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

A quote from Mark Ode of NFPA in Electrical Contractor Magazine from 2000: "The metal wireway dissipates heat better than the nonmetallic wireway, which is likely to be affected by excessive heat from the conductors."

I guess your assumption about maybe melting the plastic gutter was correct.