Derating Conductors

[LUD ELECTRIC]

Can you use the 90 degree C colume in Table 310-15(B)(16) when you are applying the derating factors for THHN copper wire from table 310.15(B)(3)(a) or do you have to use the 75 degree C colume?

 

[George Stolz]

Your answer lies in 310.15(B):

The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).

We start with the highest temp rating of the conductor.

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