Low and high voltage in the same conduit

Discussion in 'Electrical Forum discussion & Blog' started by alternety, Oct 23, 2007.

  1. alternety

    alternety Like an engineer

    Messages:
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    Location:
    Washington
    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?
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    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.
  3. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

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    Yes it is, with few exceptions. This is common in commercial applications where everything is conduit.
  4. jwelectric

    jwelectric Electrical Contractor/Instructor

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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.

    This is a prime example why untrained people should stay out of electrical components.
  5. alternety

    alternety Like an engineer

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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.
  6. jwelectric

    jwelectric Electrical Contractor/Instructor

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    is the transfer switch auto or manual?


    Unless the transfer is rated as service equipment then a disconnect can not be eliminated.

    If the transfer is manual then going outside is no longer an option.

    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?
  7. alternety

    alternety Like an engineer

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    Location:
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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.
  8. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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.
  9. alternety

    alternety Like an engineer

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    Location:
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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. kd

    kd New Member

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    207
    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.
  11. alternety

    alternety Like an engineer

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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. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    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.
  13. alternety

    alternety Like an engineer

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    Location:
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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: Nov 5, 2007
  14. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689

    How?

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

    alternety Like an engineer

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    Location:
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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.
  16. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

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    I assume you mean 800.53 since there is no 800.52 .

    Also, this does NOT apply. 800.53 in in section "II. Wires and Cables Outside and Entering Buildings" of Art. 800.

    You are looking for 800.133(A)(1)(c), but this applies to communication conductors. I believe the OP is asking about control circuits.
  17. kd

    kd New Member

    Messages:
    207
    The reason to keep comm circuits and control circuits and other circuits below 60 v. away from high voltage is that the high voltage wires create EMF that interferes with the comm wires. The insulation on the comm wires could be 25,000 v. insulation--same problem. Also there is the possibility of short circuit or error that will destroy sensitive equipment. Do not put a 24 v. system in a conduit with a 120v or 480 v. system.
  18. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    Rtfm

    I have a 200A whole-house transfer switch (Generac RTS) inside the garage. Installation instructions for the switch specifically prohibit running the control system interconnection leads in the same conduit as the AC power wires. Generac tech support did say, however, that for lower-power installations (my generator is only 16KW) it would be OK. It's mainly an EMF interference issue, as ked pointed out.
    Last edited: Nov 6, 2007
  19. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    It's done every day with motor control wiring.
  20. NuetronBill

    NuetronBill New Member

    Messages:
    1
    We do not put a circuit 50 volts or less in the same conduit with single phase 120 volts to ground (240 volts phase to phase).Also we do not mix 120/240 with 277/480 in the same conduit.
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