately 15 min time was allowed to elapse after the final electrical current adjustments.were made to ensure that the cable conductor temperatures were stabilized. Upon. reaching and maintaining the steady-state temperature of-90*C i 0.4'C over the 15 min time period, the electrical current I was' recorded and the temperatures of each thermocouple in-the .j test set up were measured and recorded at 1 min intervals for a .i 60 min time period. During the 60 min time period, the i electrical current was monitored to ensure that it. did not change. -j i I i ~ 4*
R6002/86NK23826 4 Page 9 l F. M tjNFORB#u.paOpu viuL1 u t ( January 21, 1987 ( _..a/_. RESULTS The temperature data from each test is on file at Underwriters Laboratories Inc. in Northbrook, Illinois. The results of the ampacity tes'ts are summarized in the following tables nestews Ambient Conductor Temperature. 'C__ Teasersture. 'C T.C.
- Current, Ampacity Test Configuration Start 50 ein Start 60 min No.
A Cable tray without protective 40.3 40.1 90.3 90.3 16 32.1 enclosure (ikeveltae) Cable troy with regular density 40.0
- >0. 0 90.0 90.3 17 23.1 1/2 in. thick panel enclosure Cable tray with regular density 40.1 40.0 90.2
$0.1 18 22.1 1 in. thick panel enclosure 7 Cable tray with low density 40.0 40.2 90.2 90.1 17 21.7 1/2 in. thick panel enclosure Cable tray with low density 40.3 40.2 90.4 90.3 19 19.5' 1 in, thick panel enclosure Conduit without protective 40.1 40.2 90.2 90.2 2 34.1-enclosure (Baseline) Conduit with regular density 40.2 40.2 90.1 90.1 2 34.8 1/2 in. thick preform panels Conduit with regular density 40.1 39.9 90.1 90.2 2 30.9 1 In thick prefore panels e .]
n R6802/86HK23826 Page 10 January 21, 1987 For each of the ampacity tests, a representative of the Bechtal Power Corporation made the determination as to when the ampacity test sample had reached a steady-state condition. One or more representatives of Thermal Science, Inc. was also present for each of the ampacity tests. Very truly yours, Reviewed by: NT J MARK T. FAVA K. D. RHODES Laboratory Assistant Engineering Group Leader Fire Pro ection Department Fire Protection Department e; JOHNSON Senior Engineering Associate 7 Fire Protection Department CJJ:qz GZ5:2 w . A ,r" '+, ' e i e m ( s' v-,-
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- 43 7*
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TYPICAL INSTALLAT!ON DETAILS FOR CABLE TRAY ENCLOSURES 1. The Type P1001 Unistrut support channels were each covered on the bottom and sides with a "U"-shaped section formed by slitting a nominal 10 by 38 in. panel with a razor knife and folding as shown. The "U"-shaped panel sections were each secured to the support channels with a stainless steel banding strap on each side of the cable tray. 2. The cable tray siderails were covered using four nominal 4-1/2 in, wide by 6 ft long sections of panel. The bottom edge of each panel was notched approximately 1/2 in, at the support channel locations such that its top edge was flush with the top of the cable tray siderail. Pairs of siderail cover panels were secured in place with stainless steel banding straps passing around cable tray. Each 6 ft long pair of siderail cover panels was secured by a banding strap near its center and near each end. For configurations using 1/2 in. thick panels, the ribbed surface of the siderail cover panels was placed against the cable tray siderails (flat surface exposed). For configurations using 1 in. thick panels, the flat surface of the siderail cover panels was , laced against the cable tray siderails (ribbed surface exposed). The panel sections on the underside of the cable tray were cut to extend 1/4 to 3/4 in. beyond the siderail cover panels on both sides of the cable tray. Sections of panel, with the ribbed surface toward the cable tray (flat surface exposed), were secured to the underside of the cable tray with stainless steel banding straps passing around the cable tray and spaced 18 to 24 in. OC. 3A. For configurations employing the 1/2 in. thick regular or low density panels, the panel sections on the underside of the cable tray were additionally supported along the longitudinal centerline of the cable tray using stainless steel tie wires. At each rung location (12 in. OC), two holes were pierced through the 1/2 in. thick panel using a Phillips screwdriver. A length of wire was passed around the rung of the cable tray with its two ends extending through the pierced holes in the panel. The panel was then pushed against the underside of the cable tray and was secured in place by twisting the ends of the tie wire together using multiple tight twists.
- 4. ~
The nominal 6 ft long panel sections on the top of the cable tray were cut to the same width as the bottom panels and were placed stop the cable tray with the ribbed surface against the cable tray (flat surface exposed). The panel sections were l secured to the cable tray using stainless steel banding straps in conjunction with stainless steel corner protector angles at each corner with the bands spaced 12 to 18 in. OC. R6802 /L L. Al
4 The openings'along the top and bottom of mach cable tray 5. sidera11. panel, formed by the ribs of the top and bottom panel sections, were covered with a thin coating of trowel-grade joint sealant. No attempt was made to fill the openings through the thickness of the siderail panels. The seams of the top,. side and i bottom panel sections at the center of the cable tray sample were also covered with a thin coating of the joint sealant. S W g e N i O i ~ ) R4802 /LL. 5 j j
1 4 9 1 ENCLOSURE 6 l 1 AMPACITIES FOR CABLES IN RANDOMLY FILLED TRAYS j
_~ .= ~ e 4 AMPACITILS I'OR CAllLES IN RANDOMLY FILLED Td AYS J. Stolpe Southern Cahfornia Edison Company Los AngeIcs,Cahfornia AttST H ACT P! 011LEM DErlNITION The allowable current which may be carried by a given conduc-The first of many variables to be examined is the extent to site cable has been thoroughly igvestigated in almost every tot conceivable type of cable installation. One arca which has not had which the cables in any tray are packed. lt is apparent that cablesin a very loose arrangement are essentially immersed in air which can .I much attention up to now is the allowable current which can be freely flow through the vacant space in a tray. As the space between carried by cables in cable trays, or troughs. This paper presents a cables is reduced, by packing cables closer together, free flow of air completely general rnethod for calculating the ampacities of cablen in cable trays;it has been derived from elementary heat transfc: theory through the pack is gradually restricted. Taking this to the point. g and amply verified with rnany fulbscale tests.The method shows that where adjacent cables are toucidng cach other on all sides, the continuous free space between cables becomes practically non-currently published ampacities for small cables in highly filled trays existent and only small air pockets remain between the cables. t j snust be reduced, but the large cable ampacities can be safely increased. Applying this reasoning to heat Dow from cables in a cable tray INTRODUCTION we see that a loose packing is desirable since air can naturally flow. around each cabic. The heat will then rise out of the pak and be In the studies which have been made on the current carrying replaced with cooler air from the ' bottom. When cables become l abihty of electric power cables, the most simpic case of one cable tightly packed, there is no air flow through the bundic, and thus heat cannot be carried out of the bundle by natural air flow. in fact,the operstmg in air has been expanded to multiple cables in a conduit,I only way for heat to flow out of the light bundic is by heat multiple cables or conduits in stacked banks,2 and several cables pulled mio steel raceways.3 The results of these studies are incorpo-conduction through the conglomeration of cable conductors, insula-r tion, and air pockets, rated to various extents in both the AIEE lpCEA Power Cable Ampacities and the National Electric Code. Cable ampacitics in randomly filled trays must be based on the The ampacities, or derating factors, which have been deter. assumption that cables are tightly packed and that we cannot depend mined'so far are for cables which are in some form of an orderly ' on heat being carried out of the bundle by air flowin arrangement; a further simplification which has been easily justified Without question, this tightly packed condition does not exist in every cable tray, but it does randomly occur often enough that.for - in the past is all the conductors considered were the same site. safety, each cable tray must be designed as though it was poing to bc Unfortunately, this simphrying treatment cannot be justified when tightly packed. It is not even necessary that the entire cable tray be considering ampacities of randomly arranged cables in trays, tightly pxked, since a packed width of only about three inches is { A typical cable tray installation which is found in the electric sufficient to produce a hot spot in an otherwise cool tray, I I } power generation and distribution industry can be visualized as a With the criterion of tight cable packing established,it is then 3 incl > deep 24 inch wide metal trough containing anywhere from 20 required to determine how the heat generation is distributed in the to 400 randomly arranged single or multi-conductor power and tray cross-section. The many cable sizes possible, both single and control cables rangingin size from #12 AWG to 750 MCM This array of cables is usually secured along the cable tray with some ties to multi-conductor, and each carrying a diflerent current apparently makes it quite difficult to place allowable currents on such a prevent the cabks already in the tray from shifting if additional cables should be pulled into IW tray, During con.truction as cables heterogeneous mixture llowever, looking at the problem from the - l are secured in the tray, group b) yroup, they can become packed standpoint that we do not want any hot spots in the cab lc tray, the problem can be solved. i together tight enough that air a ur.able to circulate through the mass i of cables. %th the physical Iks and the normal vibration which is llot spots in a thermal system are produced by locally intense prewnt in tuost plants, even many of the initially loose cable arrays heat sources; thus, in every area of the cable tray we must chminate can h expected to settle and thus become more or less restrictmg to such conditions. In other words, the heat pencrated in every area of a i air flow. j l cable tray cross-section must be uniform. This is the Key to the entire { problem of ampacities for randomly arranped cables in cable trays, ) Several other vanables tend to compheate the determination of and the concept of uniform heat generation cannot be over-ampa:atics of caMes in trays Lorne of the more apparent ones are the emphasized. fullnen of a tray, dnessity of loading of caldes in a tray, determining t the location of the hattest spot over the tray cromection, and the amount of power cable Iwhich cencrates heat) m proportion to the Consider !~igure i showing a hypothetical slice of area from a amount of control cable twhich generates neghtable heat)in a tray. typical, tightly pxke'd cable tray. The heat intensity within each unit All the above variables can be, and are accounted for in the method area expressed in watts /ft, per square inch of cross-sectional arca, descritied herem. must be comtant all the way down to the smallest unit area inside the tray. which is the smallest cable in the tray. We therefore place ampacities of cables, such as shown in l'ipre 1,in proportion to the Paper 70 TP 557 PWR recommend:d and approved by the Insula-overall croswctional area of the sndividud cables, includmg the ted Conductors Cornmittee of the llIL Powet Group for presenta-conductor a, a + u oon' tion at the IEEE Summer Power Meeting and Lily Conference. Los Angeles. Cahf., Juh 12 17, 1970 Manusenpt submitted Septernber II we know the allowable heal intewty for u given cable 18,1969;made avadable for pimting Aptd 2L 1970. tray, we etn immediateN place ampacitics on every cable in the tray 962
by knowmg the cross 4cetional area of each compoute cabic. Thus, overell heat trensfer mechamsm is possible. A simple analytical the problem now remains to estabbsh the allowable heat intensity for solution to the heat transfer from the pencral, hypothetical cable various cable tray configurations. tray in Figure 3 has been made, and some rather subtle findmgs from the analysis will be pointed out. The reasoning presented thus far is significantly different from that used for cable tray ratings we now use.To show this, consider a large cable tray randomly filled with, say 300 tightly packed 00 volt cables of assorted sizes. According to the ratings pubhshed so far, every cable in thss tray must be derated to 50% of the ampacity for a 3<onductor cable in air.W Figure 2 shows that seven single W conductor #12 cabics can occupy about the same area in the tray as one 4/0 cable. Comparing the heat which is generated within the AT equal areas of cables it can be seen that three to four times more heat 0 / is produced in the bundle of seven fl2 cables as in a single #4/0 ATg cable, even though the two configurations occupy the same area in the filled tray. This effect is exactly what we want to chtninate in a C ABLE MASS cable tray installation because it is possible to get bundles of small WITH UNIFORM cables which produce locally intense heat sources and result in hot HEAT GENERATION spots within the cable tray cross 4cction. Fig. 3. Simplified analytical model for heat trans/cr from a tsghtly packed cable tray contatning all power cabic. Before proceeding with the analysis, two additional conditions must be specified. The first condition is cables in any tray must be t installed at a constant, or uniform, depth. This is to prevent cables from being heaped on one side of a tray with a resulting vacant space on the other side. The second condition is to assume, a? first, that all y the cables in the tray are power cables which will uniformly generate heat throughcal the tray. These conditions allow the random mixture of calle to be treated as a homogeneous rectangular rnass with uniforra heat generation. The task now is to simply find the allowable heat intensity (Q) Fog.1. Cross-section slice from a randomly arrapged. closely pacled for trays containing variable amounts of cable.Onec we find the heat cable fray. intensity, the heat which can be generated by each individual conductor (q) can be calculated from /~ s GRQUP / OF 12 QA \\ q-(1) CABLES n \\ /s i Si tlG L E ' A > CABLE where n = number of conductors in cable g i A = cross sectionalarca of the n-conductor cab!c I [ / \\ Q = allowable heat per imit arca generated in the tray \\ y,' \\ and, of course, ~ ),, 2 9=1R (2) Dettecuve u\\ w het c I = maximum allowable current for a conductor R = a.c. resistance of conductor at the maximum operating temperainae of the insulation materialin the cable tray, lig :. l'hysicalsi:e compartson of t.ipocalrul>ber insulated cables. lleat generated in any tightly packed cable tray must pass This comparison CJn be rnade ovef and over with the present through two media: 1) the cJble mass, and 2) the air immediately opacities for cables in trays. The result is that small conductor size around the tray; $ince heat flows through the media there is a cables are allowed to " work" harder than the large size cables when resulimg temperature drop in each. as shown in Figure 3, 6T, they are all placed in a comrnon random tray. Actually, all cables through the cables and AT throurb the air, 3 suuld be worked uniform 4 by conung to the same operating temperature n the tr.iy, To determine the total amount of heat (WI which can be dmirated by a cable tray m an aminent temperature tT ), and a AN A LYTICA L.\\!OD1'L mamta+n its highest temperature at or below the operating temper, ature 1Tm) of the cable insulation m the tray, we must hmet the Wheneser cable ampacities can be estabbshed with calculationi, system temperature drop 16T) to instead of an empirical approach, a better understanding of the LT = Tm Ta (3) 963 = 'Wd-
.~ e + as The system temperature drop is the sum of the drop through t the packed cable mass (4T,) and the drop through the air (6T ) test results to be presented later show this value t'o be vahd for either around the cable tray. rubber or polyethylene snsulated cables which are tightly packed. Therefore At this pomt we must defme cable tray percent fill as the sum of the cross sectional areas of all cabics in the tray (including conductor, insulation, and jacket) divided by IFe total available AT = AT + AT, (4) c cross sectional area in the cable tray (width times height). It can be The drop through the cable mass (6T ) can be obtained from seen that a cable tray which is packed as tight as possible and level for a rectangular slab with uniform across the top is filled to about 75%, because about 25% of the tray g the equation given by llolman8 internal heat generation. area is void area between the circular cables. From the above percent tray fill dermition it is apparent that a 6-inch deep tray with 20% fill AT = ~ad has the same depth of packed cable as a 3 inch deep tray with 407, W ($) rill c 8w where p = effective thermalresistivity of cable mass d = depth of cable rnass in applying equations (1) and (2) to get the ampacity of specific w = width of cable mass and tray conductor sizes in a given cable tray, an interesting observation can W = the total heat generated in the tray per unit length be made.The cable ampacity (1)is given by - p I= (8) Equation (5)is specifically for one dimensional heat flow out the top "N and bottom of the tray and it ignores any heat flow out the sides of the tray. This is a realistic simplification which is accurate for 6-inch and substituting for the circular cross sectional area of each cable IA) we get and wider cable trays. !=D Or The temperature drop through the air (4T )is obtained from a (9) ~ ~2 nR s heat balance between convection and radiation heat flow. Using basic 9 equations from McAdams we find W = hA 6T, + cA ctT 4 T,d) (6) 25 3 3 c N where hA,oT, the heat loss from the tray due to a 20 convection c A,elTe T,4) = the heat loss from the tray due to d g 33 \\ y radiation ' E and h = overallconvection heat transfer coefficient .) for tray y, j'o \\ A = surface area of cable rnass per unit 3 3 o tray length \\ \\ e-90*c i g \\ \\ /f 75'c o = Stefan-Doltzmann constant \\ \\ V/j GO*c 8 ~ = cffective thermalcmissivity of yI e cable mass and tray surface / w 6 T, = average cable mass surface temperature h \\ \\ 3 The three equatsons (4),($), and (6) have three unknowns and they can be sohed to get the total allowable heat which can be generated 4 I in a cable tray (W). Since equation (6) is quite non-linear, the \\ - l solution to the three,cquations must be obtained by iteration; thus, 3 \\ m 3 for general application the solution for W is donc snost easily on a \\ g a computer. 3 A \\ Having the total heat generated in the cable tray, the heat generation per unit area is simply h h h w t.S \\ \\ i (d)(w) The ampacity of each cable in the tray is finally determined t I sith equa' ions (1) & (2). to 15 20 2b 30 40 50 60 70 80 l PERCENT TRAY FILL TilLORETICAL RESULTS The solution to equations 14L ($). and (6) fur W and several degrees of cubic tray fill ujil resuh in curves enular to those shown in Figure 4. It is seen that as the cable tray percent fillincreases. the { allowable heat antensity decreases due to greater temperature drop in Ag. 4. Allowubic hrar mirnuti (Of in mamtnin rubbcr hAc or the tightly packed cable mass. I'ngurc 4 was made for an cffectivt thermal resistivity of the cabic mass being.t00*C.cmiwatt; and the Imh erhylenc cohles or the spenfied temperature m Linches deep by N4nches mJe trays operating m a dry'C' ambient. 9 f>4 4 m 1
~,e r-i i 14 is seen th.it the ampacity of a cable is directly proportioncil to its too low for the large conductor mict Note that for the thm wall overall diameter (Dh Thus, mcreasms lhe msulation thickness on a XLP insulated cables, the ampacitws are even lower tiun for the given conductor increases its diameter and thus increases its ampacity thick wall rubber cables, and the safety of the present ampacities I when mstalled in a cable tray, for a given percent tray fill and the would be even rnore questionable. same temperature hmsts. This point is made 30 supplement one of thc favorable pv I llcre it must be pohted out that the ampacities of the bulky erties of the small diameter XLP cables. Specifically, mm P l rubber insulated cables in trays are not at all the same as ampacities cables can be installed in a cable liay than other kinds of i wd a for the small crossimked polyethylene msulated cables with very tiun cables, and thus there is economy m using fewer cable tray:, Along insulations. For example, a number 12 AWG rubber insulated cable with being able to instati more cables in a tray it is essential that the with a diameter of.24 inches may have an allowable heat intensity tlun wall cables carry less current than the heavier insulated cables, if (from Figure 4) to give an ampacity of 24 amps;the same conductor tius is not done, there will be overheating of the XLP cables and the insulated with crosshnked polyethylene would have a diameter of accelerated loss of cable hic tesulting m premature cable failures, only about.16 inches and therefore, from equation (9), an ampacity of 16 amps. It thus becomes necessary to distinguish between thin The best observation to be made from the theory is related to wall and thick wallinsulated cabics;throughout this paper. reference heat generation in cable trays' being in proportion to the cross-to polyethylene cabic imphes thin wallinsulation and rubber implies sectionalarca of each cabic. A somewhat evident justification for this thick wallinsulation. requirement can be seen from the following reasoning. The above difference in ampacity cornes from the fact that for a The most elementary equation describing convection heat flow given percent tray fill, more crossimked polyethylene (tlun wall) is insulated conductors can be packed into the tr,ty than rubber (thick wallJinsulated conductors. Smcc the total amount of heat which may q = hA AT 3 be generated in the tray must remain constant, the heat per con-ductor must be less for the small diameter cables than for the large where h is the convection heat transfer coefficient, A is the surface 3 ones. area convectmg heat to the air, and 6T is temperature difference between the cable surface and the ambient air. The basic equation i With the allowable heat intensities from Figure 4 and using for conduction heat transfer is them in equation (9), the ampaci'ies of several cable sizes and 6T percent tray fills can be obtained. The results are shown m l'igure 5, q=LA g which is a graphical ampacity table for typical single conductor rubber insulated copper conductors installed in 3 inch by 24 inch where L is the thermal conductivity of the heat conducting medium, [ cable trays. For comparison, the presently published ampacities for A is the cross-sectional area through which heat flows, and AT is the c the same type cable are also plotted; they are for the assumed case of temperature drop over a distance as in the direction of heat flow. l maximum deratmg which is for 43 or more conductors in the tray, Note that convection heat flow is proportional to surface area while ar.d thus are SOW of the ampacity of a three conductor cable in air, conduction heat flow is proportional to cross-sectional area. Since conduction is the r.overning method of heat flow within a tightly j packed cable man, we should be concerned with eromectional areas of cables rather than peripheral or surface arcas. WOO - & , j =
==M'--:: t?tt 20%TILL 4 30 W t TEST PROCEDUltE I l f
- CEA 4
NEC -E Mgp Five different cable tray arrangements have been thoroughly j J f /g tested in order to deternune the heat transler properties of cath - 10 0 A p-- arranrement. Two of the tests involved randomly arranred cahtes of b M fM various sacs in 24 inch wide trays and three tests were perfortned on j z e' y g g 12-inch wide trays with only one cable site in the tray. Table I se I H sommarves the various tests which were performed and sture 6 ' d, / h shows the overall test setup. m s e d,* " 3 TABLE I - Surreary of lests Conducted to Support j g 4 3 Analvtical Results 8 i lii l l hig I F.AY PE RCENT CA3LE 5!2E5 INSU9T10N 517E F il.t Tf51[D TrPE 't IO IOO 1000 CONDUCTOR SIZE (MCM) ,, x ft thick 3"il2" 40 3/C-al2 Rt.ber s fig. 3. Amparsnes of typuul ruhhcr snudarcJ rapper cables or 3"x 3"x12" 40 3/C"12 LI \\ thin N" tran us determom J h.: Ilus study und comparcJ wah 3'*\\2* 50 llc 003 M ) wol1 7 IKl:rl und N1.C calncs for tras s contunun.e more than 4) cuuductws, WP Coversting temperature un a # Combu nt Some d: tails of tid testmg wHeh mere es>mmon in all tests can be ven f rom Figure 6. t 00 solt rated copper (onJuetor cables were Tlns graphual comparnon, along with test results presented laid in a N-foot long cable stay and termperatures were measured at later nukes it quite clear that the prewnt ampacities f or trays with three different tray cross vetums; one was in the mid. length of the high per6ent fA are too high t'or small conductor sacs, whde bems tray and two others at the gustler lengths. In nuny cases cables %5 t- +f e
- e cstended out both ends of the tray in order to rnake up connections more casi y. To ensure that there was ne heat flow from the tray All testmg was e:onducted uth smgle phas'e 60 herti alternatmg l
center out the ends of the tray, a ring of fiberglass buddmg insulation current. Other insoliptumd have shown that eswntially no dif-was wrapped around the cables at cach end of the tray. This served ference exists hetween three phase and single phase test results. Thus. to rnake a short hot spot at cach end where the cabks tan about 5'C int current was apphed to rach conducior sue by pawns it through hotter than the cabks mside the tray, and thus no heat could flow a long contmuous length of wire folded back and forth m the tray out the tray ends. the reqmrrd omnber of times to ret the proper quantdy nf cable in each test tray The voltare apphed to the cable was only enourh to UYtrCome the s'ktlHealifnped.ntce of the long Contuluout wire. 7 ~' T19. Temperatores produced by the test currents were rucasured I with No. 20 AWG iron-constantan thermocourtes connected to a 24 ".. h.a f. ' point therrnocouple recorder. Cahbration 2. cach tScrmocouple was (g checked orainst a st.mdard thermometer dy comparint. thermocouple 1 j
- J, @,',
readings at room temperature and in bahng water. The reviation was L less than 1*C from the known ten.peratures in every case. The ~ thermocoupks were placed on the i<.st cabics by making a narrow slit j j.' in the insulation just wide enough to accept the twisted end of the thermocouple, thus cantwddmg it in the cahh insulation. This j pernuts accurate measurement of the snanimum temperaturc in a cable tray, provided thermocouples are placed on the side of cables s at the mid4cpth of the packed cable mass. / g m, g. Finally,in order to closely pack tiic cables in each tested tray. ~ ~ ^ plastic tic-straps about 0.2-inch wide were used where required. The tic straps were generally passed through one of the ventilating hoics Frg. 6 Overall eiew of test area showing 24-ft. long cabic trar. in the tray bottom, over the cabics to be held down, and batl l loadmg transformers on right. and thermocouple recorder through the tray bottom and secured. i in the center. I i ALL CABLES LOADED TRAY BOTTOM SEALED i o A UL TEST ON #6 CABLES - 6 LAYERS g CORRECTED TO 40*C AMDIENT ,/ a / g,0 i / 3 I' / [ w 20--- !I g / "12, MO
- p
- 6
- 4 1/(
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J 5 6 7 8 9 to 20 30 40 50 60 00 100 20) 300 CURRENT ( Anap s ) }ip 7 7n: Roults for the 33 h rirnt IillNom h h'ade Trar Conorunne the I<>lhowung Thu i Is'alltw alairJ Cobh t C E E 517E OUTSIDE D1 AMETER 1 O'.Wti ! T ) U: T RAY
- 12 j
10 .25" i E .27" 78 1 6 .36" 9 l 17 4 .40" 1/0 45" 43 2/0 .65" 8 4/0 .70" 6 3 Multi- .E" 6 (cnductor el2 To fill Tray to 55 Fercent i 9 6 f, m-
e TEST ItLSULTS The set of trungle points m Figure 7 is data taken from an unpublished report made by the Underwriters
- Laboratories The data from each heating test is summarized in Figures 7 Incorporated in October 19$7 The report is substantially the basis through 11. In each figure the theoretical steady state temperature for the cable tray derating factors published thus far for trays in nse for the indicated cabh sizes is drawn as a solid line and has been which cable spacing is not maintained. The two triangle points are obtained from the allowable heat intensities in Figure 4. The taken directly from Figure 2 of the U.L report, and are for a 6-inch theoretical ampacities are all based on an average cable mass thermal wide tray filled with six even layers of single conductor No.6 rubber resistivity of 400*C.cm/ watt. Test data is indicated by the plotted insulated cable. The correlation between the U.L test data and the points.
theoretical calculations is remarkabic. The first test which was run to estabbsh the validity of this The data in Figure 8 for the 20% filled tray shows much less method was on the $5% filled tray. Figure 7 shows an appreciable scatter than the $$% data, with the majority of the points being amount of data scatter which can be attributed to air flow through nearly coincident with the calculated values. All the cables came up the tray. But it must be noted that when a sheet of.003-inch to the predicted temperatures since care was taken to lay the cables thickness polyethylene was placed under the tray to seal the close enough to prevent air movement through the cable mass it is ventilating holes in its bottom, the temperatures came up to the important to note that the temperatures in the 2(yh filled tray cateulated values, as shown by the open data points. The reason for remained essentially constant when a layer of polyethylene sheet was the air flow through the tray is that when it was assembled, all the placed on the tray bottom. cables were first laid loosely in the tray and then later tied down. This sequence did not effectively form trapped air pockets in the Possibly the most irnportant information which came directly areas where thermocouples were placed. Subsequent test trays were from the data is in reference to diversity within the tray.The triangle assembled by placing and tying a large handful of cables at a time, points plotted for the No. 6,1/0, and 4/0 cables are for only those which is more representative of how cables are installed in the field, three cable sizes carrying current, and the No. 12,10,8,4, and 2/0 Note that even though the majority of cables in the $$% filled tray cables being unloaded. The No. 6 cables ran about 15'C cooler than ran cooler than calcuated. there was a group of No. 6 AWC cables when all cables were energized but the 4/0 cable only ran l'C cooler. within the tray which did reach the calculated maximum temper-ature. This points out the fact that all cables in a randomly arranged It is from this experimental finding that it appears to be unwise tray cannot be expected to have the most thermally adverse environ-to increase cable ampacities on the basis of diversity. The cabics in ment, but some of them will. the above diversity test were separated by about 6 inches of" dead" . ALL CADLES LOADED o TRAY DOTTOM SEALED A DIVERSITY WITH ONLY 3 SI2ES LOADED 60 y - 50 y% / l / Y ./ // Y / / e m ,/ M y / g j W 20 g 2 90 "B '6 84 I/O 2/O 4h w .? I 10 20 30 40 50 60 CO 10 0 200 300 400 CURRENT ( A mps) i ) ng. 8 Test Raulu Jar the.'u Tcrerni nit N.r sch n' ode Tror Cuntaining the DsII.m#g Thick h%ttInudatal cabkt I CA3LE $12E OUTSIDE DI AMETER QUANTITY IN TRAY
- 12
.25" 10 14 .27" 8 6 .36" 6 6 40" 10 4 45" 8 1/0 .65" 6 1 2/0 .70" 3 4/0 .80" Multi-6 Conductor #12 To Fill Tray to 20 Percent i 967 +
- v
~
4 cable, but st is conceivable that the No. 6 cabics could be picced respond the same if twier the number of cables would be in it. Usmg adpcent to, or between, some 4/0 cables. If the cabics in tids the present desating factors for more than 43 conductors in the tray conLguration had increased ampacities based on assumed diversity, (46 in this case) one-half of the three conductor cable ampacity there would undoubtedly be a local hot spot in the cable tray.Thus, would result; llus would be one-half of either 487 amps. or 365 it sectns impossible to apply a general mcrease in the ampacities of amps, dependmg on whether li'CLA or National Elecitic Code smalkt cables due to diversity because there is no general way to respectively is used. !!ath of these values are sigmGeantly kss than assure that small cables would remam separated from large cables in the 324 ampere ampacity which comes from calculations and testmg randomly Elled trays, on the 50% hit tray. Cigure 9 shows results for two different tests on trays with 40% Figure 10 also shows execilent correlation between calculated 011. one with 3/C 62 thitk wall rubber insulated calde and the other and tested ampacities for only one lightly packed layer of twelve with thin wallX LPinsulate:d cable contained within a neoprene jacket, cabics, which is a 26% fill.This exampic again shows that the present The same total amount of heat was generated within cach cabic tray, derating factors for cabics in trays are much too low for the large but the smaller diarneter cables generated less heat per conductor cables installed in wide trays, because there were more small cables in the tray. Tjus shows that all 3/C.12 cables do not have the same ampacity when installed in trays. and for a given tray Gil the smaller the cable diameter the lower its cable tray ampacity,in accordance with equation (9). 50 / e j It is interesting to note that in the tests producing 50'C rise in / Figure 9, the two thermocouples closest to the tray sides only ran 2 ~ / to 3'C cooler than the other five in the tray. With the vertical M 30 temperature gradient through the cable mass being on the order of E 15'C. it is quite evident that most of the heat in a cable tray nows w vertically, rather than horimntally, as assurned in equation (5). From h20 the above Gnding,it becomes clear that cable ampacitics developed <r 50 % 26 % with slus method are vahd for trays 6-inches or more in width. E a 2w F-60 10 0 IW 200 2 % 300 @ 500000 WO CURRENT (Amps) 50 0 40 Fat JUrst results for the 360 itlCAI thin wall insulated coirics in 124nch wide troys filled to 30 percent and 26 percent as w shown bclow: g 30 cc / $0% FILL 26% FILL to THERMOCOUPLES TEIN THICK k'My)) e h
- g tr W
Coble O ll - 1.01" Cable uly. I til" y Quunitry - 23 (inantury - 12 6 T/8 910 . IS 20 30 One bst example of this same idea can be taken directly from CURRENT ( A mps) the data of the unpubhshed Underwriters' Laboratories report,which is the basis for the present derating factors for cables m trays. l'irure il shows calculated ampacities for rubber msulated, single conductor Fig. 9 Test resufr> Jar 12-inth woJr trays Jt!!cJ so 40 percent wn. 500 Mal cables in a 30G hil tray and a 602 Gil tray. In both cases ranting thick well and thm wellinsudated 3/C#12 cables as the data from Vi ure 4 of the U.L report shows the cable ampacity t shown bcfme to be even peater than the calculaled ampacity.The reason that the cables tan cooler in the test is most hkely due to air now through the THIN WALL THICK WALL tray because of air gaps betwecn the cables. THERMOCOUPLES - MMNi$ bkMEff the rauhn presente ms aN r ca e im win haw achieved steady state thermal egmbbrium. Fyure 12 shows that it reymres about sis hours Inr a cabk tray to reach steady state Coble O 11 .475" Cal;te OJA .70" conditions, whether it is Glkd to 20 or.3011hese roults can be Umtery - XI sjuontity - 33 used to calculate transient ampacities of cablo in trays which may see loadmg lur. say, on!> one hour 4 a tuneJI his w ould he pomble. 't he Imal test was run on singk conductor 500 Mol tiun wall of course. only it all the cables in the tray were loaded and unloaded XLp insulated cabk. liyuse 10 shows excellent a;rcernent between sirnultaneously, and if a precise knowledge of the maximum loadmg cakulated and lest ampuities for the XLP cables. Ahhough the duration couhl be auured. results are for a l>mth wide tray. a 24 m.h wide tra) would %8
I e i Another precaution can best bc explained by visuahring a tray with many small cables and three 750 MCM cables. When the tray is filled to a uniform depth up to the top of the Lage cables (about i [ ' 31%) the criterion for the theory is fulfilled. BJi II a tray would { 50 contain the threc 750 MCM cables and enough semaimng small cable U 40 / t bring the fill in only about 15% the large cables would stand I e about twice as high as the small cables.and the theory would not be w satisfied. Using Figure 4 for a 15% tray fill would assume a uniform i {30 depth and would effectively flatten the large cables into a rectangular E shape, rather than circular shape. The calculated arnpacity would i h20 then be for a " rectangular cable" of 750 MCM cross section which would be meaningless. e 60 % 30 % ia g For the above reason a limitation must be made that unicss g specifically crmincered, no cable in any tightly packed cable tray shall be allowed to carry a current greater than that of the same size three conductor cable in air operating at the same temperature limits. This is because it turns out that one layer oflightly packed single CURRENT (Amps) '"*"E* conductor cable given in reference 5. Fig.11. Underwriters
- Laboratories test resultsfor a 2* inch wide tray consauring one and two laycrs of //C-300 NCM rubber-Ide it must aho be made clear that the percent fiHs used in this roble with an assumed oretallr/rameter of f.16 inches.
paper are specifically for 3-inch deep trays only. The tray width is variable without error, but a 3-inch deep tray with 30'A fill would g only have half the depth of cable in it as a 6-inch deep tray with the same percent fill Obviously, for the same percent fal the frinch deep / s h / N26% FILL (500 MLM) tray would run much hotter than the 3-inch deep tray because heat - g 80 would have to flow through twice as much packed cabic. Thus, to N l3 simphry the applicatico of this method, percent fdlis best figured by E \\ 40% FILL /C 12 dividing the total area of cable in a tray by the area available in the S l l l same width but 3-inch deep tray, even if the tray to be used would j h 20% FIL'L RA'NDOPl \\ " " #8 # E' ex 3"@ The results presented,in this paper are for open trays without y any cover. In locations where covers must be used, the Underwriters' h laboratories found that open cable tray ampacities must be reduced by about 4 to 51 Since covered cable trays are usually found g 20 y/ outdoors where they may be exposed to the sun's radiation, care L-should be taken in specifying the ambient temperature for ouldnor 0 trays. Specifically, ambient temperature fnr a cable tray is the 0 1 2 3 4 5 6 7 8 9 10 '#'"I#### * '# W ELAPSED TIME (HOURS) cxternal heat sources except the 1 R heat from the cables within the tra y. Dr /2 Temperaturc respone of three dif/crent cable tray assembhes. This definition of arubient temperature can be useful for a first / DISCUSSION approsimation in handhng cases of rnutual heating of several trays stacked in a vertical row. The extent to which lower trays wiH affect !! is seen that the ampacities of randomly arranged, tightly trays above them wiH depend on how much total heat is generated packed cables in trays can be cateulated with abnut 5% error. The within each tray, For trays containing both power and control cable, method is safe for any number of cables in a tray as long as they are the effect of mutual heating wdl usuaHy be snuch less than with trays ps: Led to a uniform depth across the tray. (it should be noted that containing all power cable. An ambient increase of about 5 to 10*C this is a condition w hich is easy to inspect as constructiun progrcoes for moderate and extreme cases respec:ively, would probably be the in the field.) Although this method was developed and tested for simplest way in account for mutual heating,if necestery, many 600 vult (lass cables in a tray it also yicids reahstic ampacities for 4 L V class cables mixed with low vohage cable, The last observation to be made involves the idea of deter. mining some " optimum" percent tray fill figure 5 and Table il To apply this snethod and form a workable ampacity table, show that cable ampaeny drops off hiemlicantly when going frorn some precaulsons snust be obsersed. Figure 4 is the most general way low to high percent tray idis. Combmmg this with the fact that the to present cable tray ampaalies but it is awkward to use since heat cost of instalhng a tray is usually less than just one large copper int:nsities must be converted to arnpxity for a given cable dumeter conductor which would lay in the tray,in some c; net it may be poor with equation (9). This has been done in Tabte 11 for 20,40. and econnmy to fitl trays more than one cable deep. Tins wouhl vary 60 a fdled trays contaming typical durneter XLp and rubber in-with exh installaimn and would he prinurdy dependent on the sulated cables. For actual cables whkh may have a different di-available head room in a particular area of a plant. Ilut in cach case, am:ter, the actual ampacity is obtamed from the simple proportion, an optimum tray idl would probably esist. lactual, livrical Dactual D ypical i 969 m.
, ~.- e i TAULEll Ampanties for Copper Cables in 3 inch Deep Cable Trays, 90'C Operatmg Temperature in a 40*C Ambient Typical Cable Ampacity for Each Conductor l Conductor Outside Dumeter 207. Till 40% Fill 60% l'ill Size Rubber XLP Rubber XLP Rubber XLP Rubber XLP 1/C14 .22 .17 11 9 7 6 5 4. 3/C.14 .57 .46 17 13 11 9 8 7 1/C 12 .24 .19 15 12 10 8 7 6 3/C12 .62 .51 23 19 15 12 II 9' 8 1/C.10 .26 .22 21 18 13 )I 10 9 3/C.10 .69 .57 32 26 21 17 16 13-1/C. 8 .36 .28 37 28 24 18-18 14 3/C. 8 .94 74 50 43 36 '28 27 21 1/C. 6 40 .32 51 41 33 26 25 20 3/C. 6 1.00 .82 60 60 48 39 26 30 1/C. 4 .45 .37 72 60 47 38 35 29 J/C-4 1.15 93 80 80 69 56 52 42 1/C 2 .51 43 104 87 67 56 51 43 3/C. 2 1.28 1.07 105 105 97 81 73 61 1/C 1/0 .65, .54 167 139 108 89 81 68 1/C2/0 .70 .59 202 170 130 110 98 83 1/C 4/0 .80 .69 287 252 188 162 142 - 123-. 3 1/C 250 .92 .77 320 304 234 196 177 148 1/C 350 1.03 .88 394 394 310 265 235 201 !!C 500 1.16 1.01 487 487 419 365 317 276 1/C 750 1.38 1.24 615 615 610 548 461 415 Notes: 1) Ampacities are for any width tray filled to a uniform depth. 11 A 6" deep tray with 20% fill has the same ampacities as a 3" tray with 40% fill 31 Correction for different ambient or different operating temperature is done by the estabhshed IPCLA methods in reference 5.
- 4) The above ampacities are specifically for the cable diameters shown; account for deviations with enuation 10.
CONCLUSIONS from a closely spaced cable tray ampacity to a higher ampacity once cables are separated to about one. fourth diameter, it has been demonstrated that the temperatures procluced in tightly packed cable trays can be predicted with good accuracy.and A surprising fmding which has been made is that typical rubber ampacitics can pc calculated for randomly arranged cables packed to insulated cables have sigrificantly higher ampacilits than typical a umform depth across a tray. Cables are permitted to generate heat crosslinked polyethylene cables when packed in trays. This seems in proportion to their individual cross-sectional areas.and thus cable contradiciory to what would be first expected. but a closer analysis ampaaty is directly proportional tu cach cable diameter, shows that more XLP tables van be placed in a given tray becauw they are smaller in diameter.Therefore,the heat per conductor must The derating factors pubbshed thus far for cables in trays can be less for the XLPinsulated cables to keep the total heat generation lead to serious overheating on small conductor sizes while resulting in within a pnen tray constarit. i significantly underloaded large conductor sites. Even though the presiously published deratmg factors have distinct lunitations the RLFERLNCES I unpublished basic data which was used to form the f actors agrees 4 scry well with ampacitics calculated in this report.
- 1. S. J.
H ow h. "1 he Cm re.f rarr) mg - Capasit) of Rubber-Insulated ConJudors. "All.L 'l ransactions" Vol. 57, pp. l A simple table of dcrating factors which can be apphed to 155+7. Wrch 19h. existing arnpacity tables to get cable tray ampacities seems impossible. Table 11 is about the only way to simphry ampacities of
- 2. IPCL A Conunintee on Researth, " Current Rating of Cables as.
cables in trays. and it can be expanded for different tray fith with Aliceted by blutual lleatmg in Air or Conduit,"'"All'E 'Irans L + ? ligure 4 or for datierent temperature junits wah the methods Jdmia." Vul t 3.1944. pp. 354 365. f described in reference 5.
- 3. 51. AL lhandon. L. hl Khne. K. S. Geiges. F. V, Paradise,"The -
The ampacities in Table 11 are consistent with derating factors lleatmg and hlechanical Effects of instalhng Insulated Con. for cables with maintamed spacing in that a logscal transinon is made duetors in Steel R acewa ys," "AIEC Transactions." pp. j 970 t i n -r w .. ~, -n .e
s 1$531569. February 1957. .W o the total amount of heat which is generated m a saMe tray in watts /lt
- 4. National Fire Protection Association " National Electnc Code,"
= width of packed cable mass or tray in inches. Boston Mass.,1968 edit on. w
- 5. AIEE IPCEA," Power Cabic Ampacities," New York, American e
= the effective thermal emissivity of the packed cable mass and Institute of Electrical Engineers.1962. tray (dimensionless).
- 6. Cencral Ekctric Company, " Wire and Cable Selection and p = the effective thermal resistivity of the packed cable mass in Technical Data." lindgeport, Conn., April 1967,
('C f t)/ watt. 2 4
- 7. Simplex Wire & Cable Co.,"The Simplex Manual," Cambridge, o = Stefan-Boltzmann constant in (watts /ft )/* K.
Mass.,1959. Discuwion
- 8. Iloiman. J. P., "llest Transfer " New York, McGraw-liill Book Co.,1963.
Marshall Morris (Consohdated Edison Company of New York, New
- 9. McAdams, W. ll.."lleat iransmission." New York, McGraw-lldi York u 1000h Die author is to be commended for a very ingenious solution to a complex problem. Where applicable. Table 11 000k C0" I9E gives a quick and convenient method for rating such groups of cables.
llowever, pracheal considerations do not usually result in the
- 10. Underwnters' Laboratories incorporated Report E-28078 uniform dntribution of watts throughout the mass of the cables October 14,1957 (unpubbshed).
which is anumed by the author. In our expenence many of the. circuits lpve htlie or no inad while a few key circuits may be heavily I aded. The IPCEA method Iakn this into account by giving. NOMENCLATURE recognition to load dnernly m estabbshing their tactors. The author's statement, "that it appears to be unwise to increase cable A = cross sectional area of a single or multiconductor cabic ampacities on the basis of diversity" is a good rule for conservative - includmg conductor, insulation. and jacket in (in*). design, although there are many caws when the application of diversity would seem to be justified. surface area of cable tray per umt length in (ft2 )/f t. I w uld suggest that load diversity could be incorporated into the A a 5 author s method by adjusting the value of percent fdl used in the calculations. T his could be done very simply by inscrpc5 tion m the overall diameter of a cable in mches. D = Table IL The proper value to use would be a maner of judgment dependmg on the relative location of the heavily-loaded and hghtly. d = depth of packed cable mass m a tray in inches. loaded cables, overall conv,ection heat transfer coefficient for the cabic tray Manuwript recened July.10. !"70. h = in (wa t t s/f t * )/' C. the umpacoy,or maximum allowable currcnt for a conductor t = Ralph 11. Lee (1L 1. Du Pont de Nemours and (.ompany. Inc..
- d PCTC5-Wihnington. Del.l: l be author nas perf ormed a highly commendabk service to our technology in demonuratmg that cables in trays the number of conductors m a cabic.
perform Ibermatly in a nunner spute ddferent f rom that in raceways. n = Recopmtion of this fact should be the starHng point for adophon of the allowabk umform heat per unit area which can be rational derating factors for cables in Ir.rys and reabstic loading of Q = W ^ " ' d '"I '8 h ',h the k" f or many users.' generJtCd within a cubk tray in Iwatta/ft)/In. responuble negled of deratmg. Ihn, wH absens e of realnhc appliutum rules has c.iused much of the the heat generated by each conduttor in a tray in watts /ft. m.dlunenons of tray systems m power use. 't he San Onofre e.ne h a q m prime etunple of thn. the n.c. resi tance of a conductor at the operating temper. While we agree m reneral with the author, on the b.nis of interim R = ature of the insulation material m the cable tray in ohnn. resuus of a ten we have m progm mere aN a few pmnts of dharreement. first h the anthiuN awomption that the tranuerse thermal conductante of all able wes n the ume t ItHi"P em. watt t LT the total temperature drop t. rom the hottest point in the tray = in Imer sires die condosior.co unulahon ratio is grealer; also, te ambient in 'C. there are fewer but larger air spaces lloth of these factors would lopeally unpose a rennlance factor whkh h lower for the large sires, the temperature diop through u.e packed cable ma>s in 'C. Freater Du smaller wes t he raho of snull to large cabks in a trav J.T = c would markedly atfect Ine ocer.att tiermal reshtony, makiny any '"""'"'I"'"'""'""""""dd'#'"'d oble or uses hberal and dangesous H the till n!c idh of niosdy large AT the temperature drop through ths air surroundmg the able e a ruostly small ables tray m
- C.
Secod me Nb of N.0 nerem bhm On m m mur stow witm t. n sc6ognued by the anthor m coniunction wHh T, - the ambient temperature of de tray due to all heat suunes ha 1 wre 2. In thn. sesen = t 2 3 C obks are show n lo rs ocr.oc j to oui 4Je the cabk tray in 'C. J tune 3 ihe heat on one 84 0.I C uhl+ hasm; the ume crow wetmn area. Ihn fut, howescr. n submp endy not (onudered ' by' the - aserjee ubt/ man surface temperature m *C. d"d'"' di'P "'"di "" d'# d 'C' " ) '"di U"' wunid be equatoed - T, throuch the t hermal (ondu bon of the hil 1)cparluie os obsened s T the nusunum operating temperatme of the able msulahun a m in the tray in 'C. Wnuwnpt resened Jul, 2S PD0 971
l ? e dalJ 1 rom throrcheal calculation % lor larger percentage idi, rnay be Where the hu!L of the sonduttors or cabisk are of the smaller due to thn and the presious puent. 1barJ, en Figure 7. the author's observed data for cable temper. vanet). it appears thJI the deraung factor needs to be greater than atores lunsealedt faH mell below the theorchealimes sencept Ph g,s.L where the installation tonsnh uf average or larse-surd cables. j Calmlahom on the rateo el heat los at rated current. to the surtaa sera rif 3,0 cables shows 86 p tu be the most heavdy loaded of all a It ],,T y way nues, m lernn of *Jilge per uml se surface area. lhe 1%N NL,C taungs pernut its surface thermalloadmg to 4.I'J h her than #12 p. / T and 7W higher than P4/0 and 500 MCM. It n sigmheant that the author's data for Pb conduefors show its temperature nw at rated u / / current 10 be highess of all nues tested. - 510 0 / .4 i / " " " " * ' ~ " in out tesh tusing cables of only one site for each testi for tb and / a P: p. results agree rather weH with the authori observahons, in / / Figure 1 for f2 p., our 23.79 ful falls weH between the aulhor's j [ / / i f t/0 and P4. 20V Idi data. Aho our #2 p. 47M (dt data is [, l / !' p [ - intennedute between the author's fi/0 and $4. 55% fill data. ~Ihe pneral slopes. hkewise. agree rather well y9 / .s/ #y ,7 / h*/gi ci N'/ ? wn v - E' b j eM A - s.. T v tan 3 -[ , se l W v / [ ,,/ u ( / j a 8 / ,/ p' $.* / p _ -. - - t u m n p / ?= l f j d-l f r { / L= / # (, .x 'g. h/ / / / d.__.7 i, s, u a !% / 1 curmur (a+ > 1= l i-! / c / / / ng )
- ~
/ / Unavoidably, our tests all uhlue 3/C type TilWN conductors with j / ova all vmyl iasket. Ibh cable type h lhe one approced for tray use for more than 3 conductors. and n typical of our generalinstaHa-n. tionA The authori data is largely based on thicker insulation and s, eveas.: (4 ) hg /. pnneipaHy for smpte condvetors rather th.in 3/C cables. lle indicatch that muluple condoclor cables ni tray have mherently higher dm. pacity, but that conductors with tinn implahon base reduced
- 6-IaT s* 1o n
'[ ampaen). T he rather good agreement of hn data unh ours appears 7~ to indicate that the amlueHy mtrease lur muHiple conduelnr cabhng b and the dette.nc for ihm insulahon are approumately equal sa- -/ - f The good agreement should lend credence to the authorichserva- / ..m f j hom, in tontrast to the theorcheal knet especiahy those shown in J j M - di -1, [7y/- his Figure 7. Founh. the presmse of a mulorm level of tray I6 n not /.-. compaDNe wah the use of a few su> tuge awe, te g.. Jfc 500 m n sea th.a ih o,r ofin ~ h mea hoseihe ne,ghiofihe .- e d _ if. hatma oi the uay hit Likew~. ihe con
- 'n
-[..-[.-- --l~ r ni oi paanine hn of iray, n.o s.nunce wah a fundameulal dehnnion of trap. i.e.. a [ /- --/- / / cable support, nul a raamay Variaison m tray side height n baucalf {> for the purro,e of spannmp inier suppori dinanus. or macaong the f weight to be supported. t.dher than los conlanunent. As the aulhori / j f' j [j resnhs mrix.ne, the degree os der. amp reymred w hen the depth of I [ hil re.whes about ' is so sermus that addomn.d tra)s and louer fdh are cionomically justihed.1he conapt of averge hH depth, rather f Ihan pm a,e on. aas g,e lu m.sihikty. aed ~ h ar-mm, w l f ,j the basic crueram. should retene precettence. 1 f, f a l'aflh. m e w ould 6.mtion uwts that Iray h not miended or me +o /k J recomruended lor use u nh smgleconductor u nes or calles. espe-cially the smaller wet As t s !" tal name 'tonunuou3 Rigid Cable Suppoth" mJicates. trav st mhl be used onk w Hh cables inempo-in I yure 2. lor en p.. our data ha 1.4. 30 an i E t'dl aree quur rahng a proweine unermg ol' wmc 13 pe mer the mdantual wed H ilh thJl of the aulbVr. l he) are, hou der. colhiderJM) higher conductor unula hon. \\ tan > l ;! ares and ume hre> have been than the UL 21.' data, ahhop;;h apreemp rather ucil u nh the llL imtuted froni destruilsim ut the retsinely tion insulatiosi of single $ $' ' un pomt. c ond u.t or s m tray s m common wuh larger eaNo 't he medunical in I r;ure 3. tor 512 p our dJIJ inh.atch tyors 01 notalhomn. and snegulanhes of the nmer suriices of trap t onsiderabh rreater lemper. dure trw lor hke hil t han the..albori data. our 2, 3
- hit peally cheed those of comhut, for wlueh sm;le tondoefor impla-tion h mtended Iorhaysaun a meelumeal lacket or protectne i nh... hag lempeiahav nws ascrgny "
preater than that of the sheath h mmt uwtul.md b n quned by the Niional 1.les tihal auhmt :or 20 ; hti ihn n hebes ed due to the iondun m outtmed m hlh of smaller canes h.ne lugna thernut Code in our ua n espaiensv. uith oser 600 nules ni tray in serviev. the hra somment, that the onh.ondus lor laibacs h.n e os coned u ttere small smgle-remindy llun thuw of larger.nerage pre, sonduitms w ere madwrknll) unlalled m tr.n s u dh larger canes. 972 ~ ~-
e f $ nth. our ow n mterim results and apparently thow of the author. indian a unif orm ratio ut temperature rue to watts lost per unit per umt area, the Mtuation shown m Figure 2 is not only torretted, ' plan area of the tray. for tray fiU excludmg through-ventilation. but it is impossihic to occur with the rnethed developed m this paper. w. II) thn crescrion. the deratmg for a 3" htl wouhllogically be such I that the 1.'R low would he no more than HiJl ul a 1" fillin the same The teamn for the poor correlation between the unsealed tray data and the calculations in Figure 7 6 due to the manner in which tray. So current deratmg woulti be mversely proportional to the square root of the fill depth. While detalme by thn means would be the tray was ;nsembled, as explained in the paper. Every ampaaty in too comphated lur direct uw,it would be posuble to factor it into a the tray was pre calculated on the b,nh of undorm heat generabun derating table cunudering cable dmiensions and actual heat loss of for cach cable size, and the 1/C-#6 cables ran hottest because all 4N the range of wire sires. of them were located together. The air happened to be more effectively trapped in that particular portion of the tray. The fact that the NLC permits the #6 cables to generate more heat per nmt surface area than other conductor sites had nothing to do with the test results in Figure 7, because the test currents were calculated without any reference whatsoever to the NEC, Actually, heat per J. Stolpe: I would hke lo thank kit. Morrn and Mr. Lee for their urut surface area is not as good an indication of cable loading in dneumons of the paper. They have raised some interestmg and valid randomly filled trays as the heat per unit cross-sectional area. pomis regardmg the method of cakulatmg ampacities for cables in The data which hir. Lee has obtamed is most valuable, and it randomly filled trays. Mr. Morrn is absolutely correct in slaimg that sems to point ut the WIficuu{ts evaluation. htr. Lee has wNch can be encountered it an adequate theory is not used,th 1/C cable data. and this g there are many caws when the application of diveruly would seem to mi c mpanng /C cable data wi be justined, lhe difference between the IPCLA Ireatment and the one herem n the IPCLA ratings awume that diversity always exists. cannot be done. He has also compared data lrom ilun wallinsulated and that omtrol and power cablo are umformly spaced throughout a _ w% data hom thisk wallinsulated cables, without taking into c cable tray. ljus is generally a very optimatic anumption. account that the larger the cable diameter the higher its ampacity 11 seems that better judgment would dictate general ampacities wd, l be, for a given conductor site, anummg thal dneruly dnes nut renerapy esist. because all it takes is lhe ampacities of the cables tested by Mr. Lee can all be two large umductor, heavdy loaded ciremts located side byside in a calculased. Using Figure 4 of the paper to find the Allowahk Heat tray to produce a local hot spot m the tray crosssection. If these two IntensHy @ conespondjng to each percey M labeled in his Dgures circuits were the only power circuits in a tray filled lo, say, 40 I'3+ cach cable ampaoty is calculated with equation (9). The i pereeni, it wonhl he l osuble lo account for diversity in determining diameters of the cables tested by Mr. Lee were obtained from private the ampanty of each circu t. Thn n only if the engineer is absolutely conopondence to be; sure the cables wdl not he bunched together when they are finally 3/C'.#12 .38" O.D. placed in the trJy. 3/C #6 .69" O.D. In aws where only a few power cables are in a highly filled tray.it 3/C- #2 - 1.0l" O.D. may be feaubte to partition the tray so the power and control cables A sampk caWanon for the ampaoty of the 3/Cf12 eables which would be nulated. Thus. all the control cables could be heaped filled the tray to 26.3 percent proceeds as follows, assuming a 90*C together wnhout dangt of any heating whatsoever, and the power conductor in a 40*C ambient. or 50*C ri>c. cablo could then be plated in the remJinder of the tray. at & relatnely shallow depth. The ampa:ity of each power cable would be From Figure 4, Q = 7.9 determmed from the percent fill of the side of the tray winch in2 contamed the power abk only. Sir Lee has made the sahd observation that the effective thermal resi>tnity of la'ge eunductor size cahks is less than that of the small From equation (9) r wattVft sundmtor sier, lhe value of 400*Cwm' watt was chosen to fit the go r ~ 3* 7 *y s w here large and small cabks are mternused at randum. In 2 I" ' caw specul ases as with dnouty, ampacities of large cahics can be 3 x 2100u0/11 alsted using lower ulues of efleetne thermal resistivity if the engmeer n absolutely sure of all relevant parameters. liut for general I
- 115 amps ampacuy tables, the salue chosen will work the best because it scenunt4 for the case w here a large 6nndueror cable h surrounded by Th6 ampaoty compares very meely wHh the intersettson of the many small cabin in a tray, 50*C rhe coordmate and the 2hJ.i fdl hne ir Figure 3 of the The comment that the variabihty of heat generation h treated in diwunion. w hkh n approsimately 12 amperes. Sunilar alculations Figure 2 and is subwquently not conudered in the paper is rather for all the testa (unduced by Mr. Lee are shown in labk 111. and dxousemg. and indicates that the whole pomt to the uniform heat gnersuon concept haspeen mhsed 1 he sakulated ampacities in ll:is TAllLF lli paper are all based un'il. reqmrernent that the heat per unit area of escr3 cable site in the tray h the sone, lot a ysven percent tray fdl.
Comparbon of Lee Data and Calculations Thh an be cauly cheded in labk 11 by akulaimg the 121< for cash Cond uc tor Pera nt Measured Cakulated able md dividm; by the respectise cable (ruwscetional area. for Sir.e Tray Fdi Ampaat> Ampaoty exampk. the 3/C 44 and liC-500 rubber msulaird cabks have about the s ~ae cable diameters of 1.15" and 1.11." resredne!y. and thus From uraph they pecupy the same approumate tulal area m a cable tray. Usmg 3/C.yl2 8.7 23 i 23 1,he turautin for 40 rereent tray blh show n in Tabk II. the 3/c.gl2 17 r, in I rj 3 33 tollow mg idential hat generalmn m each eahk is oblamed 3fc.pl2 20.3 1; i J3 3/C.sa 3 x sp9)2 (330 pH 115 e Jy waigvig 1/C 500 - 1 x I41982127.4D'11 = 457 walts lt r 3fc..n 15 39 g,o 1he dgSt dnsrepaaey h due to the IJrger cfow-sectionalarea of the $00 KC\\1 able. 3jp.en 30 a} l g, ; - 49 3jc..o an 3; 39 li n pinnt an be f urther emphasued f rom the fact, as wdl be shown laser, shat able ampautres an be caholated trom %gure 4 3/C =2 24 1101 106 and olaauon tN or 191. For a tray with a pnen percent till. ligure 4 3;c..; a Le Allowdle l'mform lleat Generahun for any cabk we m u }t g g i gy now-ty. 2 7[ 37 g; t he t. Ibn sonitant plue h then uwd m equauun to or lo s to arrne t an ampacit) tur each conduttor we m the tray. Ence all amp.u: es for a gnen percent tray hit are bawd on one touform heat they are compared with me.nured.oupaohes.1 he agreement n very I good when all the subik det.nh are taken mio account. lhe matter of how to gener.sily dewribe how much abk h in a i Eriugnpt recened August 31,1970. ohk tray n debatable, and a common -agreement n not cauty 9D r m
~ - " r e e cbtamed. I sery rne.m. tulti winsh the aulbor n farmbar has l AllLL IV adiantages wdl an dmtisantages, and u hatever (onvention n as fmally adopted by the endustry should be adaptable lo the ruethod Comparnon of lotallray llcat tW) with Various !'ercent iray fills desrloped herem. Per6ent fdl w.is chown as the gange lur quanhi) of (12" Width n T Depth. lf!cclitc p
- 400'C-cm/watti cabk en a tra) for uns study besauw it n anrpied b) the inajonly of Percent Alhowable Cable Area 1 otal tray uwts. In alus h the avihor has knowlvi ge-l 1 ray
- 1) cal Intensit y in 1 ray 1 ray The problem of large cables proir,idmg above Ihe average depth pf rdi a tray fill n accounted for by hnnemy the maimum ampacity of any
!Icat cable in a tray to that correwsmdmg to the i layer percent fdl of the nitdfL 10 24.7 3.h in.' 89 watts / particular cable site. A cable sire winch would result m a 25 percent 20 I l.1 in, 7.2 MO tray it. fill f or i layer in a tray, would have the same ornpacity when 30
- 6. 7 10.N 72 mstalled in all tra)m of le>s than 25 percent fdl. and it wonid suffer a 10 4.0 14.4 66 loss in ampaoly only when mst.dled m trays idled to mure than 25 50 3.4 18.0 61 percent. This restnetion apphes to single conduelor, as well as r,0 2.7 21.6 58 ruulti conductor, cables winth are installed m randomly fdled trays.
70 2.1 25.2 53 The sisth poini made by hir. Lee is that for a given width tra>, the 80 1.7 28.8 49 lutal watts per foot of f ray length should be the same regardless of the pertent tray fill. That n. if a tray filled to 20 percent can generats a total of 70 watts /tt of length and just bring the hottest Mr. Lee's ob ervatum is good, but it does not hold for large changen conductor in the tray up to ils rated temperature then the same tray in percent fill, Analyeing his data in a similar manner yields results similar to Table IV. Titled to 40 or even ou penent can also generate a total of 70 ' watts /It, but the heat generated per cable must be reduced propor-honately. This observation hulJs with fairly good accuracy. Sirectly weaking, for a given amount of heat generation in a given width cable tray, the temperaltire rise of the outer surface is contrant with all pertent Iray fdis; but the temperature rise through the cable John inan a directly proporlional to the depth of the cable mass. as Stutpc (M*MI was t orn in Glendale, Cahf., on Uetuber 10,1943. lie received the desaibed by equation (5). lherefore, as the percent tray Idl 11.5. degree in mechanical engineering from increases, the temperature riw through the packed cable rnass j San Fernando Valley State College, North-increases relative to the temperature rne of the tray outer surface., vV sidge, Cahf., in 19th. lie is pursuing the for a given amount of total heat generation. Thus, in determinmg )f M.S.M.E. degrec at the Umversity of Southern cable ampacities,il n necenary to reduce the total heat generation in g { j Cahfornia. Los Angeles, a tray as its percent Idl is increased. Upon graduation he jomed the Southern 7 he amount of total heat reduction can be seen by taking Allowable lleat intensities f rom Figure 4 for v,inous percent tray fdis Cahlornia Edaun company, Los Angeles. and arid mukiplying them by the cabic area in the tray for each percent has been primardy concerned with heat trans-a hil Domg this for a 12 inch width tray yielJs the results shown in fer studies in the Underrround Research and labk IV. For smAl changes in pereeni trav filt Development Departenent, Mr. Stolpe is a member of the Pacitic Coast Llectrical Association. / i i 9 74
ENCLOSURE 2 CALCULATION 13-EC-ZA-300 REVISION 3 >}}