Conductor Derating

[stew]

What is the code reference for conductor derating where you use 90 degrees as your starting point for derating?

 

[infinity]

310.15(B)(2)(a) directs you to the values in table 310.16 through 310.19.

 

[stew]

trevor I know what the table values are my question is what is the specific code reference that allows you to start with the 90 degree table when derating thhn? Just cannot put my finger on it.

 

[stew]

looking for something that dosnt exist. thanks trevor i answered my own question.

 

[infinity]

The important thing to remember is that the ampacity of a conductor is listed in those tables. We typically use 310.16. What is the ampacity of #12 THHN? If you forget about 240.4(D) and 110.14(C) for a moment, the answer is 30 amps because the insulation, THHN, is rated for 90 degrees C. This would be the basis for the starting of your derating because 310.15(B)(2)(a) tells you to use the values in the tables.

 

[charlie b]

looking for something that dosnt exist. thanks trevor i answered my own question.

Yes it does exist. Look at the last sentence in 110.14(C). It's not very well written, and it may take a bit of thinking to work out what it is telling us. But it is your answer.