Ampacity Derating Question (4), 7ft, 250kcmil THHN, 75° Termination, Underground RMC

[Longarch]

I'm not following the answer given for an ampacity derating question, which is 255A.

What is the ampacity of (4) 250 kcmil THHN copper current-carrying underground feeders enclosed in a 7 foot riser of rigid metal conduit and terminated on 75°C lugs?

I understand that 310.16, 310.15(C)(1)(c), and 110.14(C) are applied but I'm not understanding the entirety of how.

310.15(C)(1)(c) states adjustment factors shall not apply in this case. So I do not derate at 80% for the quantity of 4 conductors.

Is my answer adjusted per 310.15(B)(1) due to 75° temperature?

 

[david luchini]

They don't give a ambient temperature for temperature correction...the 75°C is the lug rating.

Also, THHN can't be used for underground feeders.

Sounds like the question is trying to trip you up on the 4 current-carrying conductors, but as you've noted 310.15(C)(1)(c) doesn't require adjustment factors for the conductors, so the answer is 255A per T310.16.

 

[infinity]

Terrible question. It even says 4 feeders not 4 conductors.

 

[Longarch]

They don't give a ambient temperature for temperature correction...the 75°C is the lug rating.

Also, THHN can't be used for underground feeders.

Sounds like the question is trying to trip you up on the 4 current-carrying conductors, but as you've noted 310.15(C)(1)(c) doesn't require adjustment factors for the conductors, so the answer is 255A per T310.16.

Thank you. That's what I thought but I desired verification.

 

[Longarch]

Terrible question. It even says 4 feeders not 4 conductors.

Agree!!! Misleading and straight out of a book.