I was told by the inspector today that When derating THHN I couldn't use the 90C column. He said I had to use the 75C column because the breaker terminations are only rated at 75C. I know the termination rating limits the ampacity when selecting the proper size conductor for a given circuit, but I thought you could derate from the highest temperature rating a conductor has. If not, what is the 90C column for?
