table 310.16 column question

[mjmike]

I'm sure there is an answer for this, but maybe I am overthinking. Taking a piece of copper wire, say #3 THHN / THWN. Using as THHN, this wire is good for 110 amps under the 90-degree C column. If using on a piece of equipment where the lug temperature are rated for a temp of 75 degree C, then the same wire is rated for 100 amps. However, the 75 degree column only lists this wire for 100 amps if used as THWN (as in a wet location). What if the wire is intended to be used as THHN for a dry location? What would the amperage rating be per the 75 degree column? It's almost like the THHN designation is missing from the 75 degree column. Can THHN still be good for 100A with respect to the 75 degree column? I would appreciate it if this could be explained. Thanks.

 

[david luchini]

Yes, #3 THHN would still be good for 100A with respect to the 75 degree column.

The answer is found in 110.14(C) "Temperature Limitations."

See for instance, 110.14(C)(1)(b)(2) "Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75degC ampacity of the conductor sized used."

 

[jumper]

The 90C ampacity of THHN in a dry location is used for derating and correction factors, your final ampacity would be limited by the 75C terminals.

 

[mjmike]

Thanks for the info, but I am still confused why THHN does not show up in both the 75 degree column and the 90 degree column. THHW and XHHW does.

 

[david luchini]

Thanks for the info, but I am still confused why THHN does not show up in both the 75 degree column and the 90 degree column. THHW and XHHW does.

See table 310.13(A). THHN is listed at 90deg for dry and damp locations only. THHW and XHHW are listed at 75 deg for wet locations and 90 deg for dry locations.

 

[jumper]

And any conductor marked -2 is rated for 90C for both wet and dry locations for derating and correction factors.