What is the ampacity of eight current-carrying 10 THHN conductors installed in an ambient temperature of 100°F? (I am using NEC 2017)

[Electricallearner]

I kept getting wrong values, and finally found the solution online.

Derating value x ampacity value
Derating value for 8 conductors = .7 (Table 310.15(B)(3)(a))
Ampacity value for #10 conductor in 75C column = 35 (Table 310.15(B)(16)
This gives the correct Answer of 25Amps -

So for this problem I have couple of questions.

1) The questions states THHN conductor - And in NEC 2017 - Table 310.15(B)(16) - 75C Column - there is NO mention of THHN insulation- and instead NOT use 90C column - which clearly includes the THHN insulation.
2) It says ambient temperature of 100F - so why don't I include this in the calculation and use Ambient Temperature Table 310.15(B)(2)(a)? Instead JUST use Table 310.15(B)(3)(a)??? I thought it's imperative to use the Ambient Temperature Table.
3) Finally - why don't I multiply times 8 - The question clearly says (... the ampacity of 8...) LOL

Any help - cause I feel like I am missing some obvious information, or this question is poorly written. Now I am worried about how to do these on the actual exam?

THANK YOU!!!

 

[augie47]

THHN is a 90° conductor so use the 90° amaopcity (40 amps) X the 8 conductor detrating (310.115(B)(#)(a) X the ambient adjustment from
310.,15(B()2)a) using the 90° column (0.91)

 

[Electricallearner]

THHN is a 90° conductor so use the 90° amaopcity (40 amps) X the 8 conductor detrating (310.115(B)(#)(a) X the ambient adjustment from
310.,15(B()2)a) using the 90° column (0.91)

THANK YOU!!! LOL - that's the first thing I did that and then I multiplied by 8 conductors and got 203.84 - after that, I just went on a rabbit hole. LOL.. Thank you for the quick reply!!

 

[wwhitney]

The answers to (1) and (2) are "you're right you do." Your first calculation happens to give the correct answer, but it's a coincidence, it's the wrong calculation.

As to (3), you can say "25A each" if you like. #10 conductors are not allowed to be paralleled, so there would be no compliant way to use 8 of them to carry a current of 200A. And a circuit requires more than 1 conductor, so putting all 8 in parallel wouldn't let you have a circuit.

Cheers, Wayne

 

[augie47]

Welcome to the Forum

 

[infinity]

Derating value for 8 conductors

Also note that when counting the conductors you count only the CCC's. If these 8 conductors were two 4-wire MWBC's your derating value would be 80% because you would have 6 CCC's not 8.

 

[Electricallearner]

Also note that when counting the conductors you count only the CCC's. If these 8 conductors were two 4-wire MWBC's your derating value would be 80% because you would have 6 CCC's not 8.

Good to know - Thank you!

 

[Electricallearner]

Welcome to the Forum

Thank you!