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Wednesday, May 25, 2016

Question:

My question concerns 376.22(B) regarding the derating of conductors in metal wireways.  376.22(A) limits conductors to 20% fill in the wireway and does not limit their number.  376.22(B) does not require ampacity derating until the number of conductors exceeds 30 at any cross section. How is this calculated?  If there are thirty 12 AWG conductors, rated for 20 amps, no derating is required. If three additional 12 AWG conductors are added, it would seem that table 310.15(B)(3)(a) now requres a reduction to 40% for all conductors in the wireway. Is this correct? Thanks for keeping up the good work,

Craig Perkins

A

Answer:

Hey Craig thanks for your question. Yes, it is determined just as you have done. The 20% fill in 376.22(A) is not modified by the number of conductors. Ampacity adjustment in 310.15(B)(3)(a) are not required until more than 30 current carrying conductors are installed. Add one more current carrying conductor and the adjustment factor is 40% for all of the conductors based on TABLE 310.15(B)(3)(a).

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