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Thread: 705.12 (A) or (D)?

  1. #1
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    705.12 (A) or (D)?

    Hello All,

    I am currently working on a project where 2 inverters will be combined and interconnected with a supply side tap. The sum of the inverter OCPDs are 100A using 2 breakers, (1) 40A and (1) 60A. The bus bar rating of the combiner is 100A, MLO with a 100A fusable disconnect ahead protecting it. All of this will be mounted outside on a backboard adjacent to the utility meter.

    There is additional production metering equipment that requires a 15A reference breaker to be installed somewhere on the circuit. My original plan was to install the reference breaker in the PV combiner which would result in 115A of breakers on a 100A busbar which would violate 705.12 (D)(2)(3)(c). My question is, do I have to comply with 705.12 (D)(2)(3)(c) since I am interconnecting on the supply side and (705.12 (A)) "the sum of of the ratings of the inverter AC inverter OCPDs do not exceed the ratings of the utility service (200A)", and the bus bar of the combiner is still being protected at 100A by the fusable diso. As I understand 705.12 I must comply with one of either (A) (B) (C) or (D).

    Originally I thought to upsize the combiner to a 125A bus, which I'm told will be physically larger and we are space limited.

    Thank you all!

  2. #2
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    Assuming this is 240V, Murray makes a little 4/8 subpanel that's rated 125A and I'm sure would fit your space. It's a bit of a pain to feed with 100A conductors, but easier than arguing the point with an inspector.

    I suppose that one could argue that your fused disco isn't a service disconnect and that therefore (D) doesn't apply to the load side of it. Very disingenuous, in my opinion, even if legalistically it follows a justifiable interpretation of the code. But whether that fused disco is a service disconnect is a controversial point you can't count on the AHJ to agree on.

  3. #3
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    Quote Originally Posted by jaggedben View Post
    Assuming this is 240V, Murray makes a little 4/8 subpanel that's rated 125A and I'm sure would fit your space. It's a bit of a pain to feed with 100A conductors, but easier than arguing the point with an inspector.

    I suppose that one could argue that your fused disco isn't a service disconnect and that therefore (D) doesn't apply to the load side of it. Very disingenuous, in my opinion, even if legalistically it follows a justifiable interpretation of the code. But whether that fused disco is a service disconnect is a controversial point you can't count on the AHJ to agree on.
    Interpretations differ, of course. The only AHJ in Texas that I know of who considers a line side PV interconnection to be a service entrance is CPS Energy in San Antonio. I don't think the rest of the state is being "very disingenuous".

  4. #4
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    Quote Originally Posted by ggunn View Post
    Interpretations differ, of course. The only AHJ in Texas that I know of who considers a line side PV interconnection to be a service entrance is CPS Energy in San Antonio. I don't think the rest of the state is being "very disingenuous".
    Does a combiner behind a PV disconnect not have to meet the 705.12(D) rules because it is not on the load side of a 'service disconnect'? That's what I think would be a disingenuous argument, since it would be arguing that no rules apply to such equipment. I wonder if you've tried using that argument outside San Antonio.

  5. #5
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    Quote Originally Posted by jaggedben View Post
    Does a combiner behind a PV disconnect not have to meet the 705.12(D) rules because it is not on the load side of a 'service disconnect'? That's what I think would be a disingenuous argument, since it would be arguing that no rules apply to such equipment. I wonder if you've tried using that argument outside San Antonio.
    Oh, maybe I misunderstood you. "Ahead" and "behind" get a little muddled dealing with PV; they mean one thing under normal operation and the opposite in a fault condition, but PV AC combiners always must satisfy 705.12(D)(2)(3)(usually c) irrespective of where they are and whether the system is load or line side interconnected. Incidentally, in 2017, 705.12(D) is moved to 705.12(B).

  6. #6
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    Quote Originally Posted by _Dylan View Post
    Hello All,

    I am currently working on a project where 2 inverters will be combined and interconnected with a supply side tap. The sum of the inverter OCPDs are 100A using 2 breakers, (1) 40A and (1) 60A. The bus bar rating of the combiner is 100A, MLO with a 100A fusable disconnect ahead protecting it. All of this will be mounted outside on a backboard adjacent to the utility meter.

    There is additional production metering equipment that requires a 15A reference breaker to be installed somewhere on the circuit. My original plan was to install the reference breaker in the PV combiner which would result in 115A of breakers on a 100A busbar which would violate 705.12 (D)(2)(3)(c). My question is, do I have to comply with 705.12 (D)(2)(3)(c) since I am interconnecting on the supply side and (705.12 (A)) "the sum of of the ratings of the inverter AC inverter OCPDs do not exceed the ratings of the utility service (200A)", and the bus bar of the combiner is still being protected at 100A by the fusable diso. As I understand 705.12 I must comply with one of either (A) (B) (C) or (D).

    Originally I thought to upsize the combiner to a 125A bus, which I'm told will be physically larger and we are space limited.

    Thank you all!

    The easiest answer would be to put the monitoring breaker in another panel and you are done. What I see a lot is people using 705.12 (D)(2)(3)(c) to build AC combiners and if a small PV related load is also there most AHJs are letting it go. Depends on the AHJ. If your AHJ is holding you to a strict interpretation of 705.12 (D)(2)(3)(c) then you have to up the rating of the busbar or put the load somewhere else.

    The first draft of the 2020 NEC adds a new section 705.11 that give much more "guidance" on supply side interconnections. Still does not call the PV AC disconnect a service entrance.
    Last edited by pv_n00b; 07-02-18 at 05:52 PM.

  7. #7
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    Quote Originally Posted by pv_n00b View Post
    The easiest answer would be to put the monitoring breaker in another panel and you are done. What I see a lot is people using 705.12 (D)(2)(3)(c) to build AC combiners and if a small PV related load is also there most AHJs are letting it go. Depends on the AHJ. If your AHJ is holding you to a strict interpretation of 705.12 (D)(2)(3)(c) then you have to up the rating of the busbar or put the load somewhere else.

    The first draft of the 2020 NEC adds a new section 705.11 that give much more "guidance" on supply side interconnections. Still does not call the PV AC disconnect a service entrance.
    We have the ability to move the production meter to the interior and install the reference breaker in the MSP, thanks all for the input, definitely cleared up some of my confusion.

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