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Thread: motor circuit condutor derating

  1. #31
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    Quote Originally Posted by kwired View Post
    Does a conductor carrying say 10 amps for a motor produce any more or less heat within a raceway than the same size conductor carrying 10 amps for a resistive load? That is what the derating is supposed to be about is to keep heating effects under control.

    Maybe it is not written correctly, and I am having a little bit of a hard time trying to see what some of you are seeing, but I think it is intended that all "current carrying" conductors be included for number of current carrying conductors in a raceway. A motor circuit conductor is a current carrying conductor unless it qualifies as a neutral that only carries unbalanced current in a multiwire branch circuit.

    If you don't think motor conductors produce heat I can show you a few that obviously do, and they are sized according to the deration factors in 310.15 - the raceways still do warm up enough you don't need instruments to tell.
    There is no question that the motor conductors produce heat just like any other current carrying conductor.

    That being said, it is my opinion that the code wording does not require that the motor circuit conductors be derated. If they would delete the list of articles at the end of the code section, then I would say that they have to be derated, but with the current wording, the code does not require derating. The other issue is the fact that the motor circuit conductors are already derated to 80% based on the rules in Article 430.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  2. #32
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    Quote Originally Posted by don_resqcapt19 View Post
    There is no question that the motor conductors produce heat just like any other current carrying conductor.

    That being said, ...the code wording does not require ...motor circuit conductors be derated. ... The ...motor circuit conductors are already derated to 80% based on the rules in Article 430.
    So the wording leaves the rationale irreproachable? How is having [SDS] control conductors in the same conduit going to reduce the amount of I²R heat, compared to a similar scenario where the control conductors are run separately? That is, conduit run with control conductors and more than three motor circuit conductors requires no derating, versus a conduit run just with more than three motor circuit conductors does require derating...? I must say, that's defies the logic and purpose of derating.

    Now you have me wondering how 110.14(C) terminal temperature limitations applies to Article 430 motor circuit conductor ampacity. Chapter 2 continuous load conductors must also have an ampacity of 125% of the calculated load current... but we are permitted to use the conductor ampacity rating before adjustment and correction, i.e. the higher temperature Table value (e.g. 90°C rating) [210.19(A)(1)]. There is no such clause in Article 430 (that I'm aware of). Are we permitted to determine 125% rating from the higher temperature-rating value? If so, where is that stated in Article 430. If not stated anywhere, the 125% ampacity seems to have to be based on the termination temperature limitation ampacity...??????
    Last edited by Smart $; 07-17-12 at 04:46 AM.
    I'll never get there. No matter where I go, I'm always here.

  3. #33
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    Quote Originally Posted by don_resqcapt19 View Post
    The other issue is the fact that the motor circuit conductors are already derated to 80% based on the rules in Article 430.
    Only continuous load motors - which in general we treat most of them as though they are continuous load, same basic rule, different code section applies to conductors for other continuous loads.

  4. #34
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    Quote Originally Posted by Smart $ View Post
    So the wording leaves the rationale irreproachable? How is having [SDS] control conductors in the same conduit going to reduce the amount of I²R heat, compared to a similar scenario where the control conductors are run separately? That is, conduit run with control conductors and more than three motor circuit conductors requires no derating, versus a conduit run just with more than three motor circuit conductors does require derating...? I must say, that's defies the logic and purpose of derating.
    Sure it defies logic, but I have never said that the code is logical. Of course the motor conductors make heat and have the same heating issues as any other current carrying conductor, but, in my opinion, under the conditions given in my original post, the code does not require those conductors to be derated. If I remember, I will submit a proposal to delete the article reference at the end of this section for the 2017 code. If that is no longer there, then I think that motor conductors would be power conductors for the purposes of derating.
    Maybe we also need the term "lighting and power conductors" defined. I think that term appears in more than one article.

    Now you have me wondering how 110.14(C) terminal temperature limitations applies to Article 430 motor circuit conductor ampacity. Chapter 2 continuous load conductors must also have an ampacity of 125% of the calculated load current... but we are permitted to use the conductor ampacity rating before adjustment and correction, i.e. the higher temperature Table value (e.g. 90°C rating) [210.19(A)(1)]. There is no such clause in Article 430 (that I'm aware of). Are we permitted to determine 125% rating from the higher temperature-rating value? If so, where is that stated in Article 430. If not stated anywhere, the 125% ampacity seems to have to be based on the termination temperature limitation ampacity...??????
    I think you are correct.

    Many of these issues come a result of the rules being looked at by multiple code making panels and sometimes the panel members don't even think about how a rule that is in their relm interacts with rules in the other parts of the code.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  5. #35
    Join Date
    May 2006
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    345
    I think that "different systems" as used within the context of 300.3 means "other than power and lighting". This is the way this section read in the 1947 NEC.

    3011. Conductors of Different Systems. Conductors of signal or radio systems shall not occupy the same enclosure
    with conductors of light or power systems except as permitted for elevators in sections 6216 and 6217; for sound recording in Section 6405; for remote-control lowenergy power and signal circuits in section 7292; and communication system in section 8031. Secondary wiring to electric discharge lamps of 1,000 volts or less may occupy the same enclosure as the branch circuit conductors. Conductors of light and power systems of 600 volts or less may occupy the same enclosure, without regard to whether the individual circuits are alternating-current or directcurrent, only if all conductors are insulated for the maximum voltage of any conductor within the enclosure. Conductors of light and power systems of over 600 volts shall not occupy the same enclosure with conductors of light and power systems of 600 volts or less. Control, relay and ammeter conductors used in connection with any motor or starter may occupy the same enclosure as the motor circuit conductors.


    In the 1968 Code, derating factors for conduit fill did not apply to branch circuit conductors that were already adjusted in size to compensate for continuous loads. That exception was replaced with the language "before the application of any adjustment or correction factors" in later code editions.

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