conductor sizing question, NEC table 310.16 vs 310.17

[antigfk]

Hello all,

This is a hypothetical situation:

I have three #4/0AWG type-W 90C copper single-conductors installed an uncovered cable tray per 392.10. Cables are terminated to a transformer at one end and a breaker with 75C lugs on the other end. Ambient temperature worst case is 40C.

The base ampacity of the cable is 405A per table 310.17. Due to ambient temperature the cable is derated to 405A x 0.91 = 368.55A. Due to 392.11(B)(2) we have to derate by 0.65 so 368.55 x 0.65 = 239.55 which is about 240A.

Per NEC 110.14(C) I must determine the ampacity of the termination based upon NEC table 310.16. The base ampacity of a #4/0 termination at 75C is 230A. Multiplying by the derating factor of 0.88 gives me 202.4.

So in this situation the conductors have a rated ampacity of 240A while the terminations are rated 202A. Therefore, the ampacity of the circuit (ie. terminations and conductors) are rated for the lower of the two which is 202A.

I've been searching around and reading the NEC and UL standards all day now, and I was wondering if any of you guys can tell me if I am doing this right or wrong and/or offer any tips or links to some good reference material?

 

[wareagle]

The calculations you have done appers correct. However I have a question about the following ststements.

"per NEC 110.14(C) I must determine the ampacity of the termination based upon NEC table 310.16. The base ampacity of a #4/0 termination at 75C is 230A. Multiplying by the derating factor of 0.88 gives me 202.4."
The termination always have a 75C rating. No need to derate it. Where did you get the 0.88 factor?

"So in this situation the conductors have a rated ampacity of 240A while the terminations are rated 202A. Therefore, the ampacity of the circuit (ie. terminations and conductors) are rated for the lower of the two which is 202A."
If you use 4/0 conductor with a load of 230 amps, it is likely that the conductor/termination can reach 75C. If you have a load of 240 amps, the temp will be greater that 75C. You therefore need to use a larger conductor or ensure the load is not greater that 230 amps.

 

[antigfk]

Thank you for taking a look at my question Wareagle (and for helping out a member of Corn Dog Nation).

"The termination always have a 75C rating. No need to derate it. Where did you get the 0.88 factor?"
The 0.88 factor is the derating factor from 75C column of 310.16, 40C ambient.

"If you use 4/0 conductor with a load of 230 amps, it is likely that the conductor/termination can reach 75C. If you have a load of 240 amps, the temp will be greater that 75C. You therefore need to use a larger conductor or ensure the load is not greater that 230 amps."
Wouldn't the value of 230A (base ampacity for #4/0 from table 310.16) need to be derated for the high ambient temperature?

Now I will throw a wrench into the problem. The #4/0 is "extra-flexible" and has 2059 strands of copper. The lugs we use are mechanical lugs good for 500kcmil... #4/0 lugs WILL NOT fit the cable. Does this mean that we should, when determining cable ampacity, use the 500kcmil 380A rating for the lug when determining the termination ampacity (and of course, use the lower of the conductor and the termination ampacity)?

 

[davidbeach]

The terminal ampacity is based on the ability of the conductor to carry heat away from the termination. If you have 4/0 conductors and 500 lugs, your ability to carry heat away is still based on the 4/0 conductors.

 

[wareagle]

The 0.88 factor is the derating factor from 75C column of 310.16, 40C ambient.
Ok. That is for adjusting the ampacity of the conductor.
note the conditions at the top of table 310.16

Wouldn't the value of 230A (base ampacity for #4/0 from table 310.16) need to be derated for the high ambient temperature?
Yes. That is correct.

Now I will throw a wrench into the problem. The #4/0 is "extra-flexible" and has 2059 strands of copper. The lugs we use are mechanical lugs good for 500kcmil... #4/0 lugs WILL NOT fit the cable. Does this mean that we should, when determining cable ampacity, use the 500kcmil 380A rating for the lug when determining the termination ampacity (and of course, use the lower of the conductor and the termination ampacity)?
If the termination is good for 500 it would surely be good for 4/0. The question is if the 500 termination is ok to be used with 4/0. Generally it will have a range of conductors. The temperature your concerned with is the conductor temp. In table 310.16 the conditions are for an ambient of 30C and the other information at the top of the table. The ampacity shown under the 75C is saying that for 4/0 conductor with a continuous load of 230 amps, the conductor will rise to 75C.

 

[antigfk]

Davidbeach,

Is there a document put out by NFPA or UL that states that even with an oversized mechanical lug, the ability to carry heat away from the termination is still based on the #4/0 conductor?

Wareagle,

The mechanical lugs have a range of "#4/0 to 500kcmil". They are Burndy catalog #KA34. Our #4/0 cable does not physically fit inside of a #KA28. It seems to me that if the #4/0 cable terminated to a #KA34, and the set-screw in the lug is torqued properly, that we would be able to use the ampacity of 500kcmil per table 310.16 for the termination (but not for the conductor). This is because the lug is big enough to carry 500kcmil worth of current and not exceed 75C. Right? Wrong? What are your thoughts? Is there any official documentation to back a right or wrong response?

 

[REDDOG]

What is a type W cable?

 

[antigfk]

Type-W is a heavy-duty mining cable with 2kV insulation. It is pretty rugged and is used in the temporary portable power industry, as well as in mines. You can look up the type-W cable on General Cable's website for more info.

 

[davidbeach]

I'm sure there is, but nothing I have at hand. Try the NEC handbook.