De-rating wires in a trough

[jonny1982]

How does derating work in a trough? Typically, there is a whole mess of wires jam packed in a trough. Do the same rules apply as to conduit? I know if conduit is less then 2', no derating required.

 

[Smart $]

A trough in layman's terms is a wireway in NEC terms. There is no derating in wireways until you exceed 30 current-carrying conductors at any cross section. Those wires are then derated per 310.15(B)(3)(a).... e.g. 31st wire causes a 40% derate.

Also keep in mind there is a 20% limit on wire fill.

 

[iwire]

A trough in layman's terms is a wireway in NEC terms. There is no derating in wireways until you exceed 30 current-carrying conductors at any cross section. Those wires are then derated per 310.15(B)(3)(a).... e.g. 31st wire causes a 40% derate.

Also keep in mind there is a 20% limit on wire fill.

I agree with smart but have bolded this part at any cross section as that is a very important thing to remember when trying to comply with the rules.