ML20065G719
| ML20065G719 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 10/29/1984 |
| From: | BECHTEL CORP. |
| To: | |
| Shared Package | |
| ML17310B177 | List: |
| References | |
| NUDOCS 9404130162 | |
| Download: ML20065G719 (27) | |
Text
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CABLE DERATINC,
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A A 10-29-84 Revised Deratino of Covered Trav GAR EL ,
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43 w STANDAMD om'otN ELE CTRICAL / l DESIGN GUIDE l mE-sFPO Cable Derating Practice E.2.6.4 l 2 BPC b 8 osoow oi 41'[
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FIVISION ' 2 1i
SUMMARY
OF REVISIONS
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}j 1. General editing of entire document.
1.3 8j 2. Changed " ventilated" cable trays to "open cop" cable
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trays throughout to conform to !CEA.
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sk 3. Paragrar' 4; Deleted section on derating 4" for N )l solid t. ,
wvers,addedreferencetoparagra%s3.7,
- 51 3.8, 3.9 where 3.9 is a new section titled "Addtional
') l Derating for Cables Routed in Open Top Tray with Solid
{e Covers".
.k k
,A 4 Paragraph 3.6.6, b): Deleted derating for solid tray
- [ covers from sair.ple calculation.
Added notations that tray covers should be removed
]Egn 5.
prior to applying firestop materials or enclosing glc o
raceway with fire protecting material.
.1 L
$! 6. Paragraph 3.8; Revised to include derating for a 3-hour
{ fire rating f6r Thermo-Lag. General revision to
{2
. separate discussion of Thermo-Lag and ceramic fiber I blankets.
11
- i 7. Renumbered "3.9 General Precautions" to "3.10 General d Precautions".
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NUMBE R E2. 6. 4. P.c' 1 A .,
SHEET 1A CF
- gl DATE Cet. 29. 1-k Eo.22
, . - - ~ . _ . . . . -
- 1. SUBJECT-
?
3 .
i Cable Derating Practice
.)
,l >1,
- 2. PURPOSE 1~
-2 o establish a design guide to determine cable ampacity :
}j , ratings for esble directly buried, in underground ducts, ,
- . embedded and exposed conduits, and in ope.n top cable trays.
IJ (For industrial projects and utility buildings not inc lude d ' in'-
I~ the sover block, the National Ilectrical Code should be used
~
for.ampacity values and calculationaE methods).
- 3. DESICN CUIDE
.t l :
8 3.1 Ceneral j8 The following is to set forth a definite and uniform procedure y 8 to determine cable ampacity ratings. It encompasses various i types of cable installation, namely:
E 31s a) Underground I 1) Directly buried 1L
$I 2) In duc'ts -
)
ii b) In Conduit g
N A>
- 1) Embedded in slabs or valls
.Jl 2) Exposed. Conduit wg c) In Cable Trays
~$
- E .
- 1) With Maintained Cable Spacing
- 2) Random Fill of Cables in Tray L
N Publication No. P-46-426 of the ICEA contains tabulated jg ,
- ampacities for a variety of cable voltage classes, thermal fratings,"and installations,~and'provides the basic-ampacities 1 (for' cases'~a),b),andc)1)above. Two volumes comprise this
,, 4 publication.- Volume 1 deals with copper conductors and.
contains an Introduction and Appendices applicable to both volumes. Volume 2 contains'ampacity tables for aluminum.
~
N- conductors.
23 li ! Sections II.D.2 and 3. of the Introduction Section in Volume 1
,, 'E ^o f'the above' publication set forth a.' method for calculating
.}
lI .
sampacities for case c)2)-Cable tray installations with random
), ,
'E it W* .
NUMBER E2d, t. Rev, g[C SHEET 2 0F'27 DATE Oct. 'S . - i@
l 3
,, m:- m -- - - , _ ._
4 fill. This saction has boen supars0ded by a navar ICEA gg publication, Publication No. P-54-440 "Ampacities for Cables in Open Top Trays." The latter should be used exclusively g}
.O since the older method, as set forth in Sections D.2 and 3.,
i is in error.
>e 33 If shiel.ded medium voltage power esoles are installed so enat jj individual conductors are not sym::ietrically disposed.
T. , circulating currents in the insulation shields of the cables
!j ' may reacn t magnitude anere the thermal effect of the I~R IE factor requires derating of the caole conductor. he - r eve r
,d, such cases occur, a third ICIA publication, Publication No.
- P-53-426, "Ampacities Including Effect of Shield Losses for Q. -:
Single Conductor Solid Dielectric Power Cable 15kV through
, 35kV " should be consulted. The title could be misleading in 3 that this effect applies at other voltages as well. Whenever l85 shielded power cables are installed with individual conductors jj in a non-symmetrical arrangement, these effects should be
- [ l investigated and either taken into account if the losses are "i -
, s i gni f ic an t , or steps snould be taken to eliminate the
- 3 circulating shield current. (See Design Guide E-2.6.5 " Power l .g .
Cable Shielding and Shield Grounding")
1,1 This design guide contains samole amoscity calculations.
{f sa Although these are primarily based on copper conductors, the same procedures and considerations are applicable to aluminum.
1{5 3.1.1 It is important to remember that current-carrying espacity, voltage regulation, and short-circuit capacity of cables must b be considered independently in order to ansure proper selection of cable sizes with various types of insulations,
]{
wg voltage classes, and modes of installation.
= .:
83 3.1.2 Although it is not specifically called for in the Introduction
!j Section of Volume 1 of P-46-426 (nor elsewhere in this
- s y standard), combining circuit " sets" of power cables in the same conduit or underground duct requires that the tabulated
{E a3 ampacity be reduced (derated) accordingly. The derating 1* effects of mutual heating is addressed in other sections of d the Introduction -- derating for adjacent conduits in air or
- g in concrete-encased duct banks, etc.
!I Since the methods set forth in Section II.D.2 and II.D 3 of 3 the P-46-426 Introduction (including Table VIII) have been superseded (and should be crossed out in all copies of the g
standard presently in use), the table is reproduced in this 5) 1.t Design Guide in Section 3.5.1.
3-g 3.1.3 Ampacity calculations for underground cables, whether run in
- i duct banks or directly buried, can be rapidly and conveniently 1,0 made by means of a privately developed computer program nov
] available at some Bechtel offices.
( ' il 1 -
D '5 NUYCER E2. 6.4 Pav .
. , SHEET 3 OF 2" E,, DATE Oct. 29, 198-l e
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wo bw07U3EUGUMENOTD4NWTGEEBEisaEE9 ~GosErnaGe6F"t'U-9@')) .
is run on s Texas Instrunents II-59, a hand held, cad programmsble calculator, and i printer euxilicry unit.
. It calculates ampacities for cables in a large number of ducts
$8 or cable groups, with complete flexibility regarding duct I I size, spacing,'and bank configuration. Where some cables are '
I.I Rd loaded to less than their permissible ampacity, this reduction in mutual heating is taken into account to persit higher f2 loading of other cables in the particular run. Consult with j3 your local office . Chief Electrical Engineer's staff regarding
'] possible use of this program.
\ g.I
. Wen this program is available, it is recommended for use I' instead of the methods set forth in Sections 3.2 and 3.2.
I"l
[ 3.2 Direct Burial Cable
- 1
'a g 3.2.1 Types of directly buried cable configurations with typical lg dimensions as per ICEA Pub. No. P-46-426 are shown in Figures 1, 2 and 3. Final detail with respect to trenching and JE backfill are to be supplied by the project.
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- g Figure 1 - Buried 3/C Cables Figure 2 - Buried 1/C Cables ai gj
- Note that the arrangement in Figure 2 may cause significant IL
- shield losses if shielded cables are used and shields are
.[] grounded at more than one point.
Ii fy WM/M%O 4 ,WNMM l
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h 03 s (f) 11 8
P 24 P m 'i Figure 3 - Euried Triplex Cable i ~)
NUM BE R E 2. 6_.L Rev._ .
- SHEET 4 Of 27 I
9
~
DATE Oct. 29, 195;
't F.D 22 (34
1 3.2.2 Information required to enter ICEA Pub. No. P-46-426 for
. ,3 y- Direct Burial Cable:
g fy a) Cable Description (e.g. 1/C, 3/C, or Triplex)
I.
En b) Cable Operating Voltage (e.g. IkV, SkV, 15kV, or 25kV)
=s j3 c) Cable Insu.lation (e.g. Rubber or Thermoplastic, Varnished
- ,a. Cloth, Paper, LP Gas or Oil-Filled) 11
- ~
d) Conductor Temperature (e.g. 60, 65, 70, 75, 30, 85 or 90C) j
\
t 'c I e) The ampacity values tabulated in ICEA for direct burial '
IE .I cable are for an ambient earth temperature of 20 C. l
- l Adjustments must be made for ambient earth temperatures l
,33 whicharesubstantiallydifferentfromthis. A frequently l lg used guideline is to assume the 20 C ambient for ;
g* " Northern US" locations and 30*C for " Southern US" l
- 8, locations. While this approach may suffice for feeders in
,g which ampacity margin factors. offset the importance of o this item, important or critically loaded underground
}y g ' cable systems should utilize testing or'other methods (per EEI " Underground Systems Reference Book" - Chapter 10).
}E. An important precaution in this regard is to ensure that 3f
.s cable trenches or duct banks are not affected by close proximity to other underground systems creating a higher-than-normal earth ambient. An example,of this
( l3
.E might be the installation of a cable run from the power a block to the intake station or cooling tower in the same il excavation with the circulating water discharge line.
d If ambient earth temperatures above 20 C are Ex encountered, one method of derating the cables is to lower -
8} the conductor operating temperature by the same amount as "j
the increase in ambient temerature (e.g. to find the i
- g
,, ampacity of cable with condugtor temperature of 90 C and I an Ambient temperature of 30 C, find the'ampacity #
of a h3 cable with 80 C' conductor temperature and a 20 C E* ambient temperature which can be read directly from the
$]
2 tables). ;
- k , f) Load Factor:
II 83 Ampacities are tabulated for 30, 50, 75 and 1007. load ij factors. These are indiceted as 30LF, SOLF, 75LF, and
. 100LF. .It is recommended that 100LF be used for all
-{l i calculations involved with generating station applications.
d .9 '
Ili
- 3- 4) Earth Thermal Resistivity:
l'gI ICEA Pub. No. P-46-426 ampacities are tabulated for ;
( gj in-earth thermal resistivity, EHO, in degree
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,NUMBE R E2. 6. 4 Rev
- [ SHEET $ OF
~
! ' OATg Oct. 29, 19 1
$ s e 'n C ,
1 . . . _
centigrade-centimeters per vatt, for RHO-60. REO-90, and RHO-120. ' Procedures are given for interpolation and J s' ,. )_
- jl extrapolation, if other than indicated values of RHO are aj encountered. ICEA recommends that RHO-90 be used when earth I. thermal resistivity is not known. However, in the instances jf of major cable installations, where engineering judgement and
-3 econostes dictate, we should determine REO as closelv as ,
Ij possible. Some of the factors which must'be considered are j.s type of soil, type of backfill, moisture content of soil, eJ denen below surface, and presence of nearby concrete slabs or structures. In addition, the '. baking" of the soil by the
,, g current-carrying conductors can cause RHO to change (for the
- worse) with time. *Two reference articles on this subject
{. are: " Rapid Measurement of Thermal Resistivity of Soil" by V.
g-h V. Mason and M. Kurts, AIEE Transactions Vol. 71, 1952, page 8 570 " Soil Theriaal Resistivity Measured Simply and Accurately"
! by' John Stolpe, IEEE Transactions Vol. PAS-89, Number 2,
}j February 1970, p, age 297.
1>
!! 3.2.3 Semple C41culacion:
0 5
31 Civen: Directly Buried Cableg 3-1/C 4.16kV, Rubber Insulated, Conductor jj Earth Temperature 30, C. Temperature 90 C Ambient jg Y$ Find, Ampacities for 2/0, fe/0, and 500 kemil at 100LF.
l2g ICIA Pub. No. P-46-426 asipacities of directly )'
gi Solution:
buried cable are tabulated for 20 C Ambient Earth Temperature. To maintain same temperature g; difference between conductog and earth, use a gg conductor temperature of 80 C.
- t .:
dI ICEA Pub. No. P-46-426 index page iii refers to table on page (j 202.
.s
? Wire Size Ampacity 1~ 2/0 303 11
- 3 4/0 393 R80-90, 100LF, gg 1 Circuit, 8kV Il fj 500 kemil 629
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e NUMBER EJ ._e . 4, Rev.
3 SHEET 6' OF 2 o
DATE Oct. 29t 195
{
3.3 g M es in Underground Ducts
-3.3.1 Type of duct configurati:ns and typical dim 2nsions as per ICEA Pub. No."P-46-426 for 5" duct are shown in Figures 4, 5, 6 and
, f3 7. Duct bank overall dimensions are approximate, to give l
$ ~ 'f. ' minimum 3" encasement coverage l jj I l' 5 \
i 3 . ~
(
3_3 2 ii I .i I*
2+
XWAWXW /, AV/zgy4M i{ m owam ,x < w n c .< c. , 30" l la cither encased in f 5j 30" concrete or not a& Y encased, typical { Y 5 _
p Q ,' $ 78" p u._. - . _@@j3
=
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-- 4 <- 5 "
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- I 1,
\
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( !i Figure 5 Figure a j] 11.5" by 11.5" overall 19" by 19" overall !
r g. Duck Bank JB Duct Bank El 5i {\ve WaTA 4 g. g <<zu ,,gg Ej 4V/4WA\\ A '4\Y/QY/AW
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30" h$ V 11 V
79 ,
, Q Q jy 5" h --
Q g( 7b" l
-aw)
_j (TYP)[ l 4] __--
Figure 7 3 Figure 6 33.L" by 33.4" overall j8 19" by 26.5" overall (Not a feasible design)
E
- Duct Bank ,
8b
( 4j 11
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NUMBER E2.6.4, nev n
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@ $_HEET DATE 7 OF Oct. 29, 19 EO 2- ";
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.No t e s : .
j
'I a )' Cable in individual duct can be 1/C 3/C or Triplex. To i Jg find ampacity, use the appropriate ICEA Pub. No. P-46-426 ) J
- table. If 3-1/C cables are used per duet, the table for . 1
[a 1
Triplex Cable is recommended for use.
75 b) For any normal duct bank configuration, phase and ;
~2 conductor imoalances vill result if multiple paralleled !
yj
- 3 caoles for each phase are installed eacn in a separate duct. To preclude this, paralleled runs of cable should ;
!j be designed with all three phases installed in each of !
5~ multiple ducts (3/C, triplex, or 3-1/C), with sizes and l N lengths of all cable matched. For large loads, such as l II the secondary connections for station service l N
transformers, this may require several more (smaller) conductors per phase but compares favorably from a cost
,l l 33 viewpoint and avoids a possibly serious problem. If for I
!3 some reason paralleled single 1/C cables per duct must be !
j.E used, the individual ducts should be transposed at j y[ intervals along the duct run to balance the impedances of )
g the three phases - a slow and expensive duct installation l og method. Another way is to ayumetrically arrange the ducts l
}3 as shown in the Underground Systems Reference Book Figure j m: 10-39, arrangements 4,5,6,7, 8 and 16.
3,1- -
9E c) If cable sizes larger than cabulated in ICEA are required E! or more than nine occupied ducts per bank are required.
lj extrapolation of ICEA Pub. No. P-46-426 tables may be ]
It is reconnended that the extrapolation, gg considered.
y either ampacity versus cable size or ampacity versus number of ducts in bank, be done on log-log paper since an 2a l
- i approximate straight line will be obtained. As with any i d extrapoltion, this method is limited - the further the '!
E- extrapolation, the lower the accuracy. For duct bank ;
d arrangements other than those shown in P-46-426 extrapolation should be limited to smaller duct banks with j
?J l
- s not more than two, or at most three, layers of power l 1 conduits. Beyond this, the Neher-McGrath analysis should !
h3 be applied, manually, if necessary, or preferably by means I E* of the EE-700 (WE-80) computer program. If duct banks are Il run in parallel, the normal ampacity tables must be further derated. The derating vill never be more severe :
cI thans
!T Additional
- j 8
i Distance Between Burial of Depth of Also Applies to Any Ratio of Derating for All Cables In 5 *i . Nearest Ducts Lovest Ducts Distance / Depth Both Duct Bank 41 2.{
~
1 ft 3 ft 1/3 0.79 gy 2 ft 3 ft 2/3 0.87
.s 2 ft 3 ft 1 0.91 !
E8 4 ft 3 ft 1-1/3 .- 0.94 3 ft 1-2/3' O.95 s f 5 ft
~E y ia i- 1 i NUMEER E2. 6. 4, Rev.
g g7fl SHEET 8 OF 27 k DATE
~
Oct. 29. 198/.
- g. . . . . . . .
, , , , ~ ~ -
When a horisontal separazion of 6 ft or greater is j g maintained, the mutual heating effect of adjacent duct l gg banks can be safely ignored.
'b "I , d) In particular situations where the available tables and E5 '
extrapolations are inadequate, the general equations for 1
32 ampacity, as stated in
- CIA Pub. No. P-53-426, Section
~
fj F.4' ' are acosasended as the best available approach.
7 .2
!J ~
e) niormation required to enter ICEA Pub. No. P-46-426 Taoles for Caole in Uncerground Duet is the same as that h
required for Direct 3urial Cables, see Section 3.2.2.
N f) When 1/C cable installations are designed, care must be 38 exercised to avoid placement of steel or other magnetic material between or around conductors.
b!
jj g) Tabulated ampacities apply only to a single esble or j( single set of cables in each duct bank conduit. Where
- i acditional circuits are installed in the same concutt, the o' 1 ampacity factors tabulated at the end of Section 3.5.1 g3 must be applied.
i 3.3.2 Sample Calculation:
.R L 3 *s Given: Underground Duct Installation 13.8kV Rubber Insulated Cables Conductor Temperature 90 C;
[g {*, Ambient Earth Temperature 20 C 1600A (full load current requirements).
Il
]{ Finds Size, number and configuration of cables required. l 51 Solutions a) First consider 3-1/C or 1 Triplex per duct in gj w= order to obtain balanced currents in each I$ individual phase.
- E. l
~1 b) ICIA Pub. No. P-46-426, index page tv refers to hE the Table on page 242 for Triplex for the given E* conditions stated above.
I]
- " 3
[g c) We see the 3. Triplex vill not carry the current for the maximum size tabulated. However, 6 Triplex vill give the required ampacity (17e.
f]5 500 kemil, AHO-90, 100LF ampacity is 288; 6 x Ij 288 = 1728 which is greater than the 1600A l required). j E .I d) If duct size vill permit triplexed cable of Ti larger sizes, interpolation of the tabulated
' 'i data indicates that 4-750 kemil vill be ;
marginally satisfactory and 4-1000 kemil vill '
l0 ,
provide a conservative application.
< 'E 4s1
'A*
NUtABE R g2,6,4, gev .
3 0
- g SHEET 9 OF-:
2 DATE CCt. 29e 196 v
e ED 22 (3 -
m .
e) Note that tha tabulated data for triplexed ecblos applies to corresponding sizes of 3-1/C ccbiss instc11cd in a duct.
1i 5
.p 3.4 Cables in Conduit Embedded t - ,
l
_8. f j
t' 5 3.4.1 Types of Embedded conduit Installation I 43 Imbedded conduit refers to conduit in concrete alabs end l valls. Normal configurations of conduit in underground duct
}{!j}3 installations are illustrated in Section 3.3.1. All provisions of Section 3.3.1 are equally applicable to embedded 3~ conduit. ,
} .3 l 5! 3.4.2 Information required to enter IPCEA Tables for Cable in '
E3 Embedded Conduit l II 3 a) The ampacity of cable in embedded conduit should be taken
[j lc from the ICEA P-46-426 tables for similar cables in JE underground duct. The same data set forth in Section 3.2.2 for entering the tables for direct burial cable is y *[
required for cable in embedded conduit.
]5gc b) It is recommended that RHO-60 he used for cables in embedded conduit (RHO-60 is typical for "hardrock" I structural grade concrete).
2L
(! c) An ambient temperature of greator than 20 C is
[3
. .E frequently the case for embedded onduit in a power or industrial plant (i.e. conduit in a concrete slab with a
)
1E heated room above and below may have ambient te=cerature 40 C or greater). Thus, it vill be necessary in most
- cases to derate the ampacities given in ICEA Pub. No.
d P-46-426 for cable in underground duct, since these ampacities are for an ambient temperature of 20 C. The E}-
$ procedure outlined in 3.2.2e may be used to derate for s',, ) ambient temperature greater than 20 C, or the may be found for an ambient temperature of 20,a=pacitie C and the s 8
,5 1 derated by the equation shown below.
E* 7 -
T'
{} I' = I where i
!5 T c
T a
g g-lj I' = derated ampacity (amperes) 2 .c 8r. I = ampacity tabulated for T cand T *(amps tr.si il j ,I T = rated continuous conductcr temperature ( C) 11 = tabulated ambient temperature (20 C)
T
)I T'= actual ambient temperature ( C) a 1
iL "I NUM B E R E2. 6. l. ,_ h=>.
SHEET 10 OF :
( DATE Oct. 29, 19E F D.77 13
3.5 Cable in Conduit Exposod 1i
.j [ 3.5.1 Information required to enter ICEA Pub. No. P-46-426 Tables 1j
= for Cable in Conduit is the same as set forth in Section 3.2.2
,, a, b, c, and d for directly buried cables.
75
$3 [ Consult _.-he.index of CEA ?ab. No. ? .6-426 for abulated f .j. l ampacities for cr:,plexed or three conductor cables in isolated 13 ,
conduit. Nore that the tabulations are based on an ambient
!j air :e.toerature of a0 C. 2f ambient air temperatures higner 3i chan a0'C are encountered, then one or the same deracing procedures outlined in 3.4.2c snould be followed.
s-ga
- Note that tabulated ampacities are for a sinele three
! conductor or triplexed cable in an isolated conduit. If more 38 conductors are in ene same condui.c and concurrently leaded, l* the following ampacity factors (100% ampacity HINUS the j5 percentage derating) must be applied:
.B J: >
- i Total Numoer Ampacity 6
3 of Conductors Factor l1 1.00
} .j 3 4-5 0.80
.21 79 o,7o Y! 10-24* 0.70 25-42* 0.60 lj{
g 43 & up* 0.50
- 7
. 2
- Includes the effects of load diversity.
- 3l-Jwg .B Where a fourth conductor is included as the neutral in 3 Phase E: 4 Wire systems, the neutral is not counted as a current dd si .
carrying conductor and no derating is required.
-6
- s,. Where nominal load diversity cannot reaannably be assumed, an
- ?. appropriate Ampacity Faccce can be calculated using the h!
l' methods set forth in Appendix 1 of the Neher-McCrath paper, "The Calculation of the Temperature Rise and Load Capability
$)
- of Cable Systems." The matter should be reviewed with the office Chief Electrical Engineer.
- 0 < Note . When derating approaches 30%, an alternative cable I1
< routing'or raceway" arrangement abould be considered.
33
- i, 3.5.2 Derating Factors for Cables in Exposed Groups of Conduits in S- Air:
41 f .I a) If the vertical and horizontal spacing between surfaces of 311 conduits grouped on racks or other supports equals or exceeds the outside diameter of the conduits, the j'0 g, ampacities for cables in isolated conduits in air should be used without derating.
( b}
rk i
NUMBER E2.6.4. Rev.
- g g7N SHEET 11 OF 2' k OATE Oct. 29, 196 v
1 ED42(M
. l b) Table I shows cupacity factors by which ctpacities yg
-g tabulated for cable's in isolated conduit in air should be multiplied where conduits are grouped more closely than g)
I- outlined in a) above. The table is based on separation
$3 between adjacent conduit exterior surfaces not less than one fourth of the outside diameter of the larger of the h ! two adjacent conduits, (d/4). THIS SHOULD 3E CAREFULLY j NOTED IN PROJECT RACEWAY INSTALLATION " NOTES AND j{ )3 DETAILS." If separations are less than these minima, a .
{j complex beat transfer calculation is required to ,
I~ accurately dotermine ampacity. I
}a 1 il TA3LE 1 .
a! l
- 1 l
'I l CABLES IN CONDUIT, AMPACITY FACTORS E
Number JEI Vercically Number Horizontally y,
t 5j 1 2 3 -4 5 6 h 1 I
l'e! 1 1.00 0.94 0.91 0.88 0.87 0.86
.NE l 2 0.92 0.87 0.84 0.81 0.80 0.79 ii!
,. 2 bL 3 0.85 0.81 0.78 0.76 0.75 0.74 )l j 4 0.82 0.78 0.74 0.73 0.72 0.72 l 5 0.80 0.76 0.72 0.71 0.70 0.70
!! 6 0.79 0.75 0.71 0.70 0.69 0.68 2 1 i
o
..~E es 3.5.3 Sample Calculation:
1 e 8, , 1* Civen: Conduit installation in air of 3 vertical and 4 1 j} horizontal conduits, each conduit separated by 1/2 l
- g. conduit diameter 3/C, 600V, rubber insulated: 1
". E conductor temperature 85 Ci ambient air j
!T temperature 40 C. )
*)1 8
Find: Ampacities for f4 AWG and d6 AW cables. l
.5 . !
El Solution: ICEA Pub. No. P-46-426, index page V refers to table f .] on page 312 for isolated conduit. ; T$5 l 1 l'
.i )
.5 r- } -l i
NUMBER E2.6.4 Rev. 4 g b' SHEET 12 OF 2: ggg
$ M DATE Oct. 29, 198 i t, rn.n n.7:
1
Ampacity in Ampacity Ampacity in Isolated Conduit Factor Grouped Conduit J 3' Size of Each Cable j$ 87 x 0.76 = 66 1j
= #4 50.2 46 66 x 0.76 =
EE 13
$3 3.6 Cables in Ocen Too Cable Trav i
lj 3.6.1 Cable may be installed in tray with " maintained acacing" or In the maintained jy randomly pulled or laid in the tray. spacing method, cable spacers of plastic, impregnated wood, or g porcelain are inserted to maintain a selected vertical and ha horizontal spacing dimension between adjacent cables in-the
;j tray. Rows of such spacers are installed in the tray at intervals, depenains on the stiffness of the cables involved, l8 sufficient to ensure that the design spacing is effectively 3j r
" maintained". The labor required to do this type of j( installation is many times that required to install the same
~i caoles randomly La :he same tray. It can only be economically 3
J justified for large, important feeders involving comparitively 13 heavy electricat toads. The offsetting benefit is substantially higher ampacity. It is suggested that cable
}j duct be considered whenever conditions are such that ,
jg maintained spacing appears to be a desirable option.
.cen3 If cable duct is selected, the ampacity used should comply l lj 3.6.2 If )
r g1 with the reconenendations of the cable duct manufacturer. field-fabricated maintained spacing is to be used and the g g, spacing is maintained to exceed the full cable diameter, the ampacity will be the same as for the same cable isolated in 3y For maintained spacing from 1 diameter (cable o.d'.) to
)
gg air. 1/4 diameter, apply the ampacity factors tabulated in Table 5j (Copper), of the ICEA g= VII on page V of Volume.1-
.g $
ampacity tables (P-46-426) to the ampacities-tabulated in the -I rf book (s) for isolated cable in air. e 3.6.3 For power circuits in tray other than the major, heavily loaded runs which justify the expense of maintained spacing, ; 6g a the method used is " random spacing" or " random tray fill". Sections II.D.2 and 3 on page V of ICEA P-46-426 describe a l Ig method for determining ampacities for this condition, using
- g. Table VIII from these same sections, but the results are 1
*3 unsuitable for our applications and SHOULD NOT BE USED. The
]
, correct reference is ICEA Pub. No. P-54-440 (NEMA WC-51) g; ~
entitled "Ampacities - Cables in Open-Top Cable Trays". g E2, 3.6.4 Ampacities for power cables installed in trays without 11- maintained spectus should be based can the methods and data contained in ICEA Pub. No. P-34-440. The ampacity tables in 2} . this publication are generally based on the calculated depth - l3 a i 4 is
.*- 4UMBE R .E.2. 6.4, Rev .
SHEET 13 OF : E 8 DATE Oct. 29, 191 E 70220
. l of cables in crays carefully packed to cpproximate maxicum i cable density-of-installation, considering this as the " worst i j g- case basis" for conservative design. The tables are further ) i based on 1007, load f actor and no diversity. As the title gg indicates, the tables are based on "Open-Top Trays". The
,y effects of tray covers, fire protecting material vraps, or
'li , .
.E6 ' routing of tray through firestops recuire derating to the j
*3 -ampacities as determined from !CEA P54-440. *he additional derating required for each is covered in the following
))I sections:
b 3E - 3.7 Additonal Derating for Tray Cables Transiting Firescops . 5 - 3.8 Additonal Derating for Cable Trays or Conduits
- Inclosed in Fire Protecting Material h3 - 3.9 Additional Derating for Cables Routed in Open Top Tray vith Solid Covers. o 5
g = E 3.6.5 Scopa of ICEA Ampacity Tables for Cable Tray 9 I U[ a) . Data is tabulated for single conductor, triplexed, and E three conductor cables. For multiconductor power cable
})3 other than three conductor, a conversion formula is provided in the Introduction section of the Tables.
}j Ib b) Data is tabulated based on the overall cable sizes
*EI (outside diameters) corresponding to the more common cable lj constructions, Since cable ampacity in random-fill trays }
g generally varies directly as the cable outside diameter (other factors being equal), a simple proportion
.kl multiplier enables determination of ampacities for outside*
}{
gg diameters other than those tabulated for given conductor sizes. A special case occurs wherc cable o.d. equals or exceeds the design basis depth of fill. In these cases,
=
8:=! cables can be laid parallel in the tray, one layer deep. 8j Ampacity will be as tabulated for depth corresponding to
*5,.
cable o.d. regardless of percent fill or exset cable
~E " size".
E' c) Ampacities are tabulated for four different voltage
$) classes 0-600V., 601-2,000V., 2,001-5,000V., and gg 15,000V. Ampacities for nominal SkV class may be determined by applying the sizing (cable overall o.d.)
correction described in b) (above) to the ampacity I] 8.i tabulated for corresponding conductor sizes for 5,000V. class cable. (While this is not precise or theoretically
~'i E *. true, the resulting error will be negligible.)
il
@ .a d) Tabulations are based on cables rated for 90 C maximum continuous conductor temperature operation and 40 C 5y maximum ambient air temperature. , Correction coefficients
~
for each of these factors are tabulated in the lg ], Introduction section of the Tables. 3 4j il NUMBER E2.6.4, Kev. l SHEET 14 _OF [ g/ v , DATF. 0 : t. . 29. 195
% i r n.n 13.-
a) Tha empacities are tabulated on the basis of " depth" of **
- y. cables in the tray with " depth" defined.in the
.y introduction section of the Tables as follows:
IE "5
- l. . n,d.2 + n.d.2 .+ .... n #d 2
>c '
E}, Depch =~
" vhere i1 1 '4icth of Iray m 's !
7 .3 ' {! d,, d,, d," = Overall o.d. of different
.:{ *
- cable sizes, and le Ik Number of cables of each E? ny , n;, n, =
corresponding diameter
=5 All units are in incnes.
'a' l Ig Our usual method of calculating tray fill is to select a ii percentage of the usable cross section area of the tray i* uncer consideration, then to determine how many cables of g various cross section areas can be accommodated in that j' percentage of the tray. For this, we use the actual cross
- sectionareaofthecab}es,andsincetheseare(usually) l:E circular, we multiply d x ff /4 for each cable o.d.
.I i Using the notations of the Introduction section of the
{] Tables, our " depth" would bei 2 2 nd , d # g yg 22 ***"ad
= Depth =
}}
>=>
Width of Tray
.J I (n yd +ndyy + ......n d )
w= ,
-Width of Tray 80 J
o. d Note that our method differs from that on which the tables are based by our inclusion of the factor fl/4 hf Il Because of this dif ference, our calculated depth must be i" divided by'TT/4 (or 0.7854) to he consistent with the
*l definition of " depth" on which the Tables are based.
IT (Some find it easier to multiply our calculated depth by
$3 the reciprocal of if /4, or 1.273).
x2
}E . 3.6.6 Use of the Tables 41 'A frequent practice is to select trays for random' fill with JJ # power'eable which have a usable tray depth of 3 inches and to Qy
=3 ' design.for a J0 percent fill.'"(Vhile :he same tray usable j8 rdepth is very videly used in the power industry, both of these tparameters are selected arbitrarily. The 307. fill figure 1 44
. i!! NUMBER E2.6.4 Rev.
O IggtfffE SHEET _ If CF { 8
* ' OATE 2,S t - 29, 19!
u h EO 2 G
supposedly raproscnts filling the tray, using randos ecblo yg pulling or laying of cables into the tray, to thq. point where
-* covers may be installed wherever desired without particular )
Ie difficulty. Many others take 407. as approximating " complete" .
's N tray fill. So far, neither figure has been claimed to P5 represent a " cost-effective" optimum.)
i2 3 ~3 3 AMPLE CALC'JLATIONS
~
e2.l Horizontal tray, 3" usable depth. 1/C #2 AWG {j a) Given: jacketed. 600v. copper conductor # cable, 0.53"
}j; o.d.,airambjent temperature-50 C, insulation rated for 125 C conductor temperature, random s .E E .I fill.
25 Ampacity at 307. tray fill. n 7ind:
$j!
Solution: " Depth" of cable G 307. fill j.5 3 iY = (3" x 307.)+ Tf /4 5j =
=
(3" x 0.3) 1.15 0.7854
]g -
E Enter Tables ICIA Pub. No. P-54-440 Table 4. Use straight
.2h line interpolation to interpolate between ampacities i; 5 tabulated for #2 AWC G 1.0" depth (75A.) and 1.5" depth jj (58A.)
I 1.15 = I I.0 - 1.15 - 1.0 (II.011.5) Y3 1.5 - 1.0 *
"i = 75A - 0.3 x (75A. - 58A.)
dj =
=
75A. - 5.1A. 70 Amperes EE u w= an Note that ghis ampacity is for 40 C ambient air, where (,g ours is 50 C. Refer to page i right hand column E
" Correction for Ambient Temperature" - for 50 C ambient, h! multiply the above result by 0.90:
E=
= 0.90 x 70A. = 63A. $]
C-
*E Note that this ampacity is for 90 C rated conductor II temperature where ours is 125 C. Refer to same page, 8j sag column " Correction for Conductor Temperature" - for ~j P -
125 C conductor temperature multiply our above result by 1.24: 41 f .I = 1.24 x 63A. = 78A. II I8- ~ ii ) 4i . NUMBER E2.6.4 Rev. OF 27 ko v
@' SHEET OATE 16 Oct. 29, 1984 F0 22 (3 7s
Note that this is for a cablo outsido dicester of 0.45" where cuts is 0.53". Roca11 from 3.6.5 b) of this ossign Guide that, ignoring other parameters, ampacity is directly proportional to cable overall o.d.:
)
I I
}l cable = deable x table j table j~. = 0.53 x 78A (as calculated thus far) y 0.45 I
= 1.18 x 78A is jJ = 92 Amperes
-1a
[j 0 b) Given Horizontal tray, 3" usable depth, 2/C f4 AWG cogper conductor 600V. cable of unjacketed " singles," 90 C conductor temperature rating, 40 C ambient air f temperature, cable fillers added to make cable round in j section with overall o.d. of 0.75". Cables are installed 5j vith random lay. It-g& Tind: Ampacity for 40*. fill 5 ll Solution: Depth of cable for 40% fill k j[ = (3" x 0.40) Y Tl /4 "I 3 = 1.2" 0.7854 g ) { TL = 1.53"
> (For practical purposes, the last 0.03" depth can be
.JI ignored and tabulated data for 1.5" depth can be used
$1 without interpoistion). Inter Table 3 and note the 6j ~~
ampacity for 1.5" depth #4 AWG 3/C: E 'E = 49 Amperes'
'j;;d 2
gg Note that no corrections need be made for either g ambient air temperatures or conductor temperature y1 rating. However, we must correct for a different
-5 number of conductors and for a different cable o.d.
I) j g-These corrections can be made in one step utilizing the equation shown in the upper lef t corner of page i gg 11 of the tables: j x .c
}E ,
I' x = d' do xI 3, j j] n
$, = 0.75" x 49A 3 *From Table 3
] 3. 0.83"* 9 2
~
3
= 0.905 x 49A x 1.225 11 41 = 54.3A.
NE NUMBE A E2.6.4, Rev. SHEET 17 OF : S{ DATE OCC* 29' 190 g . ~ , , ,
c) Given: Horizontal tray, 4" usable. depth, 1/C'#1/0 AVG yg shielded cable, . g aluminum 0.97" o.d., condugtor 90 C conductor 8kV rating, nominal 40 rating,C ambient, open I top randomly filled tray.
'I
'S* Find: Ampacity at 307 fill
]3 Solticion: Depth of caole for 307. fill
- 5. = (4" x 307.) "/ /4 '
l ** Id = 1.53" Again for practical purposes we can 3 E .*l omit interpolation for
*] the 0.03" incremental }j depth.
EE jz = 1.5" i I{ Inter *able 29 for './0 AWC conductor and 1.5" cable depth:
}E.gn I g = 94A. for 5kV cable v/0.72" o.d.
E (Assume that the voltage class difference between SkV and a1 SkV has negligible ampacity effect and make correction-sj only for difference in o.d's.):
!$2 I =
d I cable x cable ga cable d table
> t-Jg = 0.97 x 94A "1 0.72 5i w = 126.6A.
- 0. 5 y2 = 127 Amperes g; d) Civen Horizontal tray, 3" usable depth, 1/C 1,000 kcm copper conductor 15kV shielded cables 2.15" o.d., 90 C f 3a conductor rating 40 C ambient open top randomly filled Ig tray.
Find: Ampacity at 30% fill sh Solution: Depth of cable for 30% fill 3i
= (3" x 0.30) TI /4 ia
]I = 1.15"
-y' h11
.2
, NUMBER E21 6.4, Rg
. SHEET 18 OF g I Oct. 29, 19;-
DATE g EO 22 0 -
Note that the cable diemster exceeds the prescribad depth of fill. The cables can, however, be laid in the tray in-
$3 a single layer. The ampacity may be calculated as 2h described in the second paragraph under Section B "Use of jj Tahles" on page "i" of P-54-440. It may more readily be sj looked *so directly in this standard by entering Table 33
'or ?.,300 kemil condue:or si:es
$~! 5
]1
~
5 3 I = 333A 11 ~4here the cable o.d. exceeds the prescribed depth, the 10 h1 ampactcy is 807. of the ampacity of the same cable in free gl air, as tabulated in P-46-426. It is therefore independent of the cable o.d. , so no correction need be g 's
- i made for the difference between the actual o.d. (2.15")
gj vs. ene taoulated o.d. (1.90"). Igf 3.7 Additional Derating for Tray Cables Transitting Firestops iI Many of the firestops commonly used for sealing vall and floor sy g 3 openings for tray cable passage aske use of a flame-retardant 4 thermal insulating material such as ailicone foam. Any solid gl a or ventilated tray covers should be removed prior to forming 33 the firestop. 3i Several manuf acturers or installers of this type of material y[2. claim that the use of their product or method does not require
- ,j derating of enclosed power cables. They base these claims on a1 data from tests, including one or two in which the cables were
}}
> g.
loaded to the full P-54-440 ampa:.gty, without the firestop hot-spot temperature exceeding 90 C. J5 El According to Stolpe, whose analyses and testing are the basis OE for P-54-440, 90 C hot spots will only occur where a number of cables are packed together - a typical " worst case" to use Ej
'E , as a design basis. Commenting on his own test results Stolpe
?? , stated, " Note that even though the majority of cables -- ran cooler than calculated, there was a group of cables -- that
{$ ga did reach the calculated temperature. This points out the y] fact that all cables in a randomly filled tray cannot be
;= expected to have the most_ thermally adverse environment,.but 38 some of them vill."? Stolpe also demonstrated that aspacity
~
5y fshould 'not be increased because of diversity (i.e., seme or j- faany of the cables in the tray are only lightly loaded or are
.= 3 completely unloaded).
$. Because of thesa facts, good engineering practice requires
.g }
yj t.that when' thermal insulating material is used as a firestop, Yadditional~derating must be considered and applied when
- necessary.
'}
8
/
t I3 3 NUMBE R E2.6.4, _Rev SHEET 19 OF
*)
- e. BI E - D ATE Oct._29 J I, F.D 22 8 m , 4
iThe Los Angeks Power: Division performed a serios of tests in i1980fo* determine'the# th'ersal"iffect of two different types of
~
E n
- y. 1
.g Efirestops.on tray cables. Onetyperepresented3heBISCO 8 firestop comprised of a 9" thickness of 17 lb/ft density. .
silicone foam. The other was the minimum thickness BPC -
}}$t - 'firestop comprised 2-1/4" thick of 17 layer of two'.b/ft of 1/2" Marinite with a la7*5:' density silicone foam iS between, plus Flamemastic coating on the cables on each j)8
- exposed face of the firestop.
Il Althoutgh the hot-spot temperature of the BPC firestop was l-I' slightly lower, the conclusions were that either type required
- I atsemina10 incremental (additional)#derating of 15%. uIt should
[* .palsobeactedthat;intheopeniastistigroofin_thisis'e' ries.
*l fas'ialStolpe's ' tests, bot spo't temperatures for trays without
') ; efirestops'of ceramic 'fibe'r' blanket' wrapping were found- wsch jg Jvirtually~ao'" thermal sarmin ubes all cables were continuously
's .n loaded-to'their F-54-440 ampacities.:ASemmarising, these testa
'j i show that cables transitting either d ihsse typical minimum mE firestops should be given an additional dorating of 157..
$l (i.e. a tray cable with F-54-440 ampacity o.f 100 amperes for open top tray should be derated to 85 amperes if it' passes
] .#g through a~firestop).
1 k'nl 3i Another approach that can be used is to analyse the I R heat (( gain (in watts).for a one foot length of tray. This is done by the following cathods l3I
)
A. Take de resistance for ONE foot of each individual stranded conductor from a cable engineering handbook such [
** as Okonite Cable Engineering Data Booklet, Table 1-3 d (tinned conductors where appropriate). r E[-
S B. Convert "A" to ac (where appropriate), by multiplying by
*J the factors tabulated in Okonite Data booklet Table 1-5.
o 1 C. Multiply each value by 1.25 to convert R tabulated for. 25 C to 90 C maximum conductor temperature (1.258 for hf IL
- aluminum conductor).
11 I D. Multiply each "C" value by the SQUARE of the current IE corresponding to the actual full load of the device being served. Short time intermittent loads (such as MOV iT 2g' operator motor loads), or loads that only occur during- ~ abnormally lightly loaded conditions, can be ignored.
.E Add all of these " watts per foot" (of tray).
$2 I] E. ,
3y F. The total vattage for each 6" increment of tray width
~
should be 24.5 'n'acu @ 40 C or less #to ' ansure hot-spot +
}c conductor temperatures less than 90 C within the firestop.
i,.1}- - " NUM BER F.2. 6. l. . ' R ev ,
; #7fl SHEET 20 OF 2-J k ' DATE Ott. 29, 19E i
- ~~
Y' . . . . . L D '
n Ixamplos 2i 70 - 3/C'#12 avg cables are routed in a 12" vide tray with gg each circuit loaded to 9.5 amps / phase. Can a 9" thick i It , silicone foam firestop be installed in the gray without creating hot-spot internal temperatures >90 C? FS f3 x 1.25 = 0.00215 (No de/ac 33 1 (each cable) 1.71 x 10' El correction required) i.
~
I .c I~ (each cable) = 9.5' = 90.25 Y s! I R (each conductor) = 90.25' x 0.00215 = 0.1940375
- n. b
,.l
- I 2R total = 0.1940373 x 70 x 3 = 40.75 ti Maximum permissible vatts for 12" v. tray = 2 x 24.5 =
ge jj 49.0 watt
.E fe 40.75 < 49.0 vaces Therefore the firestop hot-spot is less than 90 C CAUTION: Since "vatts per foot" or " watts per foot per JL unit vidth" correlates with AVERAGE temperatures, each such case should be analyzed to ensure against hot-spots.
?I If many of' the cables are lightly loaded, one or a fev
( lI
.E 3
3 small cables can be overloaded to the point of damage without the " watts per (square) foot" limitation being exceeded. The analysis should verify that the cables are Il evenly distributed in the fire stop. The review should be "i based on reasonable values of watts.per linear foot per dj unit of cross-sectional area of each of the cables of Es interest. S$ .
"E Some firestops use silicone not as a foam but as a solid !
f5
-- elastomer, either unfilled or filled with granular j metallic lead for resistance to radioactivity. Because in y hh its normal state it has thermal conductivity much greater y
E* .j than that of, foam, it dissipates the internally-generated j} heat of the cable such that no ampacity derating is g.
*I required in the 4" to 12" thicknesses of notwal Even with these materials, derating may be IT firestops.
required in substantially. thicker sections such as those
#j 8
sometimes used for radiation barrier seals. -These- ' S.
- materials pyrolize when exposed to fire, and the resulting ,
" char" is a good thermal insulator, thereby enabling the Il material to fulfill its function as a firestop.
jj li .
~}
1=
/
1
.Ii>-
NUMBER E2.6.4, Rev SHEET 21 OF ggggTfl Oct. 29, Ic
' DATE a
u ED-221: 1 i q . m,n
l 3.8 Additional Derating for Cable Trays or Conduits Enclosed in Fire Protecting Material ! 15 ? 3 . I2 In order to protect cable from damage by fire, cable trays and jj conduits are sometimes enclosed in fire protecting material.
>J Among the first such materials to be used was ceramic fiber
$j blanket material suen as Kaovool or Cara-blanket. Its js incombustibility and very lov thermal conductivity makes it
;l effective for protecting control, instrumentation, and il communications type caoles. However, the second
!}
id characteristic makes it generally unsuitable for power cables, at least as a design basis. 5 E? Another fire protection covering for trays or conduits is a yj plaster-like material named Thermo-Lag 330-1. In addition to g being easier to install and much more durable than ceramic g fiber blanket, the derating required for power cables is teore yn reasonable due to a much higher thermal conductivity. i CAUTION: i{ Tire protecting sacerials should not be used inconjunction with solid or ventilated type cable j) tray covers on power tray. If used together, the cable would have to be derated for both the tray ll covers and the fire protecting materials.
.51 g3 3.8.1 Derating Required When Using Thermo-Lag 330-1 When fire protection material is required on trays containing l 4& power cable. Thermo-Lag 330-1 is preferred over eeramic fiber
}] blanket materials'as the cable derating is substantially less F> for Thermo-Lag. Based on ASTM E-119 fire tests of Thermo-Lag j JI 330-1, 1/2" thickness vill provide a 1 hour fire rating and 1" i h3 thickness vill provide a 3 hour fire rating. Ampacity tests O} have shown that 1/2" thickness of Thermo-Lag requires that Ej 1
power cableb'e derated 12.5% for cable tray and 7.6% for iconduit. Tests using 1" thickness have shown that a derating I f 3g { of 17% is required for cabia tray. j S 1
- Since tests of conduit with 1" thickness of Thermo-Lag were l J}
not conducted, the derating for power cable in conduit is
; estimated to be 10.5% based on the 36% increase in the l
i {R dorating abovn between the 1/2" and 1" thicknesses which vere { g' g tested on cable tray. 3.8.2 Derating Required When Using Ceramic Tiber Blankets
.s. 8.
.! ,In-1980, the Los Angeles Power Division conducted tests of y* Iceramic" fiber blankets.(Cera-Blanket). These tests indicated a requried derating of 73% (2.28 vatts per foot allowable l dissipation per 6" width of tray) if :vo 1" chick layers are 18 .
h I
~
h'6 NUMBER E2.6.4. Rev 1 SHEET 22 OF f k DATE Oct. 29, 19 g roa a
I used, or 64". (4.4 vatts par foot par 6" widtn of cray) if a single 1" thick Icycr is ussd for wrapping cable trays. For ye* vrapping conduits, these tests indicated a maximum permissible vates per (linear) foot of conduit internal heat generation of E 5.98 for 4" conduit with a single 1" layer wrap. t$ For the three 250 kemil cables involved in the testing, this i3 represenIs a.derating af 23.37. from the actual measured j , 1 ampacity for the similar " unwrapped" conduit condition. is [j Since only a single case was tested, it was necessary.co 3~ :alculate the maximum permissible vatts per foot of internal h heat generation for other sizes of conduit and other thickness E .I of thermal insulation vrapping. A fairly detailed h .". mathematical model was developed and the calculations
*l 3g performed for various trade sizes of conduit, each vrapped with one 1" chick layer of ceramic fiber blanket. Conclusions lg were as follows:
8e
.2 8
Maximum Allowable Conduit Size !nternal Heat k? (RS, IMT, or IMT) M R) Watts /Ft.* g 1" 3.28
- 33. 1-1/2" 3.88 2" 4.30
?! 4.70 2-1/2"
{' l2{
.E 3" 5.18 5.54 3 3-1/2" 4" 5.88 2 5" 6.53 d* 6" 7.11 t'":-1 l
$ Where one or two power circuits are installed in a vrapped conduit, these limits can be directly applied without !
lJ exceeding the cable rating (s). Where three or more power
*s,.
circuits are installed in the same common conduit, two
~,2 ;
separate criteria must be applied, as follows: h.8 n *
}} (1) The sum of the I R losses of all of the insulated conductors shall not exceed the tabulated Watts per foot gI tabulated above, and II 3 (2) No insulated conductor =ay be loaded to more than its
*3 i ampacity tabulated in ICEA P-46-426 for isolated conduit E *i .
in air derated for the total number of current-carrying d} conductors in accordance with Table VIII of the Introduction to Volume I (Copper) of F-46-426, and J.i reproduced in this Design Guide in Section 3.5.1
- 3. ;y
*Use the method set forth in the latter part of Section "A" through "E".
))6 3.7 of this Design Guide, steps
< 4,
, .> 2) NUMBER E2. 6. 4,,Ryv n or SHEET _ 23 3 gf,C#hi Oct. 29, 19 A D.# DATE
$ ED 22 0 - - . J
1 3.9 Additicnal Derating for Ccblos Routed in Opsn Top Trav vith Solic Covers Ji ) Solid metal tray covers are often used on cable tray to {l provide mechanical protection and prevent the accumulation of mj debris. Nuclear power plant projects may consider the use of y~ g~ solid tray covers to address the separation criteria in s Jtegulation: Guide h75. w ~w g
~
jAspacity ysiEMuductedlin 1980..by_ the Los Angeles Power f.i
- Divisfon indicated that a derating of 277. is required for
~
solid aetal(covers mounted directly on'the tray sills. The
~
jI test consistedf o a tray with a 12 foot long solid metal cover
-l mounted directly on the tray sills with a 1/8" opening at each
( 15 and. For the configuration tested, a maximum allowable dj dissipation of 17.25 watts per linear foot for each 6" jg increment of tray vidth was determined. -Sul.4equent to the gf LAPD tests, IEEE paper No. 83 SM 305-0 pres- ted test results
$j indicating that a 257.-307. derating should b : sed for solid jy metal covers. The test configuration for t.it IEEE paper was a g
3 24" vide tray with a 24 foot long solid cover mounted on the A tray sills.
$1 The use of solid covers on tray containing power cable should li be avoided when it is practical and feasible to provide an ,)[ > economical layout routing the trays in areas not requiring }g
- covers. Realizing this is not always possible, projects which utilise solid metal tray covers for debris protection should j tc 3 consider means other than mounting the covers directly on the'
{IL Instead of covers mounted directly on the tray p tray sills. sills, covers or shields supported above the trays can be used _1I with no additional derating for power cables if a minimum of 0 4" clear space is maintained between the tray sill and the 3]1 cover. When adequate protection can be provided by a shield or cover suspended above the tray, (supported a long or E*E i beneath valkways, etc.) this also has the advantage that cable 33 may be added at some future time without incurring the cost of removing covers. iI IL 5 When solid tray covers greater than six (6) feet in length are y] utilised on power; trays and are mounted.directly on the tray
;=j a isills, the ICEA ampacities 'should be derated 277.. Current Bechtel practice for cable sizing and selection is such that h-g- the 27% derating for solid tray covers may not require larger m3 conductors. Based on an analysis of current SFPD projects, ,J.
cable sizing and eclection has been sufficiently conservative
,g y as to not require larger conductors to compensate for the additional derating for solid tray covers.
y-{ 1i aa 8 I i i l ij
.-. 1 NUMBER T2.6.4 key, kg SHEET 24 OF 2 '
8( ff DATF C e t ._ 20, 196
$g o n.n w
Prior to applying the 27*. dorating for solid tray covers, the jg following ite=s should be considered
-t a) Cables feeding motors are sized based on 125% of motor f1, full load current which provides a 25% margin over rated full load.
Most motors are selected as the next larger Ef " trade size" over :he horsecower requirements of the j2 driven ecutpment vnien provides additional margin. gj Mechanical ecuipment selection is also based on " worst Ta jj :ase oncitions rather than normal operating conditions, 3riefly, motor feeders usually have a margin greater than gy 27% above the actual motor load current.
,g E6 E. -%
b) Some motors are not continuous duty motors such as motor
,! operated valves, cycling sump pumps, etc. and do not contribute to heating of cables in a covered tray (ie.,
35 hi ICEA tables are based on all cables at 100% load with no diversity). j5
.B c) Cableo feeding load centers, motor control centers and
~! power panels in power generating stations are usually 5 y sized large enough to be capable of handling the full li ampacity rating of the bus. It is recoennended that this
}j criteria bw used to size such feeders as this permits the addition of loads throughout the life of the plant as long (p; es sufficient transformer capacity is available. This l
s= technique typically results in cable ampacity margins greater than 277. above the actual load currents. l(( g Il In summary, the use of solid tray covers mounced directly on the sills of power tray should be avoided. When specifically
][ required, solid tray covers may be installed directly on the g3 sills of power trays (without concern) as long as proper
- j design techniques were employed in sizing the power cables, d .=
an .
~5 When selecting the size of cable feeders to panels
*5 CAUTION:
that are dedicated to loads which are all simultaneously [1 F! energized, special care must be taken to assure that the E* conductors are of adequate size, and that all derating factors d) have been considered. Panels which are dedicated to loads such as unit heaters, EVAC equipment and freeze protection are {g typical of those which require special attention in selecting
=.
3 the proper cable size.
,, 4 g 3.10 Ceneral Precautions El The consequence of overloading power cables insulated with the
$.! high quality thermosetting elastomers generally used in our Industry is a reductice in full service life expectancy Even a rather 1 -( than sudden catastrophic failure during startup.
*1 0 seriously overloaded cable may function for many years before ll failing.
( N ia p* NUMBEA E 2 . 6 . l. , Rev SHEET 25 oc-3 g gfCd DATE Oct. 29. 11 Y, ED 22 P
l However, good engineering demands thoughtfulness and care in I performing these calculations. The relationship between } y l. conductor temperature and cable life expectancy is exponential + IE rather than linear, so that small overtemperatures over an j .5 extended period of time can seriously shorten cable life. gj Premature approach to the end-of-service-life condition due to 53 thermal aging of cables is considered by the USNRC as providing the potential for cosason-mode failure of class II
])3 q circuits under accident conditions.
ll u; Prooably the highest possibility of future problems in this Id regard is improper evaluation (or estimation) of the actual il cable environment from the point of view of ambient temperature. The " standard" procedure is to find the nominal h*l
, high ambient areas from the Plant Facilities group on the 'gj project, design for that, and consider the problem solved.
E5 'here are many tight or isolated areas in an operating plant Be through which cables are routed where the temperatures I substantially exceed the RVAC design figures. These hot-spot I{ I areas should not necessarily establish the limits for design of the whole-plant, but cables traversing these areas should
].gg be derated accordingly on a special case basis or their e premature failure should be anticipated. Raceway layouts ll should be reviewed and checked against piping and equipment
.I I. layout to minimize hot-spot exposures or verify that power
-y] cables in the area have been dorated to be conservative.-
18 Our ampacities for cable in tray are based on " percentage 1E fill" of the usable cross sectional area of a tray, but it has fl been proven that the controlling factor for fully loaded .
*t cables in tray is the depth. Our usual practice of 307. fill d8 in 3" deep tray converts to 1.15" depth per ICEA Pub. No.
g3 P-54-440 (See preceding sample Calculation 3.6.6 a), our- ._ S] design basis is conservative only if the cables- are spread mg across the width of the tray in a reasonably uniform manner.
*g Project " cable installation notes and details" should point y out to the installers the possib1'e hazard of cable piling up h5 in the trays (especially on inside corners of tray bend
&* fittings) and making the actual depth be 3", for example, for '
f} a fraction of the usable width of the tray. 5 We should also be aware that our own electrical equipment may
*T create a serious heat problem. If power cable trays are
*jb routed one above another,the upward convection from the lower tray (s) will create a higher temperature ambient for the upper -
5 ,*. one(s). The upward convection " exhaust" air from a class R 4] J,{ insulated load center (AA or FA rated) transformer creates'an ambient exceeding the total temperature capability of our 34 " standard" cables - the cables would overheat at zero
'y aspecity. Su'ch areas should be carefully avoided in racevsy j5 ,
layout (conduit or tray) or special . venti-1stion provided. ; 1 1 d i5 NUMBER E2.6.4. Rev
; fil SHEET 26 OF E DATE 0c h__29, 19 y en,,, 4
Solar radiant heating can also seriously offect the ampacity
. of cables if their rating is not to be exceeded. In most E8 casts, natural breeze or forced air circulation substantially I reduces the effect, but consideration should be given to both
$3 indoor and outdoor cable runs in tray or conduit where solar gj exposure is substantial and possibly accentuated by restricted J i3 ventilation. One method of des 11n6 vich the problem is to .
J provide -sunsh'ields of sheet metal or other cuitable paneling. i j]z
~
Another approach is to determine the maximum escinated I :emperature for an enclosed area in direct sunlight (or ' l ;- uninsulated attic temperature) from project plant facilities I2 engineers and derate the cable for operation in this nigher 3l ambient. 'A method of ~directly calculating the required D dditional deratini"is set forth'in the Neher-McGrath Paper IL?
*j J'The Calculation of the Temperature Rise and Load Capability
's j .of Cable Systems" on page 759 under " Aerial Cables". _ To lg complete the calculation outlined therein,,it is helpful to ju i refer to the " Method of Calculation" section'on'page IV of the
= Introduction to ICEA Pub. No..?-46-426 and utilize the values E{ in Table IV of that section.
]5g:
4.0 REFERENCES
)l 4.1 "The Calculation of the Temperature Rise and the Load 3k Capability of Cable Systems", J. H. Neher and M. H. McGrath; (j AIEE Transactions, Part III, Volume 76, October 1957.
18I 4.2 "Ampacities for Cables in Randomly Filled Trays", J. Stolpe i .I 8 IEEE Transactions Paper No. 70 TP 557-PWR, April 1970. IT
*g 4.3 " Engineering Data - Copper and Aluminum Conductor Electrical Cables" - Okonite Company's Bulletin ERB-81.
h 4.4 "Ampacity of Cable in Covered Tray", C. Engmanni IEEE Paper 8g No. 83 SM 305-0, Presented at the IEEE/ PES 1983 Sussner Meeting o g in Los Angeles, May 1983. 3 1, E il 11 - 33 x2 h. 41 11 11 t SP-E264#2 l 1 1 2&
>2 g NUMBE R E 2.6.4. R_ev e i-SHEER 27 OF .
?. Rf Oct. 29, 19f
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p :' ! ENCLOSURE 4 CALCULATION 13-EC-PA-210 REVISION 5 l}}