Auxiliary Gutters

[cripple]

Can anyone explain to me why sheet metal auxiliary gutters has a derating factor, for current conductors that only apply to conductors exceeding 30 conductors, and in nonmetallic auxiliary gutters the derating starts at 3 current carry conductors.

366.23 Ampacity of Conductors.
(A) Sheet Metal Auxiliary Gutters. Where the number of current-carrying conductors contained in the sheet metal auxiliary gutter is 30 or less, the adjustment factors specified in 310.15(B)(2)(a) shall not apply. The current carried continuously in bare copper bars in sheet metal auxiliary gutters shall not exceed 1.55 amperes/mm2 (1000 amperes/in.2) of cross section of the conductor. For aluminum bars, the current carried continuously shall not exceed 1.09 amperes/mm2 (700 amperes/in.2) of cross section of the conductor.

(B) Nonmetallic Auxiliary Gutters. The derating factors specified in 310.15(B)(2)(a) shall be applicable to the current-carrying conductors in the nonmetallic auxiliary gutter.

 

[don_resqcapt19]

The heat from the conductors can more easily be dissipated from the metallic wireway than from the non-metallic one. Metal is a much better conductor of heat than plastic.

 

[cripple]

Auxiliary Gutters

If metal can dissipated easier than non-metallic material why do we not get a break in non-metallic conduits? What would be the advantage of using non-metallic gutter, if we derating the after three current conductors?