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Thread: NEC Article 310-15(B)(2)(a) Conductors derating

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    NEC Article 310-15(B)(2)(a) Conductors derating

    Under the NEC, article 310-15(B)(2)(a)- Adjustment Factors. More than three current-carrying conductors in a raceway or cable exceeds three, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(A).

    I am looking for an interpetation for the Neutral Conductor as described under 310.15(B)(4)(a) - A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).

    Here is the situation, the contractor ignored the electrical plans that called for no more than 3 branch circuits per homerun and ran a total of 6 branch circuits with 6 phase conductors and anywhere from (2) to (4) neutral conductors as well as equipment grounding conductors. In most cases, the branch circuits were rated 20 amps, 120 volt. Also, the contractor did not de-rate the conductors and ran # 12 wire. One of the conduits was a 1-1/2" and had 7 phase conductors, 6 neutral conductors and 4 equipment grounding conductors.


    If the neutral is considered a current carrying conductor, the contractor will have to replace the #12 wire with # 8 wire since Table 310.15(B)(2)(a) states that 10 to 20 current-carrying conductors must be adjusted by 50%.

    Any opinions as far as if the neutral conductors should be considered current carrying conductors?

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    Quote Originally Posted by ronnieg1025 View Post
    Here is the situation, the contractor ignored the electrical plans that called for no more than 3 branch circuits per homerun . . .
    It shouldn't require plans to know that rule applies. One hot per phase max.

    . . . and ran a total of 6 branch circuits with 6 phase conductors and anywhere from (2) to (4) neutral conductors as well as equipment grounding conductors. In most cases, the branch circuits were rated 20 amps, 120 volt.
    In this case, only a neutral which serves all three phases may be uncounted; one serving one or two phases (technically not a 'neutral') must be counted.

    Also, the contractor did not de-rate the conductors and ran # 12 wire. One of the conduits was a 1-1/2" and had 7 phase conductors, 6 neutral conductors and 4 equipment grounding conductors.
    That's obviously not good.

    Any opinions as far as if the neutral conductors should be considered current carrying conductors?
    See above.


    Oh, and welcome to the forum. :smile:
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by ronnieg1025 View Post
    Also, the contractor did not de-rate the conductors and ran # 12 wire. One of the conduits was a 1-1/2" and had 7 phase conductors, 6 neutral conductors and 4 equipment grounding conductors.
    If the neutral is considered a current carrying conductor, the contractor will have to replace the #12 wire with # 8 wire since Table 310.15(B)(2)(a) states that 10 to 20 current-carrying conductors must be adjusted by 50%.
    The required use of #8 conductors may be incorrect. If the conductor type has 90 degree insulation then a #10 conductor would be fine after derating for use with a 20 amp OCPD.


    #10 THHN = 40 amps*50%= 20 amp adjusted ampacity.
    Last edited by infinity; 10-31-08 at 09:26 AM.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

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    wow can you spell NO Profit.....I would be curious of his reasoning for this action..
    Life is temporary, heaven is forever. live life like it is your only chance to make a difference..

    to do nothing is the surest way to achieve nothing..

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    If the neutral wires only carry the unbalance of 3 phase conductors from different phases, and there aren't harmonics present, then you only need to derate 70%. (For 7 phase wires).

    And like Infinity said, if the conductors are rated at 90 deg., then #12 wire would be good for 30 * 0.7 = 21 amps.

    So there may not be a problem.

    Edit: I just realized: 7 phase wires, and 6 neutral wires, the neutrals have to be counted. But the contractor might be able to fix this by combining neutral wires.

    Steve
    Last edited by steve66; 10-31-08 at 01:32 PM.

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    Quote Originally Posted by steve66 View Post
    If the neutral wires only carry the unbalance of 3 phase conductors from different phases, and there aren't harmonics present, then you only need to derate 70%. (For 7 phase wires).

    And like Infinity said, if the conductors are rated at 90 deg., then #12 wire would be good for 30 * 0.7 = 21 amps.

    So there may not be a problem.

    Steve


    I was referencing the 7 phase conductors, 6 neutral conductors and 4 equipment grounding conductors.

    In a WYE system with that scenario you will have 13 CCC's so you would have to use the 50% derating value.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

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    Quote Originally Posted by ronnieg1025 View Post
    One of the conduits was a 1-1/2" and had 7 phase conductors, 6 neutral conductors and 4 equipment grounding conductors.

    Just in case this is not already obvious, you never count the EGC. So this situation has, at most 13 CCCs. Depending on the connections, it might have as few as 8.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    Just in case this is not already obvious, you never count the EGC. So this situation has, at most 13 CCCs. Depending on the connections, it might have as few as 8.


    Charlie, how could 7 phase conductors and 6 neutrals in one conduit ever be 8 CCC's?

    Best case scenario from a 120/240 volt system would be 12 CCC's.
    Rob

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    All responses based on the 2011 NEC unless otherwise noted

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    Quote Originally Posted by infinity View Post
    Charlie, how could 7 phase conductors and 6 neutrals in one conduit ever be 8 CCC's?

    Two full boats, one 2-wire circuit, and 3 unused conductors. Keep in mind that we do not actually know how any of this is really connected.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    Two full boats, one 2-wire circuit, and 3 unused conductors. Keep in mind that we do not actually know how any of this is really connected.


    Don't you find that response in a discussion about derating to be somewhat misleading? :rolleyes:
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

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