Why 376.22, Why not 378.22?

Fred B

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
Why is the derating of conductors for "more than 3" not applicable within a metal wireway up to 30 conductors in the cross sectional area (2017 NEC 376.22(A),(B)), when in all other wiring methods the "more than 3" would become a consideration, even in NM wireways (378.22)?

376.22(B) Adjustment Factors.
The adjustment factors in 310.15⁠(B)⁠(3)⁠(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(5), exceeds 30 at any cross section of the wireway.

378.22 Number of Conductors.
........
The adjustment factors specified in 310.15(B)(3)(a) shall be applicable to the current-carrying conductors up to and including the 20 percent fill specified in the first paragraph of this section.
 
Fred B said:
That logic would equally apply to metal conduit, wouldn't it? But derating applies to conduit.
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I'm guessing but conduits with more than 3 CCC would allow up to 40% fill. Metal wireways only allow 20% fill so there is more air for heat dissipation even with the 30 CCC's.
 
infinity said:
I'm guessing but conduits with more than 3 CCC would allow up to 40% fill. Metal wireways only allow 20% fill so there is more air for heat dissipation even with the 30 CCC's.
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And there will be a lot more surface area when using the metal wireway, as the smallest I know of is 2", giving a surface are of 8 square inches per inch and I have rarely seen that size.
 
Another related question would be that it would appear that if a division in the metal wireway was to be made it could hold twice as many as conductors. (ie. a 4x4 wireway divided to a 2-2x4 spaces it could have up to 60 #10 total without derating. But a 4" EMT would be significantly derated with that many conductors.)
 
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