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Thread: Low and high voltage in the same conduit

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    Like an engineer alternety's Avatar
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    Default Low and high voltage in the same conduit

    I believe I read somewhere that low voltage control circuits could be in the same conduit with a 240V circuit as long as the low voltage wire had the same insulation values as the 240.

    Is that true?

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    DIY scratch-pad engineer leejosepho's Avatar
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    The code guys will let you know for sure, but I have never heard of that being okay. Someone coming along later might not readily distinguish one wire from another and either get hurt or cause damage.

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    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by alternety View Post
    I believe I read somewhere that low voltage control circuits could be in the same conduit with a 240V circuit as long as the low voltage wire had the same insulation values as the 240.

    Is that true?
    Yes it is, with few exceptions. This is common in commercial applications where everything is conduit.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    300.3(C) Conductors of Different Systems.
    (1) 600 Volts, Nominal, or Less. Conductors of circuits rated 600 volts, nominal, or less, ac circuits, and dc circuits shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.

    Exception: For solar photovoltaic systems in accordance with 690.4(B).

    FPN: See 725.55(A) for Class 2 and Class 3 circuit conductors.

    Quote Originally Posted by leejosepho View Post
    Someone coming along later might not readily distinguish one wire from another and either get hurt or cause damage.
    This is a prime example why untrained people should stay out of electrical components.

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    Like an engineer alternety's Avatar
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    Thanks. I have a generator to install and conduit was set up assuming that the transfer switch was outside. That makes no sense to me (other than eliminating a disconnect) because there are things you want to see on automated systems and if power is out for a long time you may want to cycle the generator to conserve fuel supplies. Going out in the storm to do that seems like a poor idea.

    The conduit size does not seem generous either for a 100 A circuit. I have to look at that. I need to look around for a fill and derating table. I believe I have them here somewhere.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by alternety View Post
    Thanks. I have a generator to install and conduit was set up assuming that the transfer switch was outside.
    is the transfer switch auto or manual?


    Quote Originally Posted by alternety View Post
    That makes no sense to me (other than eliminating a disconnect)
    Unless the transfer is rated as service equipment then a disconnect can not be eliminated.

    Quote Originally Posted by alternety View Post
    because there are things you want to see on automated systems and if power is out for a long time you may want to cycle the generator to conserve fuel supplies. Going out in the storm to do that seems like a poor idea.
    If the transfer is manual then going outside is no longer an option.

    Quote Originally Posted by alternety View Post
    The conduit size does not seem generous either for a 100 A circuit. I have to look at that. I need to look around for a fill and derating table. I believe I have them here somewhere.
    Tables in the back of the code book. I do hope you have one if you are doing electrical installations. How else are you going to know what the codes for the installation are if you don’t have one?

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    Like an engineer alternety's Avatar
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    It is automatic. I believe it is designed to be a disconnect in sight of the generator but it does not matter. That is not what I am doing.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by alternety View Post
    It is automatic. I believe it is designed to be a disconnect in sight of the generator but it does not matter. That is not what I am doing.

    Then help me a little and let me know what you are doing.

    Care sohould also be taken when installing an automatic transfer switch unless the genset is sized for the whole house.

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    The house is wired with a 100 A sub-panel that controls all circuits that need power during an outage. The transfer switch will control this panel. All loads have been used to determine and switch generator capacity.

  10. #10

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    In general, wiring over 600 volts shall not be in the same boxes, conduits as wiring under 600 volts (down to 60 volts.)
    Communications wiring, (under 60 volts) shall also be separate. NEC 800-52
    high voltage = over 600 v.
    Low voltage = 60 to 600 v.
    communications wiring = below 60 v.
    and some comm wiring shall not be mixed with other comm wiring.

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    Like an engineer alternety's Avatar
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    Thnaks to all. Ked, I now have two conflicting answers to my original question. Speedy Petey seemed to be sure it is OK. He noted that it is common practice in industrial installations.

    NEC words can be somewhat hard to interpret.

    Can one of you guys comment on how I pick.

  12. #12

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    Quote Originally Posted by alternety View Post
    I believe I read somewhere that low voltage control circuits could be in the same conduit with a 240V circuit as long as the low voltage wire had the same insulation values as the 240.

    Is that true?
    Yes. You will notice that your "240" volt wire will actually be rated at 600 volts. They don't care that the LV is in there, they just want the same protection that is offered by the isulation.

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    Like an engineer alternety's Avatar
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    Thanks Alectrician. That is how I understood it. The NEC text cited above can be read two ways. It refers to wiring; not necessairly what is actually on the wire. Hence insulation ratings. Interpreting it as the voltage imposed on the wire, if you installed a bunch of 600V rated wire and needed at a later date to put a 24V control signal on it, you would violate the code. It would appear to cause no more potential for danger than having a 240 V line in the same conduit as a 120V one. But then things are not always obvious to the casual observer.

    I do understand that the wire will be rated at 600V and wet environment.
    Last edited by alternety; 11-05-2007 at 03:59 PM.

  14. #14

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    if you installed a bunch of 600V rated wire and needed at a later date to put a 24V control signal on it, you would violate the code

    How?

    If the voltage is rated to that of the highest voltage, you are good.

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    Like an engineer alternety's Avatar
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    I agree. I was just trying to make the point that such (voltage on wire, not insulation rating) interpretation would not seem reasonable.

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