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Thread: Table 310.15 Derating Question

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    Table 310.15 Derating Question

    I'm a little confused about how many conductors I can put in a conduit.

    I have a bunch of 20A circutis and I'm wondering how many 12 THHN's I can put in (1) conduit.

    I know that 110.14(C) says that:
    Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

    I think that means that I can use the 90 degree table for derating. Using the 90 degree table I can get 9 current carrying conductors in one conduit.
    30A x 70% = 21A

    Now that's the way I figure it. I've been told though that I have to use 110.14(C)(1) which would limit me to the 60 degree table. Using that table I could only have 3 current conductors in one conduit.

    Can I use the 90 degree table for derating at all ampacities?

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    110.14 is used for sizing feeder/circuit ampacity, but the 90 deg column can be used for derating

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    For adjustment factors a thhn conductor is rated at 90 c. Your math is correct a #12thhn where you have 9 current carrying conductors is 21 amps. However look at 240.4 (D) for small conductors. Yes you can have 9 current carrying conductors in a properly sized conduit. This case overcurrent device can be 20 amperes.

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    Is 110.14 where is says that I can use the 90 degree table, or is that somewhere else? Where?

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    Quote Originally Posted by SparkyJ View Post
    Is 110.14 where is says that I can use the 90 degree table, or is that somewhere else? Where?

    It's not very clearly stated. But it is in the second sentence of 110.14(C), before you get to 110.14(C)(1). Also, if you look into the sub-paragraphs of 110.14(C)(1), you will see that you don't need to use the 60C rating, if your terminations are rated for 75C. It's a bit obscure, but I think 110.14(C)(1)(a)(3) is where it is stated.

    Welcome to the forum.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Thanks! I really appreciate the help!

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    Don't forget to read 310.15(B)(4) also. There are conditions when MWBC's are used and the neutral doesn't count as a CCC.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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