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Thursday, July 7, 2016

Question:

Concerning the derating in metal wireways, many times a wireway will be installed above a panel and nippled in. Multiple smaller conduits or cables would be entered into it and conductors dressed into panel.  Presuming wireway was mounted horizontally, the vertical cross sectional area may very well be OK at any point for fill and also ampacity but the horizontal cross sectional area must be considered also, no matter if it doesn't  really make sense.

Jack Rooney

A

Answer:

Hey Jack thanks for your question. Let's say in your example the wireway is 4 inches square and 3 feet long. The cross sectional area would be 4 X 4 inches = 16 sq inches (the actual interior cross sectional area is typically provided by the manufacturer). The adjustment factors in 310.15(B)(3)(a) do not apply unless the number of current carrying conductors is more than 30 at any cross section as stated in 376.22(B).

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