Sizing AC Combiner Panel

Philip C
Philip C Solar Expert Posts: 45 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
What are the official rules for sizing an AC Combiner panel, this will not be used for Load Ciruits, strictly to tie PV Inverters together.

Thanks
Philip
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Sizing AC Combiner Panel
    Philip C wrote: »
    What are the official rules for sizing an AC Combiner panel, this will not be used for Load Ciruits, strictly to tie PV Inverters together.

    Thanks
    Philip

    Um, as stated that doesn't make sense.
    If you're combining AC you are dealing with the output side of an inverter which inevitably will be connected to loads. And since most off-grid inverters don't stack, you're probably talking about GT inverters which have quite an intricate set of rules for integrating them to the service panel.

    The other combiner point would be on the DC side where strings of panels need to be joined up in parallel. Is that what you were wondering about?

    (Third possibility is that 'Coot has finally gone completely senile. It's bound to happen someday; why not today? :p )
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #3
    Re: Sizing AC Combiner Panel
    Cariboocoot wrote: »
    Um, as stated that doesn't make sense.
    If you're combining AC you are dealing with the output side of an inverter which inevitably will be connected to loads. And since most off-grid inverters don't stack, you're probably talking about GT inverters which have quite an intricate set of rules for integrating them to the service panel.

    The other combiner point would be on the DC side where strings of panels need to be joined up in parallel. Is that what you were wondering about?

    (Third possibility is that 'Coot has finally gone completely senile. It's bound to happen someday; why not today? :p )

    hehe :D Senility is contagious!

    My dual Grid tie inverters has an AC combiner with two 30 amp breakers that feeds the main breaker @ 60 amps. ( the inverters actually only output 23 amps each max) It was required as the local code guys would only allow one PV breaker in the main (and the utility wants the generation meter combined, and a single master AC disconnect is required)

    PS It is clearly marked, apply no loads. You can see it to the right of the inverters.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Sizing AC Combiner Panel
    solar_dave wrote: »
    PS It is clearly marked, apply no loads.

    Sounds like a non sequitur to me. :p
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,634 admin
    #5
    Re: Sizing AC Combiner Panel

    I am assuming, taking multiple GT inverters into a sub-panel, then routing the output of the sub-panel to the main panel.... I think I read that somebody here had an inspector that wanted it done this way (or perhaps it was because the main panel was near full).

    In any case, the "no loads" means not using the GT combiner sub panel to connect to other local loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Philip C
    Philip C Solar Expert Posts: 45 ✭
    #6
    Re: Sizing AC Combiner Panel

    Sorry I should have been more clear. This is a grid tied system and I need to tie the AC output of 7 sunnyboy inverters together, before entering a single AC disconnect. The AC Combiner panel will not be used as a load center, so when sizing the AC combiner, do I still have to use the 120% rule for the Buss bar?

    Thank You
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,634 admin
    #7
    Re: Sizing AC Combiner Panel

    I would suspect you would be limited to 100% of the bus bar. You cannot exceed the box/breaker capacity.

    You are not mixing loads and sources in the gt combiner box.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Philip C
    Philip C Solar Expert Posts: 45 ✭
    #8
    Re: Sizing AC Combiner Panel

    I agree, but worst case, the sum of current feeding breakers can not exceed 120% of the bus bar. So for example I have (7) single phase 40A breakers. 7*40 = 280A, but if I consider the max amps per phase will be 32*3 = 96A, so it seams like a 125A Service panel should be acceptable?

    By the way I've noticed "gt" mentioned several times, what does this stand for?

    Philip
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Sizing AC Combiner Panel
    Philip C wrote: »

    By the way I've noticed "gt" mentioned several times, what does this stand for?

    Philip

    "gt" or more typically "GT" = "Grid-Tie".

    At least that one I can answer with some degree of accuracy. :blush:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,634 admin
    #10
    Re: Sizing AC Combiner Panel

    Sorry, the previous answer was typed on my phone--so was keep short and changing case is a bit of extra work.

    And, no, you cannot run the bus bar of a sub panel at 120% of rated load when all of the "loads" are Grid Tide Inverters.

    The problem is that you have to add all of the Rated Current of the GT systems together, multiply by 1.25 (NEC breaker/wiring derating factor), then make sure that the total from all inverters will meet the breaker exiting the sub panel to the main panel...

    You cannot have 120 Amps of Rated output (20% over 100 amp bus bar), going through a 100 amp breaker (rated for box), and you cannot put a 125 Amp "main breaker" in the sub panel to supply the bus bars (that is too much current).

    If this was a single main panel with mixed loads and GT Inverters, then you could use the 120% rule (with the main breaker no larger than bus bar rating, and the GT inverters feeding from the "bottom" of the bus bar, opposite of the main panel breaker).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Philip C
    Philip C Solar Expert Posts: 45 ✭
    #11
    Re: Sizing AC Combiner Panel

    I agree fully, but when you have Sunny Boy 8000's on a 480/277 three phase system, summing the currents would be over kill (assuming you are not placing the entire system on a single phase)
    So for example if I had nine SB8000s with a rated output current of 32A,then my max current per phase would be 96A (if balanced between all three phases). 96*1.25 = 120A, therefore a 125A AC Combiner panel should be fine.

    Philip
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #12
    Re: Sizing AC Combiner Panel
    Philip C wrote: »
    ..... therefore a 125A 3ph AC Combiner panel should be fine.

    You have to use a 3 ph panel, not a "split phase" panel like us mortals use.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,634 admin
    #13
    Re: Sizing AC Combiner Panel

    From my understanding, it was the 120% over sizing of the breakers to bus bars that was the question... You cannot oversize the "load breakers" for a GT only connection sub panel.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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