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Thread: PV plan check

  1. #1
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    PV plan check

    I have a company which keeps submitting plans for a solar system with a battery back up. The plans show a 200 amp main service feeding a 400 amp panel board with a 200 amp main breaker then a 90 amp solar breaker. I've approved a couple, but I really don't like it. I keep telling them that the main service is still the end of the line and you can't have 90 amps going to it. Their claim is that the 400 amp panel is where the 90 amp breaker is and so it's fine.

    Comments?
    I can build anything you want if you draw a picture of it on the back of a big enough check.

    There's no substitute for hard work....but that doesn't mean I'm going to give up trying to find one.

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  2. #2
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    Quote Originally Posted by cowboyjwc View Post
    I have a company which keeps submitting plans for a solar system with a battery back up. The plans show a 200 amp main service feeding a 400 amp panel board with a 200 amp main breaker then a 90 amp solar breaker. I've approved a couple, but I really don't like it. I keep telling them that the main service is still the end of the line and you can't have 90 amps going to it. Their claim is that the 400 amp panel is where the 90 amp breaker is and so it's fine.

    Comments?
    If I correctly understand what you described it's totally legit. Of course it must have an ATS so that the system cannot backfeed the grid during an outage.

  3. #3
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    Quote Originally Posted by cowboyjwc View Post
    I have a company which keeps submitting plans for a solar system with a battery back up. The plans show a 200 amp main service feeding a 400 amp panel board with a 200 amp main breaker then a 90 amp solar breaker. I've approved a couple, but I really don't like it. I keep telling them that the main service is still the end of the line and you can't have 90 amps going to it. Their claim is that the 400 amp panel is where the 90 amp breaker is and so it's fine.

    Comments?
    With respect to the service, there is no limit on backfeed onto the service wires as long as it is less than the service size.
    But I think you are describing a 200A MCB main service panel. Whatever the bus rating is on that panel will limit what can be connected to it.
    If you have only a 200A feeder breaker to the 400A sub panel and the 90A backfeed is into the sub panel, that panel is OK.
    The problem, as I expect you see it, is in the main. The 200A feeder breaker must be counted as a 90A backfeed under NEC rules. But if there are no load breakers or other feeder breakers in the 200A panel, just 200A main and 200A feeder, there is no way that the main panel bus can be overloaded.
    However, if there are blank spaces in the main panel, some inspectors would argue that it would be possible to connect more than 200A of actual load above and beyond the 200A feeder, and that would force the application of the 120% rule on the main panel.
    It is similar to the case where a feeder does nothing but connect a main panel to a PV AC combiner panel. In some code revisions and some inspectors' judgement the 120% rule applies to that feeder since somebody might later cut into the middle of it to supply loads.

    I am not concerned at all about the safety of what you describe, as I interpret it.

  4. #4
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    What the code says is that you are only allowed 120% of solar load on a service panel which is calculated as the main breaker + solar breaker, so on a 200 amp panel you would only be allowed 40 amps of solar.

    I don' believe that running it to a larger panel first changes that fact. So feeding a 400 amp panel from the 200 amp panel does not change the fact that the main panel is still 200 amps even though the 90 amp solar breaker is in the 400 amp panel.
    I can build anything you want if you draw a picture of it on the back of a big enough check.

    There's no substitute for hard work....but that doesn't mean I'm going to give up trying to find one.

    John Childress
    Electrical Inspector
    IAEI / CEI / C10
    Certified Electrical Inspector

  5. #5
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    Quote Originally Posted by cowboyjwc View Post
    What the code says is that you are only allowed 120% of solar load on a service panel which is calculated as the main breaker + solar breaker, so on a 200 amp panel you would only be allowed 40 amps of solar.

    I don' believe that running it to a larger panel first changes that fact. So feeding a 400 amp panel from the 200 amp panel does not change the fact that the main panel is still 200 amps even though the 90 amp solar breaker is in the 400 amp panel.

    The main panel is 200A with a 200A main breaker. The PV feedback through the 200A feeder breaker is 90A.
    But unless there are other load breakers in the main panel there is nothing that can try to draw more than 200A across any point of the main panel bus.
    You will either have 200A in or 90A out. Any intermediate condition will lead to less than 200A on any part of the main panel bus.
    This is where different code versions and different applications of common sense and "what if" lead different inspectors to reach different conclusions.

  6. #6
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    Quote Originally Posted by cowboyjwc View Post
    What the code says is that you are only allowed 120% of solar load on a service panel which is calculated as the main breaker + solar breaker, so on a 200 amp panel you would only be allowed 40 amps of solar.

    I don' believe that running it to a larger panel first changes that fact. So feeding a 400 amp panel from the 200 amp panel does not change the fact that the main panel is still 200 amps even though the 90 amp solar breaker is in the 400 amp panel.
    Is there a 200A panel upstream of the 400A one? If so, the solar counts toward the 120% on both of them. If it is (as you stated, or how I interpreted it, anyway) a 200A service (as defined by the size of the transformer or of the size of a disco with OCPD) feeding a 400A panel protected by 200A OCPD, and there are no panels between it and the service entrance, then it's OK.

  7. #7
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    Quote Originally Posted by GoldDigger View Post
    The main panel is 200A with a 200A main breaker. The PV feedback through the 200A feeder breaker is 90A.
    But unless there are other load breakers in the main panel there is nothing that can try to draw more than 200A across any point of the main panel bus.
    You will either have 200A in or 90A out. Any intermediate condition will lead to less than 200A on any part of the main panel bus.
    This is where different code versions and different applications of common sense and "what if" lead different inspectors to reach different conclusions.
    In the 2014 NEC, 705.12(D)(2)(3)(b) - the120% rule - does not mention load breakers at all except to say that the busbar needs to be sized to accommodate them, so you can't use the fact that there are no loads in the panel to get past the rule. You can use 705.12(D)(2)(3)(c) to qualify the bus if all the breakers (load and feed, excluding the main breaker) sum to the bus rating or less.

    Upsizing the MDP busbar and keeping the main breaker the same is a pretty common way to satisfy the 120% rule, and it is legit.
    Last edited by ggunn; 09-19-17 at 04:52 PM.

  8. #8
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    Quote Originally Posted by cowboyjwc View Post
    ... The plans show a 200 amp main service feeding a 400 amp panel board with a 200 amp main breaker then a 90 amp solar breaker. ...
    Quote Originally Posted by cowboyjwc View Post
    What the code says is that you are only allowed 120% of solar load on a service panel which is calculated as the main breaker + solar breaker, so on a 200 amp panel you would only be allowed 40 amps of solar.
    The use of imprecise terms is inadequate here. In the 2014 code, the 120% rule applies specifically to panelboards. If the 'main service' is just a disconnect or breaker enclosure, it is not a panelboard. If 'service panel' refers to an actual panelboard, then the rule applies. Your language is not making it clear which it is.

  9. #9
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    Quote Originally Posted by cowboyjwc View Post
    What the code says is that you are only allowed 120% of solar load on a service panel which is calculated as the main breaker + solar breaker
    That is not what it says. Look again at the wording of the "120% rule".
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  10. #10
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    Quote Originally Posted by ggunn View Post
    In the 2014 NEC, 705.12(D)(2)(3)(b) - the120% rule - does not mention load breakers at all except to say that the busbar needs to be sized to accommodate them, so you can't use the fact that there are no loads in the panel to get past the rule. You can use 705.12(D)(2)(3)(c) to qualify the bus if all the breakers (load and feed, excluding the main breaker) sum to the bus rating or less.

    Upsizing the MDP busbar and keeping the main breaker the same is a pretty common way to satisfy the 120% rule, and it is legit.
    As noted, but in less words, the 120% rule under 2014 is but one option. The option to go with in this scenario, regarding the 200A service disconnecting means bus is (also as mentioned) 705.12(D)(2)(3)(c). Note signage is required.

    Also, using one option in the subpanel does not preclude us from using one of the other options upstream.
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