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Thread: more 120% rule confusion

  1. #1
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    more 120% rule confusion

    Ok. So I understand the 120% rule for back feed breakers pretty well. But I have encountered a new situation, which always leads to questions.

    We are installing PV to a residence. The residence has a 400 amp panel with integrated meter at the transformer. In this panel, the 400 amp is split and feeds two, 200 amp breakers. One breaker feeds directly to a guest house. The other breaker feeds to a panel in the main house as well as supplying power to a bus with breaker spaces in the original 400 amp panel mentioned above.

    The guest house has a 225 amp panel with a 200 amp main breaker, allowing 65 amps of solar input. Remember it is connected to the 200 amp breaker in the original panel by the transformer.

    So.......if I feed 60 amps into the panel at the guest house, will I be violating the 120% rule at the 200 amp breaker in the first panel (400 amps with two, 200 amp breakers)

    The literature at in the panel does read "200 amps max" which makes me believe this is the buss rating. But I am used to a buss that is connected to other loads, not just one main breaker.

    Any suggestions? Let me know if I need to clarify.

    thanks

  2. #2
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    Since all of the loads on the panel where the PV will be backfed are downstream of the 200A breaker, and as long as you can convince the inspector that nobody is going to tap the feeder between the 200A service disconnect breaker and the 225A panel, there is no way that that feeder can be exposed to more than 200A of current as the result of any PV contribution.

    The situation at the 225A panel is different, since the panel bus is fed by both the 200A feeder and any PV backfeed present. That is why the PV breaker must be at the opposite end of the bus from the main and the sum must be limited to 120% of the bus current rating.
    Nothing like that is taking place upstream of the service disconnect and the 225A panel, so there is nothing to apply the 120% rule to.

    If you add the complication of a tap (say 50A) in the middle of the feeder from 200A breaker to 225A panel, then you have a situation which might be seen to justify invoking the 120% rule on that feeder. There is no 120% rule that applies to the 200A breaker itself or the service conductors.

  3. #3
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    Quote Originally Posted by GoldDigger View Post
    Since all of the loads on the panel where the PV will be backfed are downstream of the 200A breaker, and as long as you can convince the inspector that nobody is going to tap the feeder between the 200A service disconnect breaker and the 225A panel, there is no way that that feeder can be exposed to more than 200A of current as the result of any PV contribution.

    The situation at the 225A panel is different, since the panel bus is fed by both the 200A feeder and any PV backfeed present. That is why the PV breaker must be at the opposite end of the bus from the main and the sum must be limited to 120% of the bus current rating.
    Nothing like that is taking place upstream of the service disconnect and the 225A panel, so there is nothing to apply the 120% rule to.

    If you add the complication of a tap (say 50A) in the middle of the feeder from 200A breaker to 225A panel, then you have a situation which might be seen to justify invoking the 120% rule on that feeder. There is no 120% rule that applies to the 200A breaker itself or the service conductors.
    Thanks. So, you're saying the 120% rule would not apply to the bussing of that one 200 amp breaker because that particular feeder, if exposed to to excess current, will be protected properly by that breaker.

    This differs from the bussing in a panel because, being fed by two sources at opposite ends, there is no protection for that part of the bussing in-between the two sources of current. Therefore it has to be limited by the 120% rule.

    I wonder if this would confuse my inspector.

    Thanks

  4. #4
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    Some points:

    -The 120% rule applies only to the load side of the service disconnecting means.
    -As of the 2014 NEC, it applies only to busbars in panelboards. (Before 2014, it applied broadly to 'busbars and conductors'. Things were a lot more confusing then, and I won't go there in this post.)

    Therefore...
    If you connect at the guest house, under the 2014 NEC or later, the only panelboard bussing on that output circuit that is subject to the rule is the 225A guest house panelboard, which meets the rule.

    Any bussing on the supply side of your two 200A breakers is fine as long as your inverter output isn't rated higher than the service.
    Anything on the load side of the other 200A breaker is irrelevant.

    The one thing that's confusing in your OP is this: "The literature at in the panel does read "200 amps max" ..." Which panel are you talking about there?

  5. #5
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    Quote Originally Posted by jaggedben View Post
    Some points:

    -The 120% rule applies only to the load side of the service disconnecting means.
    -As of the 2014 NEC, it applies only to busbars in panelboards. (Before 2014, it applied broadly to 'busbars and conductors'. Things were a lot more confusing then, and I won't go there in this post.)

    Therefore...
    If you connect at the guest house, under the 2014 NEC or later, the only panelboard bussing on that output circuit that is subject to the rule is the 225A guest house panelboard, which meets the rule.

    Any bussing on the supply side of your two 200A breakers is fine as long as your inverter output isn't rated higher than the service.
    Anything on the load side of the other 200A breaker is irrelevant.

    The one thing that's confusing in your OP is this: "The literature at in the panel does read "200 amps max" ..." Which panel are you talking about there?
    This is on the panel door in the 400 amp service. I guess maybe it's not technically a 400 amp service, just a dual 200 amp service? Not sure on terminology here. Anyway, there is a line drawing of the two breaker spaces and says for each of them "200 amps max" as well as for the buss bar that is connected to the one 200.

    thanks again, this is all very helpful

  6. #6
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    IMHO the busbar joining the two 200A breakers would have been subject to the 120% rule if not for the fact that the bus is actually a service conductor. Adding a 400A breaker upstream would cause the 120% rule to apply.
    Paradoxical, but that is how it is written.

    Sent from my XT1585 using Tapatalk

  7. #7
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    Quote Originally Posted by bbartmasse r View Post
    This is on the panel door in the 400 amp service. I guess maybe it's not technically a 400 amp service, just a dual 200 amp service? Not sure on terminology here. Anyway, there is a line drawing of the two breaker spaces and says for each of them "200 amps max" as well as for the buss bar that is connected to the one 200.

    thanks again, this is all very helpful
    That sounds pretty clear that each disconnect is max 200, thus 400A total. The labels on these 2x200 service panels can definitely be vague. I usually assume that the busbar in the panelboard section is only rated 200A unless it clearly states otherwise. As mentioned, if there's no 400A OCPD upstream then it shouldn't matter what the rating of bussing is between
    meter and MCBs.

  8. #8
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    Quote Originally Posted by bbartmasse r View Post

    The guest house has a 225 amp panel with a 200 amp main breaker, allowing 65 amps of solar input.
    Are you sure about that? I have seen door stickers on panels that say 225 Amps, but that is the rating of the enclosure, not the rating of the busbar. Cutler Hammer has told us that in all the panels they ship complete (as opposed to the bus and the main breaker separately) the bus and breaker rating are the same. If you do not have documentation that explicitly says the bus is 225A, you must assume that it is 200A.

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