312.9 Amp Feeder

[ptonsparky]

Working on a project that requires a 312.9 amp feeder. Previous EC in years past installed a buried paralleled set of 2/0 THWN cu with EG, one conduit. They had it fused @ 200 amp. I am guessing they were worried about VD. I am not. Using an ambient correction of 1.15 I can get roughly 322. FWIW Motor load, 3ph, 480v about 560' distance. 4 VFDs, plus some small across the line.

IIRC I am limited to the distance those wires can travel in a different ambient before I loose that 1.15 correction. I believe it was 6'. What is the NEC reference?

 

[kwired]

Working on a project that requires a 312.9 amp feeder. Previous EC in years past installed a buried paralleled set of 2/0 THWN cu with EG, one conduit. They had it fused @ 200 amp. I am guessing they were worried about VD. I am not. Using an ambient correction of 1.15 I can get roughly 322. FWIW Motor load, 3ph, 480v about 560' distance. 4 VFDs, plus some small across the line.

IIRC I am limited to the distance those wires can travel in a different ambient before I loose that 1.15 correction. I believe it was 6'. What is the NEC reference?

310.15(A)(2) exception - 10 feet or 10% of circuit length. (if circuit were less then 100 feet long the 10% would kick in)

 

[ptonsparky]

310.15(A)(2) exception - 10 feet or 10% of circuit length. (if circuit were less then 100 feet long the 10% would kick in)

:thumbsup:

10 ft at each end or in total? I think that was the argument back when. I hoped it had been clarified. Doesn't appear so at first glance.

 

[kwired]

:thumbsup:

10 ft at each end or in total? I think that was the argument back when. I hoped it had been clarified. Doesn't appear so at first glance.

Good question - IDK. My first assumptions would be total.

Example I have seen the most is with roof top conductors - in which case the higher ambient is only on one end of the run.

 

[ron]

:thumbsup:

10 ft at each end or in total? I think that was the argument back when. I hoped it had been clarified. Doesn't appear so at first glance.

Circuit length = total.

 

[ptonsparky]

"....Exception: Where two different ampacities apply to adjacent portions of a circuit..." I think that is what I want. Each end is over 500 from the other, they can hardly be considered adjacent. Max of 10' on both ends.

 

[iwire]

"....Exception: Where two different ampacities apply to adjacent portions of a circuit..." I think that is what I want. Each end is over 500 from the other, they can hardly be considered adjacent. Max of 10' on both ends.

No, not at each end, ron has it right.

 

[ptonsparky]

No, not at each end, ron has it right.

So the 10' ambient above ground 500'+ away affects the operating temperature of the 10' at the other end? Is this just one of those NEC "requirements" or is it based on something?

I can make it work. It's just $$s.

 

[GoldDigger]

-duplicate-

 

[GoldDigger]

As long as the middle segment with high ampacity is at least 200 feet long, I think that the spirit of the rule allows up to 10 feet at each end.
I do not think the letter of the exception is unambiguous on this point.

If you hold to the 10 foot total interpretation of the rule, just insert a j-box and splice somewhere in the middle of the 500 foot run.

 

[ptonsparky]

Seems a bit odd that a 1 foot section of 3/0 added in the center of that would make a difference, or a pull box.

 

[GoldDigger]

Seems a bit odd that a 1 foot section of 3/0 added in the center of that would make a difference, or a pull box.

It will not make any practical difference, but it will give you two separate segments of conductor, each of which can then have ten feet on one end.
(I do not agree with the single ended interpretation, in case you were in doubt.)

P.S. The 10% is based on the high ampacity portion only, not the total overall length.

 

[winnie]

I am curious, what ambient conditions give you an ambient correction factor of 1.15?

-Jon

 

[ptonsparky]

Buried.

 

[ron]

I think the word "adjacent" is for other crossing circuits in other conduit. It is for consideration for mutual heating

 

[ptonsparky]

I think the word "adjacent" is for other crossing circuits in other conduit. It is for consideration for mutual heating

You will need to be more persuasive. Positive.

 

[kwired]

I think the word "adjacent" is for other crossing circuits in other conduit. It is for consideration for mutual heating

Quote Originally Posted by ptonsparky
"....Exception: Where two different ampacities apply to adjacent portions of a circuit..." I think that is what I want. Each end is over 500 from the other, they can hardly be considered adjacent. Max of 10' on both ends.

seems to be talking about one circuit to me, and would be ampacity of conductor no matter what factors are used to determine that ampacity. Could be ambient temp, could be number of conductors in the raceway, could be both.

 

[Carultch]

:thumbsup:

10 ft at each end or in total? I think that was the argument back when. I hoped it had been clarified. Doesn't appear so at first glance.

I think it is not more than 10 ft AND not more than 10% of the remaining length that matters. Not "or".

 

[kwired]

So the 10' ambient above ground 500'+ away affects the operating temperature of the 10' at the other end? Is this just one of those NEC "requirements" or is it based on something?

I can make it work. It's just $$s.

As written I'd say 10 feet total, but logic does say that 500 feet away is not going to have any impact on the other end in nearly all instances.

 

[ron]

You will need to be more persuasive. Positive.

I don't think I can. It is a tough section to interpret.

Made even more complicated by the language in the un-enforceable Annex B.310.15(B)(3) which starts to talk about 25% of the total run specifically for burial depths in addition to the 10' / 10% "adjacent" business.