National Electrical Installation Standards

Standards as High as Your Own

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Wednesday, June 12, 2019

Question:

Raceway Fill and Adjustment Factor Part 2

I need to know how many #10 awg thhn wires and #18 awg dimming wires derated I can fit in a 1" conduit? How would you derate and split up the different wires to know how many we can have in each pipe.

Michael Henson
A

Answer:

Michael the other part of your question was about derating. The term "derating" has been replaced with the term "adjustment factor" and is not about how many conductors can be installed in conduits or tubing but about how we determine conductor ampacity. Using the 10 AWG THHN (copper) from your example it has an allowable ampacity of 40 in Table 310.15(B)(16) using the fifth row in column one and then going to the 90 degree C column for THHN. That table heading indicates it is based on not more than 3 current carrying conductors in a raceway, cable or earth. If more than 3 current carrying conductors are installed, the adjustment factors (formerly derating) in Table 310.15(B)(3)(a) need to be applied. Using 9 THHN 10 AWG conductors from your example gives us an adjustment factor of 70 %, 40 amperes X 70% = 28 which is the ampacity of each of those conductors based on that adjustment factor. If 10 through 20 conductors are installed the adjustment factor is 50 % which results in an ampacity of 20 for the 10 AWG THHN conductors, that is why we only chose 9 in yesterday's explanation. The ampacity of the 18 AWG conductors of 6 amperes is in Table 402.5 which is referenced in 725.49(A) if the dimming circuits are Class 1 as they are defined in 725.2. If the Class 1 dimming circuit conductors do not carry more than 10 % (0.6 amperes for the 18 AWG) they do not need to be counted for applying the adjustment factor as stated in 725.51(B). Consider using more and likely smaller conduits to keep the adjustment factors low. Or better yet use metal wireways because the adjustment factors do not apply until more than 30 current carrying conductors are installed as stated in 376.22(B) but the conductor area cannot exceed 20% as stated in 376.22(A).

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