Ampacity question

[Hermie17]

#12 is 20A since you have to worry about 240.4.

But yes. You can use the 90°C column for derating conductors.

310.15(A) says:
"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)."

That awesome. I guess I need to do a little more code diving. Ever since that guy told me to use the temp rating of the conductor for derating, I have done it that way. I just always wondered if it was legal or not.

 

[tortuga]

Unless they tell you the insulation type you cannot answer the question. If you assume a modern conductor like THHN then you would use the 90 degree ampacity to answer.

Even then unless they specify how its installed, I could go with 310.17 THHN in free air 40 amps for #12 AWG.