Hi guys i have a question for yall.
366.7(a) says when you have 30 or less current carring conductors the correction factors specified in 310.15(b)(2)(a) shall not apply.
(b)however a nonmetallic auxiliary gutter the derating factors specified in 310.15(b)(2)(a) shall be applicable.

Is this because one is plastic and one is metal. one has a chance to melt and one doesnt? any help would be great. thanks