Does anyone know where it states in the NEC that you can use the 90 degree column for derating conductor for pipe fill?
I think the question is clear enough. Let's be a bit more lenient over the use of "conversational English," as opposed to speaking our technical language. After all, nobody flinched over the use of the word "pipe," even though that is a plumbing word, not an electrical word.
The question is about derating, and the situation concerns more than 3 current carrying conductors in a conduit. James had the correct answer in post #2.
I don't know for me I'd rather ask when the question contains two different applications for conductors, derating and fill. You're probably right as to what he meant but why assume?