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Thread: Derating factors, when to count the Neutral

  1. #1
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    Derating factors, when to count the Neutral

    Does the Neutral need to be considered a current carrying conductor when pulling a neutral for each circuit and would you count it if you are sharing it with three circuits A, B, and C.

    Or maybe an easier question would be, When would you not count the neutral as a current carrying conductor?

  2. #2
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    You would count it in all two wire circuits, you would also count it when sharing it with two phases of a Wye.

    You wouldn't need to count it when sharing it between two legs of a single phase system, when used with the two phases it's common to in a center grounded Delta, and in most situations you wouldn't need to count it when sharing it between A, B, and C, of a Wye system.

    Roger
    Moderator

  3. #3
    Adding to what Roger said, you also need to count it on a 3-phase wye when you have single-phase loads of varying powerfactors.

  4. #4
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    Quote Originally Posted by Tigerracer View Post

    Or maybe an easier question would be, When would you not count the neutral as a current carrying conductor?
    Welcome to the Forum. Its 310.15(B)(4) if you would like to read it for yourself.

    310.15(B)(4) Neutral Conductor.

    (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).
    (b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).
    (c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.

  5. #5
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    Part A is what is confusing me. I would appreciate a little detailed description of what is being covered. Thank you.

  6. #6
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    Quote Originally Posted by Tigerracer View Post
    Part A is what is confusing me. I would appreciate a little detailed description of what is being covered.
    Here are two examples:

    • On a 120/240 volt, single phase system, bring two ungrounded conductors and one shared neutral to outlet for an electric range.
    • On a 120/208 volt, three phase system, bring three ungrounded conductors and one shared neutral to an office area, and feed a number of receptacle outlets for computers and table lamps.
    In each example, the shared neutral will only carry current if there is an unequal load on the two (or three) phase conductors.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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