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Thread: NEC 705.12

  1. #1
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    NEC 705.12

    We are stuck with the AHJ on a completed PV install. Everyone is having a different interpretation of NEC 705.12 #2 The question is, the size of breaker that should go in the 200A main service panel. There are 3 PV AC circuits off the roof into a combiner box where each of the 3 AC circuits land on a 20A DP breaker. So we have 39 amps to terminate in the 200A panel. One interpretations is that based on the 120% rule the max size breaker would be a 40A. Is the 40A the max to go in the 200A panel? We need at at least a 60A.

  2. #2
    Join Date
    May 2011
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    Strictly speaking, unless you're still on the 2011 NEC or older, it's the inverter output that counts and not the breaker size. But is correct that 40A is the limit (after multiplying by 125%.) 120% of 200 is 240. 240-200=40. [I'm assuming the panel's main breaker is 200A, although you didn't say that.]

    If you're nameplate total inverter output is 39A, then your backfeed calc is 39A*1.25=48.75A. So you are not compliant.

    One solution might be to do a load calculation and downsize the main breaker to 175A. Then 240-175=65 and you have enough 'headroom.'

  3. #3
    Join Date
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    Quote Originally Posted by jaggedben View Post
    Strictly speaking, unless you're still on the 2011 NEC or older, it's the inverter output that counts and not the breaker size. But is correct that 40A is the limit (after multiplying by 125%.) 120% of 200 is 240. 240-200=40. [I'm assuming the panel's main breaker is 200A, although you didn't say that.]

    If you're nameplate total inverter output is 39A, then your backfeed calc is 39A*1.25=48.75A. So you are not compliant.

    One solution might be to do a load calculation and downsize the main breaker to 175A. Then 240-175=65 and you have enough 'headroom.'
    One example where the 2014 change in this rule would help you, is given a 600A main panelboard with a 600A main breaker.

    In 2011 rules, you'd be limited to a 110A interconnection breaker (unless you have a rare situation where you can find and fit a 120A breaker), and this would mean a maximum total inverter output current of 88A.

    In 2014+ rules, you'd be limited to a 120A as your calculation of 125% of total inverter output current. This would mean you can have up to 96A of total inverter output current, interconnected to this same panelboard.

  4. #4
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    May 2018
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    question if I may, as I am trying to understand... the last panel I dealt with in Jamaica was a 200 amp main breaker, but the box itself was labelled do not use more than 225 amps breaker... so which would be used for the above calculations, the 225 or the 200? Like a 125 bus with a 100 amp main breaker? If panel says no more than x breaker then is that the rating to use? Then see if you can drop the main breaker enough to allow the back feed?

  5. #5
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    Quote Originally Posted by Adamjamma View Post
    question if I may, as I am trying to understand... the last panel I dealt with in Jamaica was a 200 amp main breaker, but the box itself was labelled do not use more than 225 amps breaker... so which would be used for the above calculations, the 225 or the 200?
    Probably 225. If it can take a 225A breaker then the busbar must be rated for that.

    Like a 125 bus with a 100 amp main breaker?
    Yes.

    If panel says no more than x breaker then is that the rating to use? Then see if you can drop the main breaker enough to allow the back feed?
    More or less, yes. Usually I simply see a label that says 'Mains Rating xxxA', and that's what I go by.

  6. #6
    Join Date
    May 2018
    Location
    Jamaica and london
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    778
    thanks.. getting to understand this pv stuff a little more. Loads to learn. Thanks again

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