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Thread: Connecting Outside Generator to Transfer Switch

  1. #1
    DIY Member SAS's Avatar
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    Default Connecting Outside Generator to Transfer Switch

    I am going to connect a 7500 watt generator to a transfer switch via an outside connection box. Can I just run 10/4 NM-B cable between the connection box and the transfer switch in my basement, or do I need to use conduit?

  2. #2

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    10/3 NM-B (you don't count the ground) is fine as long as the circuit breaker on the generator is rated at 30Amps and you don't live in Chicago where, if memory serves me correctly, everything has to be in conduit.

    Remember to use some silicone around the inlet box to help keep water out.

    -rick

  3. #3
    Test, Don't Guess! cacher_chick's Avatar
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    NM is not rated for wet, making it inappropriate for outdoor use.
    Last edited by cacher_chick; 11-06-2011 at 02:34 PM.

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    DIY Senior Member Rich B's Avatar
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    OH BOY this should be good.......LOL

  5. #5
    DIY Member SAS's Avatar
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    Thanks, Rick. I'm a bit confused about the ground, maybe because the power cord from the generator to the connection box is described as 10/4. I'm guessing that's because the ground wire is insulated. Using 10/3 I would be using a bare copper ground. Also, since I'll be mounting the connection box inside the garage, I don't have to worry about keeping any water out.

  6. #6
    DIY Member SAS's Avatar
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    I won't be using the NM cable outside; it will run from the backside of the connection box to the transfer panel. The entire run will be in the basement.

  7. #7

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    Quote Originally Posted by SAS View Post
    Thanks, Rick. I'm a bit confused about the ground, maybe because the power cord from the generator to the connection box is described as 10/4. I'm guessing that's because the ground wire is insulated. Using 10/3 I would be using a bare copper ground. Also, since I'll be mounting the connection box inside the garage, I don't have to worry about keeping any water out.
    The /3 or /4 referrs to the number of insulated conductors. And while placing the power inlet in the garage seem like a good idea, its actually a code violation. To meet code the generator inlet must be installed outdoors.

    If you are running the cable along the bottom sill in the garage you do need conduit to protect it from physical damage. Your options are to run individual #10 THHN conductors in conduit along the sill in the garage. Or if you want to avoid using conduit you can run the 10/3 cable into the garage, straight up the first stud, across the top wall plate, and back down the stud where you want to locate the inlet. This requires more wire, but no conduit.

    Or just locate the generator inlet so that the wire does not need to run thru the garage.

    -rick

  8. #8
    DIY Member SAS's Avatar
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    That's all good information to have. I can easily (just a little more work) put the connection box in the outside wall. Another option I've considered is to just run a long power cord from the generator directly to the transfer switch (through a slightly opened window or door). Is that permitted by code?

  9. #9
    Electrical Contractor/Instructor jwelectric's Avatar
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    THE RULES HAVE CHANGED

    [Engine Generators] Engine Generators for Portable Use


    See General Information for Engine Generators

    445.1702.10(A)702.10(B)445.1702.10(A)702.10(B)445. 1702.11(A)702.11(B)
    GENERAL
    This category covers internal-combustion-engine-driven generators rated 15 kW or less, 250 V or less, which are provided only with receptacle outlets for the ac output circuits. The generators may incorporate alternating- or direct-current generator sections for supplying energy to battery-charging circuits.

    When a portable generator is used to supply a building or structure wiring system:

    1. The generator is considered a separately derived system in accordance with ANSI/NFPA 70, "National Electrical Code" (NEC).
    2. The generator is intended to be connected through permanently installed Listed transfer equipment that switches all conductors other than the equipment grounding conductor.
    3. The frame of a Listed generator is connected to the equipment-grounding conductor and the grounded (neutral) conductor of the generator. When properly connected to a premises or structure wiring system, the portable generator will be connected to the premises or structure grounding electrode for its ground reference.
    4. Portable generators used other than to power building or structure wiring systems are intended to be connected to ground if required by the NEC.

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