When to use AC Sub-panels/Combiner panels?

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arcturusk1
arcturusk1 Solar Expert Posts: 26 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
When and if you guys use multiple inverters or have multiple sources of AC solar power, how do you combine them down in order to funnel them through a single meter and into a single breaker in the main distribution panel (MDP)?

As an example: Let's say you have a residential install with 4 strings of Enphase M210s. That's 4 sources of AC power. How would you folks go about combining it down so it can pass through a meter and feed a single breaker in the MDP? What if you have 5 SMA inverters? That's 5 sources.

My first thought (and what I do currently) would be what I would term a solar sub-panel, basically just an AC load center. It would be lug-fed, so no main breaker in the panel. If you have the 4 or 5 sources, you'd have a 4- or 5-breaker load center (ignoring whether or not an exact 4-5 load center exists, I don't know standard load center sizes). Each source feeds a breaker, and all those breakers feed the busbars, which goes through the lugs into the wiring. That passes through your, let's say, generic analog kWh meter, through an AC disconnect (this assumes your utility requires it), and then into the MDP via another breaker.

Is this reasonable? Is there another way? AC load centers and breakers can get pretty expensive. I wondered if wire nuts would be an option but that sounds insanely low-tech and probably dangerous.
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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    #2
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    Re: When to use AC Sub-panels/Combiner panels?

    Each source has to have it's own breaker. Sub panels are the typical way to do it unless your main panel has plenty of room for each circuit.

    For example, my system has 2 15A breakers, one for each Enphase string (9 inverters) in a 100A sub panel which feed a 20A breaker in my 100A main panel (18 inverters at 0.8A = 14.25A max).

    The sub panel was like $30 and you only need one extra breaker to feed the main panel ($10) so parts cost is hardly an issue here...

    Of course if we're talking commercial size installs parts will cost more, but either you have room in the main panel or you don't.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #3
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    Re: When to use AC Sub-panels/Combiner panels?

    Mine with dual inverters has a sub panel with 2 30 amp breakers, the next hop is the solar production meter, then a disconnect switch, then the 60 amp breaker in the main panel
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #4
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    Re: When to use AC Sub-panels/Combiner panels?

    Here in Austin the AHJ does not allow multiple backfed breakers in the main panel. They allow aggregation panels, however, which are just what you describe - load centers which collect the output of multiple inverters onto a single backfed breaker in the main, with the stipulation that the aggregation panel may not contain any load breakers.
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  • arcturusk1
    arcturusk1 Solar Expert Posts: 26 ✭
    #5
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    Re: When to use AC Sub-panels/Combiner panels?

    Ok guys, good to know. As I mentioned, a sub-panel/aggregation panel is what I currently design for and use, but I figured it wouldn't hurt to see if there are any other options out there. I'm always learning new things and finding out about new tech, so I wanted to make sure I wasn't missing an easier, safer, and/or cheaper way of combining multiple AC outputs.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #6
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    Re: When to use AC Sub-panels/Combiner panels?
    arcturusk1 wrote: »
    Ok guys, good to know. As I mentioned, a sub-panel/aggregation panel is what I currently design for and use, but I figured it wouldn't hurt to see if there are any other options out there. I'm always learning new things and finding out about new tech, so I wanted to make sure I wasn't missing an easier, safer, and/or cheaper way of combining multiple AC outputs.
    First, check and see if your AHJ will allow multiple backfed breakers in the main service. That would certainly be cheaper and easier. I'd think it would be safe, as well, but all disclaimers apply.
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