Conductor Temperature Limitations per NEC

[nightfox1925]

I am presently developing a cable sizing philosophy for a project here in Asia Pacific. I used a manufacturer catalog and using an XLPE cable I took the ampacity based from a 25 Deg. Cent. ambient (U/G) on the table. My actual ambient is 40 deg. cent. (U/G). IEEE 835 provides a temperature derating calculated by a formula:

Temp. Derating = sqrt[(Tc-Ta')/(Tc-Ta)] where Tc is the conductor temperature rating, Ta' is the ambient temp in the table and Ta is actual ambient.

For an XLPE, the catalog indicates a rated conductor temperature rating of 90 Deg. C. and for an actual ambient of 40Deg C., I am getting from the table a value of 0.88. I applied IEEE 835 calculation and got 0.877 and is approximately equal to the one in the table.

Article 110.14.(C) requires ampacities to be derated to consider the temperature ratings for device terminals where these conductors are terminated. The device terminals are rated for 75 Deg. C. If I use this limitation of 75 Deg. C on the IEEE formula, I get 0.83 (more conservative). Is my proposed approach correct. If I will use the ampacities published in NEC at 75 Deg. C, I will end up using a very large cable compared in using the catalog.

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[nightfox1925]

I have checked the NEC Table B.310.8 which is "Ampacities of Two or Three Insulated Conductors Rated 0 Through 2000Volts, Cabled Within an Overall (Two or Three-Conductor) Covering, Directly Buried In Earth, Based on Ambient Earth Temperature of 20 Deg. C, 100% Load Factor, Thermal Resistance (RHO)of 90 Deg. C"

By observation, this table is refering to a direct buried, three core cable. Here, the table has a 75Deg. C. ampacity ratings and if this is used, the selected cable will comply with the terminal temperature limitations stated by NEC 110.14 (C). For a a conductor say 70mm2, I get 236A (@75Deg.C column). I took three cable manufacturers of XLPE for direct burial applications and I get 229A (catalog A), 252A (catalog B) and 250A (catalog C).



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[wareagle]

You may use the 90C rating for adjusting the ampacity for a different temp. However, when you adjust the ampacity, you may not exceed the 75C ampacity.
Example: assume you are using 500 kcm. 75C rating = 380 amps. The 90C rating is 430 amps. Adjust the rating using the 90C rating 0.88 x 430 = 378 amps. The 500 is ok if the calculated load is not over 378 amps.
Assume an adjustment factor of 0.90. Using the 90C rating
0.90 x 430 = 387 amps. This figure is greater that the 75C rating so you would be limited to the 75C rating of 380 amps because the terminations are rated at 75C.

 

[dpc]

I'm not sure what your question is exactly.

The NEC Article 110 requirement that the cable ampacity be based on the temperature rating of the terminations is not there to protect the cable, it is to protect the equipment. It is also there to ensure that the thermal performance of the overcurrent protection is maintained. Molded case circuit breakers are tested with a length of cable attached to them. The conductor conducts heat away from the breaker and its thermal devices. If smaller conductor is used, the thermal protection may operate at a lower value of current than it is rated for. UL has requirements for minimum conductor sizes attached to molded case breakers.

But if you are de-rating a conductor because of the adjacent conductors, you can use the 90 deg C ampacity instead of the 75 deg C ampacity for derating purposes.

 

[nightfox1925]

Wareagle, does your point implies that I would rather use then the 75Deg. C table from the mentioned NEC table since it would yield me larger size of cable compared when using the 90Deg. C table from the manufacturer catalog. Considering 75Deg C conductor rating in the derating formula (IEEE 835) may even yield to a larger one. My objective is to try to see the suitability of using this NEC table so I would not be dependent to any manufacturer data especially if we are not sure which manufacturer to buy from.

DPC, is it right to conclude that if i will consider terminal limitations by using the 75Deg C table of the NEC (Table B310.8 for XLPE, multicore cable) i would be ensured that the device would be properly protected and perfomance achieved as per test?

I am trying to justify the use of the NEC ampacity tables for LV, multicore, direct buried applications.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[dpc]

I'm not sure which NEC Table you are referring to. Low voltage cables and medium-voltage cables have different tables in the NEC. My discussion of terminations, etc is strictly limited to low voltage. Above 1000 V in NEC, everything is based on 90 deg C terminations. For low voltage conductors, NEC Table 310.16 should be used.

If the concern is the sizing of conductors terminating on molded case circuit breakers, the safest approach is as follows:

Up through 110 A breakers, use 60 deg C conductor rating

For 125 A and above use 75 deg C conductor rating

But this is meaningful only for NEC/UL related installations below 1000 V. The low voltage conductor table 310.16 in the NEC is only partly based on physics. It has a lot of history, trade practices, and conservatism built into it.

 

[wareagle]

Nightfox stated
"Wareagle, does your point implies that I would rather use then the 75Deg. C table from the mentioned NEC table since it would yield me larger size of cable compared when using the 90Deg. C table from the manufacturer catalog. Considering 75Deg C conductor rating in the derating formula (IEEE 835) may even yield to a larger one. My objective is to try to see the suitability of using this NEC table so I would not be dependent to any manufacturer data especially if we are not sure which manufacturer to buy from."
If you are subject to the NEC then you use table 310.16 unless you can show by engineering calculations that you have a higher ampacity. When adjusting ampacity you always
use the 90C col if the insulation is rated 90C. The results
may some times exceed the 75C rating of the same size conductor but you can not use that higher ampacity. You are limited to the 75C ampacity rating because the terminations are rate at 75C.All new equipment has 75C or 60/75C ratings.