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Thread: BREAKER REQUIREMENT FOR GENERATOR TRANSFER SWITCH

  1. #1
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    BREAKER REQUIREMENT FOR GENERATOR TRANSFER SWITCH

    Hi, I have a generator installation 350 KW 120/208. The AHJ says I need to add a stand alone 200 amp breaker, in the transfer switch, to protect the conductors between the generator (600 amp breaker) and the transfer switch. My thought is the 200 amp main in the service panel would protect those conductors from overloading. I'm looking at it like primary or secondary protection on a transformer. Normally I would just put it in and move on but I have a 400 amp transfer going on the same generator. Any help would be appreciated. Pat.

  2. #2
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    What size are the conductors, and what do they run between? More detail please.
    The more they overthink the plumbing, the easier it is to stop up the drain

  3. #3
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    Quote Originally Posted by pat04607 View Post
    Hi, I have a generator installation 350 KW 120/208. The AHJ says I need to add a stand alone 200 amp breaker, in the transfer switch, to protect the conductors between the generator (600 amp breaker) and the transfer switch. My thought is the 200 amp main in the service panel would protect those conductors from overloading. I'm looking at it like primary or secondary protection on a transformer. Normally I would just put it in and move on but I have a 400 amp transfer going on the same generator. Any help would be appreciated. Pat.
    Not sure on the aerial run, that was done by a line co. The aerial goes to a mast on the transfer switch with 3/0 cu for the phase conductors.

  4. #4
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    It's hard to visualize the situation from description. Sketch will help.

    Sent from my Nexus 5X using Tapatalk

  5. #5
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    Hi, Rather than over think details my question is this, If the conductors are rated for 200 amps then why wouldn't the 200 amp main in the panel protect them from overload? Why would you need a 200 amp breaker in the transfer switch?

  6. #6
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    Quote Originally Posted by pat04607 View Post
    Hi, Rather than over think details my question is this, If the conductors are rated for 200 amps then why wouldn't the 200 amp main in the panel protect them from overload? Why would you need a 200 amp breaker in the transfer switch?
    Because you have a 600A semi truck ready to crush a 200A mo ped if the brakes fail.

    I'm not understanding the inspectors idea of adding another 200A breaker at the transfer switch either. The AIC wouldn't be enough would it? You need another breaker at the generator.
    If Billy Idol or John Denver is on your play list go and reevaluate your life.

  7. #7
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    You generally have to protect conductors at their source, not the load end. So a 200A breaker at the panel wouldn't protect the wiring from the generator to the panel, and the 600A breaker won't protect the wires unless they are rated for the full 600A. There are some tap rule exceptions, and some specific exceptions for transformers also.

    Also, I'm not sure its a definite rule, but I always protect a 200A ATS with no more than a 200 amp OCP in front of the ATS.

  8. #8
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    The generator breaker may be a 600A frame but have a much smaller rating plug anywhere from 200A to 600A so need more detail on the breaker, trip unit and conductors from the generator to transfer switch.

  9. #9
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    If I understand the OP correctly, he has a 200 amp ATS and it's feeder conductors protected by a 600 amp upstream OCPS. The issue would not be just that the conductors are not properly but also the ATS is not protected per the NEC and UL standards. UL standards have very specific requirements for OCPDs for ATSs. The type and size will depend on things such as available fault current.

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