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Thread: Bonding and Unbonding portable generator neutral

  1. #1

    Bonding and Unbonding portable generator neutral

    I have a new portable generator that I will be using for standby power connected through a transfer switch to selected circuits in my home. I need to unbond the neutral by removing the neutral bonding strap. However, I will also be using the generator in stand alone situations where I need the neutral bonded and a separate generator ground. Since bonding and unbonding the generator requires pulling covers off to get to the jumper, I would like to put an external properly marked switch or junction box on the generator so that I can easily connect or disconnect the neutral bonding jumper. Would that be code compliant?

  2. #2
    Join Date
    Feb 2003
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    Re: Bonding and Unbonding portable generator neutral

    You could leave the generator neutral bonded to the frame and switch the neutral with a three pole transfer switch.

    Just use a 4-wire cord, and ground the generator circuit at the transfer switch, similar to the sketch below.



    Ed

    [ February 10, 2004, 07:56 AM: Message edited by: Ed MacLaren ]

  3. #3
    Guest

    Re: Bonding and Unbonding portable generator neutral

    Ed this seems to me to be a connection which gives one an alternate neutral path no? I have always looked at a generator a separatley derived source. In that scenario one would not ground the neutral at the generator but only at the service I believe. I think a single pole switch of the proper amperage wouls be a better solution. Just my humble opinion.

  4. #4
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    Re: Bonding and Unbonding portable generator neutral

    Joe,
    If you are saying that the sketch above has a parallel path for unbalanced load current (a parallel neutral), I don't believe it does.

    It is a separately derived system (SDS) because the neutral is switched by the transfer switch.

    This creates two separate systems, each of which must be grounded.

    When on utility power, (Diagram 4) the unbalanced load (neutral) current flow is from X to Z.

    When switched to generator power, (Diagram 5) the unbalanced load (neutral) current flow is from X to Y.

    Other comments are welcome.
    Ed





    [ February 13, 2004, 10:32 AM: Message edited by: Ed MacLaren ]

  5. #5
    Guest

    Re: Bonding and Unbonding portable generator neutral

    yup I was looking at your load as the panel oops.

  6. #6
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    Re: Bonding and Unbonding portable generator neutral

    The three sketches that Ed has provided give us an opportunity to see the real reason that there is a difference in bonding techniques for the 3-way (Diagram #1) and 4-way (Diagram 4 and Diagram 5) transfer switches.

    Imagine a fault occurring at the load, from a hot leg to the case. Recall that the current is seeking its way back to the neutral point of the source that caused current to flow in the first place. Follow the path that the fault current will take from the fault point, along the external cases of various components, along the green wires (EGCs) that connect the cases to each other, and eventually leading back to the neutral point of the source that is on-line at the time. In Diagram #4, the source is the utility. In Diagram #5, the source is the generator.

    In Diagram #1, the source that is shown to be on-line is the generator. But you could mentally throw the 3-way switch the other way, and follow the fault current's path back to the utility.

    Of all the possibilities, the one that I think is the most revealing is in Diagram #1. With the generator on-line, the fault current has to pass through the Service Panel, in order to find its way back to the generator’s neutral.
    Charles E. Beck, P.E., Seattle
    Comments based on 2008 NEC unless otherwise noted.

  7. #7
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    Re: Bonding and Unbonding portable generator neutral

    Ed and Charlie make a good team!!!

    Between Ed's illustrations and Charlie's explanation, they make it easy to understand. Outstanding!!!

    Pierre

  8. #8
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    Re: Bonding and Unbonding portable generator neutral

    I know. I saved the drawing in case I loose the thread or forget where it was.
    John, Chair City, NC
    Technology: Mans best efforts to make things as good as they used to be

  9. #9
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    Re: Bonding and Unbonding portable generator neutral

    The three sketches - - - - - - there is a difference in bonding techniques for the 3-way (Diagram #1) and 4-way (Diagram 4 and Diagram 5) transfer switches.
    Are you looking at the same diagrams that I am, Charlie? :confused:

    All three are supposed to be different views of the same installation.

    Ed

  10. #10

    Re: Bonding and Unbonding portable generator neutral

    Thanks for the info. Using the configuration in figure 1, it appears to me that I could use a 60 amp breaker in the main service panel to provide the hot leads to the service side of the transfer switch plus the neutral. I would have to extend the hot and neutral leads through a conduit from the service panel to the branch panel for the circuits that I want to power with the generator (I don't want to tear out the existing wiring to the service panel and reroute it to the branch panel). Since everything is grounded together, the ground wires for the branch panel circuits can remain tied to the ground bar in the service panel. One other question, can you steer me to a good three pole transfer switch that would handle 60 amps.

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