Cable Ampacity Adjustment for touching Multiconductor cables 1

[SSKLC]

Hi Guys,

I have following query regarding NEC code requirements for cable ampacity derating.

We have following situation for calculation of motor cable sizes for cables leaving from one of the MCC in Substation.
Cable Laying Condition: Uncovered Cable Tray
Nos. of Power Cables in tray: Cables for 9 Nos. 150HP, 3Ph, 460V Motor
Cables are kept touching to each other.
Conductor Type: Copper
Conductor Insulation: XHHW (90 deg. C)
MCC Terminal temperature: 75 deg. C
Ambient Air temperature 40 deg. C

Motor FLA for 3Ph, 460V, 150HP motor = 180A as per Table 430.250, Motor Cable to be sized for = 1.25 * 180 = 225A each.

Thus, cable size selected to carry the current = 4/0 AWG Copper Conductor, which is suitable for carrying 230Amp load current, which satisfies 75 deg.C MCC terminal in line with NEC 110.14(C).

Temperature Correction Factor = 0.91 --- for ambient temperature variation as per NEC Table Table 310.15(B)(1)(1).

NEC 392.22(A)(1)(a) requires that multiconductor cables with 4/0AWG or larger to be laid in single layer, however, does not specify any mandatory spacing between the cables.

Considering the huge quantum of cables leaving electrical substation, it is not possible for us to maintain the spacing of one diameter and it is required to install cables touching to each other.

NEC clause 392.80(A)(1)(c) says “Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the ambient temperature-corrected ampacities of multiconductor cables , with not more than three insulated conductors rated o through 2000 volts in free air, in accordance with 310.14(B)”

Accordingly, above clause is applicable only in case of maintained spacing of not less than one diameter.
Also, NEC 310.15(C)(1) does not apply, as there are not more than 3 current carrying conductors in a multiconductor cable.

Thus, it is not clear to me about how to adjust ampacity of cables, in case of multiconductor cables installed touching to each other.

Any advice on above will be highly appreciated.

Thanks in advance,
SK

 

[7anoter4]

If you cannot assure a clearance between cables, you follow 392.22 (a) first part:
392.22 (a) Where all of the cables are 4/0 AWG or larger, the
sum of the diameters of all cables shall not exceed the cable
tray width, and the cables shall be installed in a single layer.
392.80 1) Multiconductor Cables. The allowable ampacity of multiconductor
cables, nominally rated 2000 volts or less, installed.
according to the requirements of 392.22(A) shall be as given in
Table 310.15(B)(16) and Table 310.15(B)(18), subject to the
provisions of (A)(1)(a), (b), (c), and 310.15(A)(2).
My remark:
Cable type XHHW (90 deg. C) it is a single core cable-not multi-core cable.
Art.336 Power and Control Tray Cable: Type TC
336.2 Definition.
Power and Control Tray Cable, Type TC. A factory assembly of
two or more insulated conductors, with or without associated.
bare or covered grounding conductors, under a non-metallic
jacket.
For instance, according TEXCAN catalogue for 4/0 3 conductors 1.743" is the overall diameter.
So, 9*1.743=15.687". I think a 10% tolerance 18" width will be o.k.
In this case according to Table 310.15(B)(16) 230 A ampacity will be correct

 

[waross]

Under the Canadian code, cables in a cable tray are de-rated based on the total number of current carrying conductors in the tray.
For 25 to 42 current carrying conductors the ampacity is 60%.
So, for 9 to 14 three conductor cables,the ampacity is 60%.
For more than 42 current carrying conductors, the ampacity is 50%
Under the Canadian code it may be safer to just derate all cables to 50% (plus temperature derating) than to calculate each cable individually.
What does the NEC say about multiple conductors in a raceway?
Possibly take that number and have a chat with the AHJ.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[SSKLC]

7anoter4,
Yes, you are right, I should have said XHHW-2 cable.
But, do you mean to say if TC type cable is used and cables are kept touching to each other, there is no need to derate the cable ampacity ?

Waross,
In case of multiple conductors in raceway, derating is applied as per below NEC Table 310.15(C)(1).
No. of Conductors %Derating
4-6 80
7-9 70
10-20 50
21-30 45
31-40 40
41 and above 35
Considering 9 cables = 27 conductors, derating ampacity is 45%, which is without applying ambient temperature correction factor. Thus, if above table is applied, cables will be highly derated.

However, this table does only apply to more than three current carrying conductors in multiconductor cable. Also, as per NEC Clause 310.15(F), grounding conductor not to be considered while applying the provisions of 310.15(C)(1). Accordingly, NEC 310.15(C)(1) does not apply to derate the cable ampacity, as there are not more than 3 current carrying conductors in a multiconductor cable.

As cables are kept touching, it does not seem logical to use ampacity from table as it is (without derating).

Am I missing something here ?

Regards,
SK

 

[waross]

Consider all of the conductors in the tray to be in one raceway.
The derating rules are based on the inability of the inner conductors to radiate heat.
The heat buildup of conductors surrounded by other conductors will be an issue in a cable tray as well as in a conduit.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

I suggest that you ask the local AHJ what he will accept for derating.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

The correction factors in CEC Table 5C are the same as per NEC Table 310.15(B)(3)(a) Adjustment Factors for More Than
Three Current-Carrying Conductors
However, in NEC 392.80(A)(1)(a) is noted:
(a) The adjustment factors of 310.15(A)(3)(a) shall
apply only to multiconductor cables with more than three.
current-carrying conductors. Adjustment factors shall be limited.
to the number of current-carrying conductors in the cable
and not to the number of conductors in the cable tray.
CEC Table 2 for 4/0 90oC 260 A
Table 5C requirement for 9*3=27 conductors reducing factor 0.6
Then the ampacity is 260*0.6=156 A
According to ANSI/NEMA WC 51-2009/ICEA P-54-440-2009 standard [not for NEC conforming installations]
Let us say the tray width=18”
Apparent depth =9*1.743^2/18”=1.52
Table 5-7 3/C Insulated Jacketed Conductors with Overall Jacket
I=228 A in 40oc ambient. For 30oC ambient multiply by 1.09*228=248.52 A
IEC 60364-5-52 Table A.52-20
for 9 multi-conductor cables [3*c] on perforated trays ,1 tray, reducing factor 0.73.
95 sqr.mm copper 90oC insulation 246 A
120 sqr.mm copper 90oC insulation 298 A
Average (95+120)/2=107.5 mm^2; 4/0=107.23 mm^2
Average (246+298)/2=272 A. Then ampacity is 272*0.73=198.56 A
However, in my opinion 230 A, as per NEC, is correct.

 

[7anoter4]

Remark: from NEC 2020 edition Table310.15(A)(3)(a) it is now Table 310.15(C1), but the point (a) from 392.80 (1) is the same.

 

[SSKLC]

7anoter4,

Thank you for your explanation. However, I would like to correct some figures in above.

As per NEC Table 310.16, for XHHW-2 (90[sup]o[/sup]C) conductor, ampacity is 260Amp @30[sup]o[/sup]C. Are your referring to 90[sup]o[/sup]C or 75[sup]o[/sup]C ampacity ? I understand that 75[sup]o[/sup]C ampacity needs to be used only for checking termination requirement as per NEC 110.14(C).

According to ANSI/NEMA WC 51-2009/ICEA P-54-440-2009 standard, as per Table 5-7a: 3/C Insulated Jacketed Conductors with Overall Jacket, for 1.5" depth in cable tray, ampacity for 4/0AWG conductor is 180Amp @ 40oC. Accordingly, ampacity @30oC =1.09*180 = 196.2 Amp

Also, as per IEC 60364-5-52, Table B.52.12,
Ampacity of 95sq.mm @ 90[sup]o[/sup]C = 298 Amp @ 30[sup]o[/sup]C
Ampacity of 120sq.mm @ 90[sup]o[/sup]C= 346 Amp @ 30[sup]o[/sup]C
Thus, Ampacity of 107.5 sq.mm or 4/0Awg = (298+346)/2 = 322 Amp
And with application of derating factor = 0.73 * 322 = 235 Amp

Now, based on above, do you still suggest that ampacity of NEC should be applied without derating factor.

Waross,
This being private installation in Louisiana state, I am not sure about who is AHJ for this facility. In my understanding, in case of public safety matter, AHJ is fire Marshall.

 

[SSKLC]

7anoter4,

NEC ampacity as per Table 310.16 for XHHW-2 (90[sup]o[/sup]C) conductor, ampacity is 260Amp @30[sup]o[/sup]C, whereas IEC ampacity of 235Amp is comparable only if one no. rack with 9no. multicore cables.

However, in case of multiple nos. of racks, derating factor further worsens.
Example, for 6 Nos. racks with 9 nos. cables, derating factor is 0.64 as per IEC table B.52.20.
So, this will further lower the ampacity. i.e. 0.64*322 = 206 Amp.

Then, it does not remain comparable to NEC value of 260 Amps without derating.

IS there any software in which above condition can be simulated to calculate cable size ?

 

[7anoter4]

You can use 90oC before reducing factors but you have not to overcome the 75oC -if the current is 100 A or more-if the connected terminations are not approved for 90oC.
5-7a is for aluminum conductor.
No such table B.52.12 .The last B it is B.52.3.
However, the Table B.52.1 is only for information and according to 523.2 it is only an example how National Committees may wish to adapt the tables of annex A to a simplified form.
I use always only annex A.

 

[waross]

You can use 90oC before reducing factors but you have not to overcome the 75oC -if the current is 100 A or more-if the connected terminations are not approved for 90oC

Not an issue for derated cables.
The derating only applies where the cables are touching other cables.
There need be no derating in the switchgear.
A single cable rated at 90 degrees must have properly rated terminations.
If the 90 degree ampacity is used for derating and the cable is derated below the 75 degree ampacity there is no need for special terminations.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!