ML20086C246
| ML20086C246 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 06/30/1995 |
| From: | Wadley M NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-92-08, GL-92-8, TAC-M85592, TAC-M85593, NUDOCS 9507060373 | |
| Download: ML20086C246 (13) | |
Text
{{#Wiki_filter:i l I Northern States Power Company l Prairie Island Nuclear Generating Plant 1717 Wakonede Dr. East Welch, Mnnesote 55089 June 30, 1995 Generic Letter 92-08 U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISIAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Response to the November 17, 1994 Request for Additional Information Regarding Thermo-Lag Related Ampacity Derating Issues (TAC Nos. M85592 and M85593) In a letter dated November 17, 1994, the Nuclear Regulatory Commission (NRC) transmitted a Request for Additional Information (RAI) Regarding Thermo-Lag Related Ampacity Derating Issues for Prairie Island Nuclear Generating Plant. In a letter dated January 31, 1995 ve provided our response to the first 12 concerns expressed in the RAI. We stated that we would respond to the remaining concerns (13 through 20) by June 30, 1995. This letter provides that response (see Attachment 2). As we stated earlier (our March 29, 1995 letter), we plan to replace Thermo-Lag on required fire barriers with Darmatt, using the Darchem supplied testing bases and derate values. In addition to our previous commitment to resolve the Thermo-Lag issue at Prairie Island by December 31, 1996, we are making one new NRC commitment. In addition to resolving the Thermo-Lag issue, we will resolve the issues raised concerning ampacity calculations for Appendix R cables wrapped by Kaowool, , also by December 31, 1996. As noted, most of the Kaowool-wrapped cables discussed in the NRC questions will either no longer require fire wrap or the ampacities will be recalculated using the methodologies attached to this letter. q J 060019 GL9208-8. DOC 9507060373 950630 PDR ADOCK 05000282 P PDR f<f#
USNRC NORTHERN STATES POWER COMPANY l June 30,1995 Page 2 s . Please contact Jack Leveille (612-388-1121, Ext 4662) if you require further , information. l k l Ga Michael D Wadle 3 Plant Manager
;- Prairie Island Nuclear Generating Plant a
i cc: Regional Administrator - Region III, NRC NRR Project Manager, NRC Senior Resident Inspector, NRC Kris Sanda, State of Minnesota l l Attachments:
- 1. Affidavit (1 page) ,
j 2. Response to November 17, 1994 Request for Additional l i Information (3 pages)
- 3. Methodology for Conduit Ampacities (1 page)
- 4. Methodology for Cable Tray Ampacity Summary (1 page)
- 5. Methodology for Cable Tray Ampacity (6 pages)
GL9208-8. Doc
l
. l AFFIDAVIT UNITED STATES NUCLEAR REGUIATORY COMMISSION i NORTHERN STATES POWER COMPANY ,
l PRAIRIE IS1AND NUCLEAR GENERATING PIANT DOCKET NO. 50-282 50-306 i THERMO-LAG 330-1 FIRE BARRIERS Northern States Power Company, a Minnesota corporation, with this letter is submitting information requested by Generic Letter 92-08, Thermo-Lag 330-1 Fire Barriers, pursuant to 10 CFR 50.54(f). This letter contains no restricted or other defense information. NORTHERN STATES POWER COMPANY By Michael D Wadley Plant Manager ; Prairie Islan uclear Generating Plant [ Onthis30 # day ofdttott 1993 defore me a tary public in and for said County, personally 4ppeared Michael D Wadley, Piant Manager of Prairie Island Nuclear Generating Plant and being first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his ' knowledge, information, and belief the statements made in it are true and that ' it is not interposed for delay. N7/241 (J M ' 62 V_b
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'f MARtLYN G. NOVEK NOTARY PUBUC-MINNESOTA GOODHUE COUNTY Wy Cm EgmJULY M 1936 1
GL9208-8. DOC l l 1 l
)
J . P ge 1 of 3 R:sponse to November 17,1994 Request for Additional information
! BACKGROUND
- it should be noted irwtiaHy that the cables referred to in major portions of this calculation have either been re routed or reconfigured as part of the Station Blackout (SBO) modification and Electrical System Upgrade installed during the period from 1992 to 1994. This modification
- resulted in the installation of two Safeguards Diesel Generators for Unit 2, the replacement of au 480 volt switchgear in Units 1 and 2 and the replacement of au 4160 volt s#3 sear in Unit
- 2. The magnitude of this modification resulted in the re-routing or abandonment of a large number of the specific cables referred to in this calculation. Specifically nearty everything j associated with Unit 1480 volt MCC feeder cables and Unit 2 diesel cables, 4160 volt motor cables and 480 voit MCC feeder cables. This effectively makes this entire calculation obsolete.
Two parallel projects are presently active to address Generic Letter 92-08. The first is a project to address the qualification of Thermo-Lag 330-1 material and the potential replacement with Darmatt KM-1 material. The second is a reanalysis of the safe shutdown , system due to the extensive electrical modifications as a result of the SBO/ESU changes. ! Because these issues were parallel projects, and because of the requirement to provide a : detailed schedule to resolve the GL 92-08 issues, the scope of the qualification project was to ! address aN existmg Thermo-Lag plus evaluate potential issues with Knowool. Sargent & Lundy i 4 has been contracted to provide engineering on the qualification and/or replacement of the ! Thermo-Lag material which includes new ampacity calculations as required. Our June 30, l 3 1995, commitment to the NRC for items 13 through 20 was based on the original scope for the , 6 Fire Areas within Sargent & Lundy's responsibility (Fire Areas 31,32,58,59,73 and 74) and . would have included an ampacity review of all wrapped power cables (both Thermo-Lag and . Kaowool) by the June 30 date. As the safe shutdown analysis progressed, Sargent & Lundy's !' scope was reduced to 3 Fire Areas (58,59 & 74) and did not include reviewing the ampacity for cables wrapped with Knowool at this time. The reason for this reduction in scope was based on a preliminary safe shutdown analysis indicating that most of the existing Thermo-Lag l protection wiu no longer be required due to improved separation provided by the SBO/ESU modification and most, if not aH, power cables presently wrapped in Kaowool could either be re-routed or reanalyzed to eliminate wrapping and therefore ampacity analysis of the Kaowool wrapped cables would not be needed. i Following completion of the Thermo-Lag replacement project per the previously established ; commitment of December,1996, new calculations of cable ampacity will be in place which will i replace both the Stone & Webster calculation dated September,1983, and the Prairie Island Ampacity Tabulation dated February,1987. The replacement calculations will follow the methodology proposed by Sargent & Lundy and outlined in the attachments to this response i and will include ampacity for both the Darmatt wrapped cables and, if any, power cables wrapped in Kaowool l Specifically to the items referred to in the RAl: ! Item 13 Motor control center (MCC) feeder cables 122-1,123-1,126-3 and 222-1 considered here were au abandoned in place as part of the SBO modification. Charging pump cable 1K2-7 is L normally not energized during safe shutdown, however during the replacement of Thermo-Lag i 330-1 with Darmatt KM1, ampacity calculations for this cable will assume it to have an
- amperage of 134 amps. The methodology for the ampacity calculations is attached.
Gr3046 TML NRC-RAl.boC
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- Response to November 17,1994 Request for Additional information Pcge 2 of 3 l
- m.m t4 Cables 1CR-99,1CY-99 and 2CR-1 supply instrumentabon Panels 112,114 and 212, j respectively. The safe shutdown analysis project is presently reviewing these panels the l cables ==aariatad with these panels. It is expected that cable re-routes and instrument re-
- configurations wiu aHow some or au of these panels to be removed from the safe shutdown list.
j Therefore, these cables wHI not be required to be wrapped for safe shutdown. If, based on j further analysis, any of the instrumentation panel cables that are required to be wrapped per i Appendix R requirements, the analysis wiu be performed in accordance with the ampacity ! methodology attached and wiu be in place by the previously committed date of December, 1996. l I Cables 122-1 and 222-1 are abandoned as described in item 13. l ! Cables 16405-1 and 25405-1 supply the 12 and 22 Si pumps, respectively. Cable 16405-1 l was renumbered 16407-1 and will be re-routed to ehminate the majority of the existing l Thermo-Lag wrap. Cable 25405-1 has been abandoned as part of the SBO modification. A small portion of cable 16407-1 will stiu require fire wrap. The calculation for ampacity of this cable win foHow the methodology described in the attachments as part of the Thermo-Lag
- replacement project and will replace this calculation by the previously committed date of l
December,1996. Item 15 ! This item refers to cables wrapped in Kaowool and are not part of the current scope as defined { to Sargent & Lundy under the Thermo-Lag replacement project. The safe shutdown analysis, 1 which is currently being re-analyzed and re-configured to accommodate the major plant j configurabon changes as a result of the SBO modification, wiH be addressing these issues. If any power cables are to remain wrapped in Knowool they will be analyzed by Sargent & ! Lundy in accordance with the methodology outlined in the attached ampacity calculation methodology. The present emphasis in the safe shutdown analysis, however, is to eliminate Kaowool wrapping on au power cables which should render this issue moot. The complete analysis wiu be completed by the previously committed date of December 1996. Item 16 The SBO modification abandoned the Diesel Feed cables from the Unit i diesels to the Unit 2 buses. Cables 25402-1 and 25406-1 were the D-1 and D-2 diesel feed cables to the old Bus 25, respectively and are presently abandoned. Cables 123-1,16403-1 and 16404-1 supply MCC 1KA2, the 12 CC pump and the 12 RHR pump, respectively. Cable 123-1 has been abandoned. The tray in which these cables are routed is presently proposed to be wrapped with Darmatt KM1, however pending analysis and re-configuration by safe shutdown project may eliminate the need for this wrap. The new ampacity calculation, if required, is outhned in the attached ampacity calculation methodology and wiu be in place by the previously committed date of December,1996. Special Calculations 7,9 and 12 refer to cables supplying instrumentation Panels 112,114, 212,214 and 1EM. These cables are presently being reanalyzed to allow some or aH them to be removed from the safe shutdown list as described in item 14. Special Calculation 8 deals with cables 1HVB-78 and 1HVB-82 supplying area unit coolers. ! The cables are presently wrapped in Knowool. These items have been removed from the safe shutdown list and are no longer required to be wrapped. Gr3046 TML NRC-RAl. doc
- Response to November 17,1994 Request for Additional Inform tion Pags 3 of 3
) ! BEDLil ! The cables indicated in Special Calculation 3 are either abandoned or have been re-routed as part of the SBO modification that installed new Unit 2 buses. The Thermo-Lag has been ; ed from these cables, therefore this calculation is no longer applicable. j Cable 164051 **=en==ad in Special Calculation 5 was re-identified as cable 16407-1 as part of j the SBO modification. Presently this cable is being proposed for re-routing to eliminate a i j majority of the Thermo-Lag replacement on this section of the cable. A small portion of the cable route will still require fire wrap and the calculation for ampacity will follow the ; ! methodology described the attachments. This calculation will be in place by the previously i { commstted date of December,1996. l Item 19 i Special Calculations 7,9 and 12 refer to cables supplying instrumentation Panels 112,114, 1 ! 212,214 and 1EM. These cables are presently under review by the safe shutdown analysis ; ! and present proposal is to remove some or all of them from the safe shutdown list as ! } desenbod in item 14. } Speaal Calculation 8 deals with cables 1HVB-78 and 1HVB-82 supplying area unit coolers. l The cables are wrapped in Knowool These items have been removed from the safe i shutdown list and are no longer required to be wrapped.
- Item 20 L The Thermo-Lag installations are being removed, therefore a review of the Thermo-Lag fire j endurance qualification is not necessary. NSP's December 21,1994 letter to the NRC
' regardog GL 92-08 and the responses to the RAI included Darchem, Inc. test reports for ! review. Fire endurance qualification documents for the Darmatt KM-1 material are: Procedure ! TIQAP 9.20; Manual TlOAM-1; and Test Reports FTCR/94/0060 and FTCR/94/0073. 6/3045 TML NRc-RAl. doc
4 k' ' Methodology for Cabla Trsy Amp: city Summary We will solve the temperature rise in each part of the thermal circuit from the outside of l l the wrapped tray, working inwards until the temperature of the cable conductors has i been calculated. From this, the cable derating factor can be determined. The initial step is to calculate the amount of heat generated by the cables. An initial temperature is ; assumed to begin an iterative solution. The temperature on the outside of the Darmatt wrap will be adjusted iteratively until the amount of heat dissipated from the outside surface of the Darmatt wrap due to convection and radiation equals the amount of heat i generated by the cables inside the wrap. The temperature drop due to the conduction , of heat through the Darmatt fire barrier, the small air gap between Darmatt layers, and
- the air gaps at the sides of the cable tray (due to the tray flanges) is calculated to give the average inside surface temperature of the Darmatt wrap. !
L The temperature drop between the cable conductors and the inside surface of the Darmatt mass will be calculated by treating the cable mass as a uniform body in accordance with the Sto!pe method and taking into account test data giving the I derating of cables in trays that are covered with tightly fitting covers. The ampacity of j the cables in the tray section will be calculated from the heat intensity for cables in a wrapped tray.
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in some cases, the calculations may indicate that some cables are overloaded. However, the Stolpe method does not consider the fact that some cables in a typical , cable tray section are lightly loaded, increasing the actual ampacity of the remaining i cables. The minimum increase in ampacity due to diversity has been determined in published testsi A revised ampacity will be calculated using the test data to consider
- diversity.
I !- 1 Stolt, H. H.; Lamkin, D. C.; Sykora, G.; Haddad, S. Z.; and Bloethe, W. G. " Tests at Braidwood Station on the Effects of Fire Stops on the Ampacity Ratings of Power l i Cables". Chicago: Proceedings of the American Power Conference (1982).
U' M:thodology for Cabla Tray Ampacity Th'scalculation of the ampacity of the cables in a wrapped tray is based on the Stolpe
- method, which has been endorsed by NEMA WC-51/ ICEA P-54-440. This method j treats the cables in the tray as a tightly packed cable mass that contains uniformly i distributed heat sources. The allowable heat generation for a tightly covered cable tray j is first calculated from the allowable heat intensity versus depth of fill curve for an uncovered cable tray. Because of the representation of the cable mass, no credit is
- taken for convection in the air spaces between cables in the installations at Prairie
. Island. When the cable tray is enclosed, test have shown that the ampacity of the l cables is decreased by 15%1 . The tray is considered tightly covered in order to
- account for the e"fect on cooling air flow when the air tight Darmatt system is applied to i
j the tray. l The derating factor of 15% for cable trays with tight fitting covers was obtained from tests using galvanized steel covers and cables with rubber-like jackets. However, the 4 condition at Prairie Island is that of cables with metal armor and non-metallic Darmatt j tovers' The change in material will not affect the heat transferred by convection, but it
- will affect heat transfer by radiation. Consider a cable mass radiating to a cover
l
/ 'M.,
N , ., c -
- n .
I Assume that the tray length and width are much larger than the spacing between the cable mass and the cover so that the view factor between the cable mass and the cover f approaches 1 (heat radiated to the sides can be neglected). Then the amount of heat transported by radiation is given by: Q'~=~{ sigma left({T sub c ^4}~-{T sub t ^4}right }} over { ((1 ~ epsilon sub c}over ((A sub c} { epsilon sub c}}}~ + ~{1 over A sub c}~+~((1--epsilon sub t}over((A sub i Stolt, H. K; Lamkin, D. C.; Sykora, G.; Haddad, S. Z.; and Bloethe, W. G. " Tests at Braidwood Station on the Effects of Fire Stops on the Ampacity Rating of Power Cables". Chicago: Proceedings of the American Power Conference {1982).
1 I' Methodology for Cabla Tray Ampacity . c}{ epsilon sub t}}}}~=~((A sub c} sigma left({T sub c ^4}~-{T sub t ^4}right }} over { ((1 -
~ epsilon sub c}over { { epsilon sub c}}}~ + ~{1}~+~((1~-epsilon sub t}over(( epsilon sub t}}}}
where: is the Stefan-Boltzman constant : Tc is the absolute temperature of the cable mass Ttis the absolute temperature of the tray cover ! Ac is the area of the radiating cable mass The denominator of the equation can be treated as a tonductance"per unit area of l
. cable mass:
G-=~ 1 over (({1- epsilon sub c}over { { epsilon sub c}}}~ + ~{1}~+-((1--epsilon l sub t}over({ epsilon sub t}}}} For the conditions of the test, the cable jacket emissivity, c is 0.95, and the emissivity of an oxidized metal tray cover t si 0.33. This results in G= 0.32. In the case of the Prairie Island cables, the cable armor has an emissivity, e, of 0.33 and the emissivity of the Darmatt cover, t, is 0.7. This results in G=0.29, which about the same heat transfer as the original tests. Therefore, the use of the test result giving 15% derating for tightly fitting tray covers is also applicable at Prairie Island. ! i- This allowable heat generation for a tightly covered cable tray is then used to calculate j i the surface temperature of the cable tray. The difference between the rated conductor l l temperature and this surface temperature is then divided by the allowable heat i f generation for a tightly covered cable tray in order to obtain an equivalent thermal l resistance from the conductor metal to the surface of the cable tray. The surface i temperature of the tightly covered cable tray, TGS, is found by iteration using Mathcad until QTGS, the calculated value of the total heat transferred from closed cable tray { { surface, nearly matches QCB, the allowable heat generation in a tightly covered cable tray. The thermal resistance of the Darmatt fire wrap is calculated and added to the L previously calculated equivalent resistance from the conductor metal to the surface of { the cable tray. The small air gaps between the layers of Darmatt and the air gap at the i sides of the cable tray (due to the flange of the tray) are incorporated in the calculation l e
, - ~ ~ , - - . - - , - - - - , . - . . . . , , . - - - , - - - . - . - . ., . .-- .,-- ~ - - - - . , .- . ~ ,.-- --,. - -
l 1 4 Methodology for Cabla Tray Ampacity by being included in the calculation of the equivalent thermal resistance. The same formulas for the convection and radiation from the tray surface to the ambiani, &a modified for the emissivity of the Darmatt material, are then used to calculate the convection and radiation heat transfer from the wrapped cable tray to the ambient. The surface temperature of the wrapped cable tray, TWT, is iterated using the MATHCAD program until the calculated maximum temperature of the conductor, TCCR, ; is nearly equal to the rated conductor temperature, TCR. The conductor temperature, i TCCR, is calculated using the formula TCCR= TWT + QTWT RTOT, where QTWT is ; the total heat transfer from the wrapped cable tray, and RTOT is the total thermal resistance from the conductor metal to the surface of the fire wrap. , Since the heat generated in the cable tray is proportional to the square of the current, the ratio of the ampacity with the cable wrap to the ampacity for the tightly covered tray is equal to the square root of the ratio of the allowable heats for each cable installation method. Power cables at the Prairie Island Station were installed in the power trays using tie i wraps to secure the cables to the bottom of the tray. The cables are arranged in a single layer in the power cable trays and the project specified spacing is maintained between cables. This method of installation will actually allow higher cable ampacities (as reflected in the ICEA standards) than tightly packing cables in tray due to the fact that the tray which has mai.ntained spacing between cables will contain fewer heat j producing cables (less heat generation) then a tightly packed cable tray. For the purpose of initial ampacity screening, ampacity for the unwrapped tray is calculated ; conservatively using Stolpe's Method which is based on tightly packed power cable ; trays. Depth of cable tray fill is calculated conservatively for the purpose of the initial [ ampacity screening as the tray width times the larger of the outside diameter of the I cable passing through the tray routing point or the tray depath of fill which results in a
- calculated Stolpe ampacity that is less than 80% of the free air ampacity for the cable
- size.
!' The equations for calculating the Stolpe allowable cable tray heat intensity (HI) in t 3 i - _ . . . _ _ _ , _ . _ . - . _ _ - _ . _ . _ _ . _ - . , , , . _ . . - . , . . , _ _ . _ , _ . _ . . .
). ' ' Methodology for Cable Tray Ampacity Wetts/(ft of tray in2 of cable cross-sectional area) for cables installed in ladder bottom cable trays are taken from S&L Standard Calculation ESI-150-2. The allowable cable tray heat intensity is calculated using Mathcad based on an ambient temperature of 40 C, and a maximum conductor temperature of 90 C. Cable diameters are used to obtain the power cable ampacities based on the following equation:
1-=~ SQRT{ {Hi-x- {pi-D SUP 2} over 4} over(( n R)}} where, 5 = cable ampacity (amps per conductor) Hi a ellowable heat intensity (Watts #t -in2 ) D = cable overall outside diamater (inches) n = number of conductors in the cable R = cable AC resistance in ohms #t J The calculated ampacity is therefore within the ICEA P-54-440, paragraph 2.2 limit of 1 80% of the free air ampacity. The 80% free air ampacity restriction is necessary to i correctly implement the Stolpe Model for the combination of a large diameter cable and 3 amall depth of cable tray fill. 1; x>me cases, tray specific calculations may be necessary. These calculations are essentially identical to the above mentioned calculations in methodology, with the exception that additional credit is taken for the actual tray heat load (Hactual), which is less than the Stolpe allowable heat load (Hmax) for the tray depth of fill. A detailed description of this calculation is provided below. In the Stolpe methodology for determining cable ampacity all cables are assumed to be generating heat at a uniform rate per unit cross sectional area. The total heat I generated under the condition when all cables are carrying their ampacities as derived I by the Stolpe method is Hmax-In the vast majority of cable tray installations most of the cables are carrying load cuirents which are less than the allowable ampacity. The total heat generated by the actual load currents can be designated Hactual-1 When Hactual is less than Hmax it is expected that the maximum conductor temperature will be less than the rated conductor temperature as long as all load l l L
?.
.. Methodology for Cable Tray Ampacity currents are less than the Stolpe cable ampacity. The maximum conductor temperature i l
j may still be less than the rated conductor temperature even if some of the load currents : exceed the Stolpe ampacity as long as Hactual is less than Hmax. It can therefore be
! concluded that the cable ampacity in cable tray can be increased above the Stolpe l cable ampacity when not all cables are uniformly loaded.
t 4 A method needs to be developed which indicates the amount by which the cable l l ampacity can be increased above the Stolpe ampacity when cable loading diversity is ! t taken into account. It can be expected that this allowable increase in ampacity depends ) on the relation between Hectual and Hmax- ! Consider the hypothetical case where half of the cables are deenergized, and half of l 2 ! j the cables are carrying a current equal to the Stolpe ampacity. Consider the off cables ; i i to be oistributed evenly throughout the cable tray (every other cable). In this case the j I equivalent thermal resistance is approximately the same as in tho' Stolpe model since f the heat flow pattern is nearly unchanged. Therefore the allowable total heat is equal to l i Hmax- i l The heat generated by each of the energized cables can be increased by a factor of 2.
- i The maximum load currents which each of the energized cables can carry can be l j increased by 2 based on the equation Q = 12 R. l in this case HmaxMactual=2. The currents can be increased by 2. The'refore the f following equation applies in this case !
1 L {l + DELTA l} over i ~=~ sqrt { {H sub { max} } over {H sub { actual} } } j 1 . I l< Alternatively, this can be written as follows: l Ii DELTA Ampacity ~=~ { DELTA I } over I ~=~ sqrt { { H sub { max} } over {H sub { actual} j j }}-1
; if the cables which are off are not uniformly distributed throughout the cable tray, then !
the increase in ampacity will be less than the ideal value given by the above equation. 1 The bunching of cables will increase the temperature rise for a given amount of heat, i I and therefore limit the allowable increase in ampacity. A factor K can be used to account for this non-uniform distribution. t ._ _ _ _ . _ - _ _ ._. _ _ _ ..._. _ _ _ _.._ _ _ _ _.. _ _ . _ _._ . _ ..
.. Methodology for Cablo Tray Amp: city DELTA Ampacity ~=~ { DELTA 1 } over I ~=~ { sqrt { (H sub { max} } over { H sub { actual} } } - 1 } over K The worst case as far as the lowest allowable increase in ampacity when half of the cables are off oex:urs when all of the off cables are on one side of the cable tray. This situation has has been studied, and the results published in the American Power Conference Paper cited above. In these tests the allowable ampacity was found to increase by 8% when all of the cables on one side of the tray were deenergized. This value of .08 can be substituted in the above equation to find the appropriate value of K {.08} ~=~ {sqrt {2} -1} over K K=5.2 The value of K is approximately equal to 5 when all of the cables on one side of the tray are deenergized. The value of K is equal to 1 for the ideal case where every other cable is deenergized. Therefore it can be surmised that a value of K somewhere in the range from 1 to 5 would be appropriate. Because of the random placement of heavily loaded and lightly loaded cables, a value 4 for K is reasonable. The use of a value of 5 for K is conservative since it is only found when the cables are installed in an unfavorable arrangement. The use of a value of K less than 4 could be justified by tests on the specific cable configuration in the tray.
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