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Thread: Protection of Conductors under 705.12.(D) (2)

  1. #1
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    Jan 2018
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    Sparks, NV
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    Protection of Conductors under 705.12.(D) (2)

    I have a plan the has a 100A disconnect wired with #1 in between of a 200A protected Tesla gateway (on the utility side of the disconnect) and a 125A breaker feeding out of a panel recieving the input from 2-30A Tesla Powerwalls and a 35A Inverter breaker (solar input side of the disconnect). 200A potential on one side and 125A on the other. If any conductor grounded out (unlikely in the 3' segment of the system) the potential would be 300A +/- to ground over a #1 THWN and thru a 100A rated disconnect. Shouldn't these components be protected to a 325 ++Amp level (125 + 200)x125%=? )
    Thanks

  2. #2
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    Your description of the proposed arrangement is a bit unclear, perhaps you could post a one-line diagram? ASCII art can work for that.

    If the only protection on the utility side of the #1 copper feeder is a 200A breaker, then the feeder would have to comply with the feeder tap rules in 240.21(B). Is the 100A disconnect fused or unfused?

    As for the utility interactive inverters, the sum of the breakers is only 95A, so the #1 copper feeder should have adequate ampacity for them (barring unusual adjustment factors).

    How are the loads connected to this system? If via breakers in the panel with the 125A main and the utility interactive inverter connections, that panel needs to have a 200A bus with proper breaker location, or a 225A bus.

    Cheers, Wayne

  3. #3
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    To clarify.

    Name:  705.12 D (2).jpg
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    Quote Originally Posted by Dcoon@cityofsparks.us View Post
    I have a plan the has a 100A disconnect wired with #1 in between of a 200A protected Tesla gateway (on the utility side of the disconnect) and a 125A breaker feeding out of a panel recieving the input from 2-30A Tesla Powerwalls and a 35A Inverter breaker (solar input side of the disconnect). 200A potential on one side and 125A on the other. If any conductor grounded out (unlikely in the 3' segment of the system) the potential would be 300A +/- to ground over a #1 THWN and thru a 100A rated disconnect. Shouldn't these components be protected to a 325 ++Amp level (125 + 200)x125%=? )
    Thanks
    I attached an image but it appears to small to identify the features. The disconnect is not fused. I will investgate the tap rule and watch for additional post.
    Thanks for your help.

  4. #4
    Join Date
    Oct 2009
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    Austin, TX, USA
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    Quote Originally Posted by Dcoon@cityofsparks.us View Post
    I have a plan the has a 100A disconnect wired with #1 in between of a 200A protected Tesla gateway (on the utility side of the disconnect) and a 125A breaker feeding out of a panel recieving the input from 2-30A Tesla Powerwalls and a 35A Inverter breaker (solar input side of the disconnect). 200A potential on one side and 125A on the other. If any conductor grounded out (unlikely in the 3' segment of the system) the potential would be 300A +/- to ground over a #1 THWN and thru a 100A rated disconnect. Shouldn't these components be protected to a 325 ++Amp level (125 + 200)x125%=? )
    Thanks
    No. A grid tied inverter cannot feed a short. In the event of a fault the inverter will shut down and the conductors will only see fault current from the service.

  5. #5
    Join Date
    May 2011
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    3,846
    I agree with that the wiring doesn't need to be sized for 325A, and that the tap rule may apply. But the plan does seem a bit weird to me. I'm not sure why the feed through the disconnect to the PV/Battery panel is not simply connected at the opposite end of the 200A distribution panel with a 100A breaker. And I'm not sure why it's main is 125A instead of 100. That's how I'd probably have done it. Maybe there are physical location constraints.

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