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Thread: Grounding requirements for new generator installation

  1. #1
    Join Date
    Oct 2008

    Grounding requirements for new generator installation

    I'm looking at somewhat of a unique situation where a temporary generator is being installed as a semi-permanent solution for the foreseeable future.

    A 480V generator is being connected to feed an existing 480V Switchgear lineup with a utility feed which has been interrupted for an indefinite amount of time. There is no existing ATS provision for this temporary generator so the plan is to connect the temporary generator in the existing service transformer secondary terminal chamber by disconnecting the existing service entrance cables from secondary bushings and somehow connecting generator cables to existing service entrance cables to feed the existing switchgear. (I don't know that these are technically "service entrance" cables since I believe the customer owns the transformer)

    Based on this setup I would consider the generator as a separately derived system and therefore need to adhere to 250.30.

    Where I'm confused is due to the fact that the generator has a generator mounted OCPD at the generator which I believe will count as the SDS first disconnect means (this differs from transformers in a SDS where first disconnect means will be in downstream location). So based on that I think the following items would be true:

    • The system bonding jumper must be at the generator enclosure itself and any existing system bonding jumper (or main
      bonding jumper at the downstream switchgear must be removed.
    • There is no Supply side bonding jumper required between generator and switchgear since the first disconnecting means is at the generator. The ground between generator and switchgear would simply be an EGC per 250.122 based on the generator breaker size.
    • With the system bonding jumper being located at the generator enclosure the grounding electrode and GEC must be located at the generator enclosure as well and not at the switchgear.

    One last question I had was when looking at 250.122 to determine size of the EGC. Can you combine the size of smaller cables to arrive at a cable size required by 250.122? For instance for a 3000A breaker 250.122 requires a 400MCM. If a 400MCM is not available can you combine multiple 4/0 cables to arrive at the same cross sectional area as a 400MCM?

  2. #2
    Join Date
    Feb 2009
    Durango, CO, 10 h 20 min without traffic from winged horses.
    Most of the time you don't have a separately derived system until you have a separate neutral. Your case is a little different because your not transferring from utility to generator. But you could pretend you are. I would leave the neutral bonded at the main on the building. There's no reason to change it, just ignore the breaker on the generator.

    No, you can't parallel EGCs. 250.122(F).
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  3. #3
    Join Date
    Jul 2005
    Take a look at 250.32(B)(2) {grounded systems} or 250.32(C)(2) {ungrounded systems}
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

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