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Thread: Wireway derating

  1. #1
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    Wireway derating

    I'm curious about the application of 376.22.
    My first question is how often is this enforced? I don't think I've seen it on a job.
    Secondly, and more importantly, how would you go about applying 310.15(B)(2)(a)? Is 376.22 saying that if I have 30 current carrying conductors no problem but if I have 31 I must use 40% ampacity?
    That seems a little crazy.
    Do you start from the top of the table instead? Maybe 31-33 are @ 80% 34-36 @ 70% and so on? With the number of inspectors in this forum surely someone has enforced it.

  2. #2
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    Derating applies after 30 CCC's. And yes you would start with 40% after you installed the 31 conductor. I agree that enforcement sometimes seems lax. I've seen many installations that have a wireway with 3 or 4 panel nippled to it and many conduits exiting the wireway. IMO most of the time this isn't a problem due to load diversity, however it is a code requirement to derate.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  3. #3
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    Smile

    Thanks for the reply Trevor.
    With the ramifications of derating that harsh in wireways I'm suprised it's not planned for more often. Usually if there will be a lot of wire guys will just use larger gutters. But there is no allowence for the size of the gutter that I see...
    Seems too scary to leave to chance.
    At least voltage drop would be covered after I put those #10's on a 15A breaker.

  4. #4
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    Quote Originally Posted by Dave58er
    Thanks for the reply Trevor.
    With the ramifications of derating that harsh in wireways I'm suprised it's not planned for more often. Usually if there will be a lot of wire guys will just use larger gutters. But there is no allowence for the size of the gutter that I see...
    Seems too scary to leave to chance.
    At least voltage drop would be covered after I put those #10's on a 15A breaker.
    Congratulations
    If you are asking the question you are part of the solution.

    there are a number of code requirements that are over looked (I believe due to inexperince)as you grow and read, talk, and question; the more percise your performance. I am encouraged by the number of junior members on the forum and the questions that are brought to this forum.

    Gutters 366 and wireways 376, 378 do have size requiremnts. The max fill (allowed) is 20% of the cross sectional area. This is without regard to the number of conductors at any cross section.
    Charlie

  5. #5
    Quote Originally Posted by Dave58er
    I'm curious about the application of 376.22.
    My first question is how often is this enforced? I don't think I've seen it on a job.
    Secondly, and more importantly, how would you go about applying 310.15(B)(2)(a)? Is 376.22 saying that if I have 30 current carrying conductors no problem but if I have 31 I must use 40% ampacity?
    That seems a little crazy.
    Do you start from the top of the table instead? Maybe 31-33 are @ 80% 34-36 @ 70% and so on? With the number of inspectors in this forum surely someone has enforced it.
    You don't state your application here. I was once confused regarding cross-sectional fill allowed in a wireway, and the master electrician on the job said, "what does your book say about auxillary gutters?"
    Safety is everybody's business.

  6. #6
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    Quote Originally Posted by infinity
    I've seen many installations that have a wire way with 3 or 4 panel nippled to it and many conduits exiting the wire way.
    Trevor I agree it may be a violation however if the majority of conductors are passing essentially straight through than there may not be more than 30 conductors in the cross sectional area

    In other words in your example if all the conductors remain vertical in the wire way it will be OK.

    If 30 or more conductors travel horizontal in any particular cross section than you would have a violation.

  7. #7
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    Bob, I thought that too but I don't see those words in 376.22. It mentions the sum of cross sectional area for fill but does not seem to include those words for the derating requirement.

    376.22 Number of Conductors.
    The sum of the cross-sectional areas of all contained conductors at any cross section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway. The derating factors in 310.15(B)(2)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  8. #8
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    376.22 Number of Conductors.
    The sum of the cross-sectional areas of all contained conductors at any cross section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway. The derating factors in 310.15(B)(2)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors.
    IMO that tells us that it is not the conductor count in the entire raceway but only in certain sections of it.

    The only way that can be applied is by the cross sectional area.

    IMO the 2002 NEC commentary has the same view.

  9. #9
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    Quote Originally Posted by iwire
    In other words in your example if all the conductors remain vertical in the wire way it will be OK.

    If 30 or more conductors travel horizontal in any particular cross section than you would have a violation.

    Bob if you mean travel vertical as in a panel nippled to the top of a wireway (which is horizontal) and exits the bottom (as in a straight pull) then the size of that section is subject to 314.28 (straight pulls), but I'm sure you know that. So what am I missing??

    Charlie
    Charlie

  10. #10
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    Quote Originally Posted by cpal
    Bob if you mean travel vertical as in a panel nippled to the top of a wireway (which is horizontal) and exits the bottom (as in a straight pull) then the size of that section is subject to 314.28 (straight pulls), but I'm sure you know that. So what am I missing??

    Charlie
    Charlie I am not talking about pulls at all.

    IMO the wire count of 30 before derating starts is not all inclusive.

    It only applies to the cross section of the wire way that contains 30 CC conductors.

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