Ampacity using Table 310.15(B)(16)

[dasarmin]

Typically we use Tables 310.15(B)(16) and 310.15(B)(3)(a) when calculating the ampacity of conductors in conduit. We also always run THHN, therefore use temperature column for 90C (194F).
If most circuit breakers, terminals, etc are rated at 75C (167F), should we use the 75C column even though we use THHN wire?
Seems like I?ve read in the NEC something about this, but can?t find it.

 

[david luchini]

Yes. See 110.14(C)(1)(b). You can start from the 90C column when applying adjustment factors, etc.

 

[dasarmin]

So if I understand this correctly; I can still use the THHN 90C column for determining the ampacity of the conductors, regardless of the 75C ratings for the terminals, etc.

 

[jumper]

Follow up on David's post:

The final ampacity must be equal to or greater than the 75C column after derating and other correction factors.

 

[jumper]

So if I understand this correctly; I can still use the THHN 90C column for determining the ampacity of the conductors, regardless of the 75C ratings for the terminals, etc.

For derating and correction factors only.

 

[david luchini]

So if I understand this correctly; I can still use the THHN 90C column for determining the ampacity of the conductors, regardless of the 75C ratings for the terminals, etc.

I'm not sure if you're understanding it or not. Lets say I have a feeder supply a 135A non-continuous load. Per 215.2(A)(1), the minimum feeder conductor size would need an ampacity of 135A, so I look at the 75C column and see that #1/0 is my minimum feeder size.

I'm running this feeder in a raceway with another feeder such that I have 6 current carrying conductors. I need to apply an adjustment factor of 80% for the 6 ccc's. I am using THHN for my feeder, so I look at the 1/0 ampacity in the 90C column and see that it is 170. Applying the 80% adjustment factor to that, I get 170*80% = 136Amps. The adjusted ampacity of the 1/0 THHN is sufficient for the load.

If the load was 155 Amps non-continuous, I could not use the 1/0, because the 75C rating is only 150A.

 

[david luchini]

The final ampacity must be equal to or greater than the 75C column after derating and other correction factors.

I don't think this statement is correct. The final ampacity of the conductor after adjustment factors have been applied must be sufficient for the load to be served. It can be lower than the unadjusted ampacity from the 75C column.