Inverter Output Connection

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SolMan
SolMan Registered Users Posts: 15
Please help clarify...
In my small-but-growing knowledge of grid-tied PV systems, I understand that if you want to connect pv to a residential panel you are limited to 120% of the ampacity of the main buss. If you need to connect more pv amps you could tap the supply side.
In 2008 NEC you can now "overcurrent" the buss by backfeeding breakers at the bottom of the main buss bar. I understand the theory that loads would take from both ends and use up the currents before pv and line side could add (right?), but I'm still confused on how much more over 20% a backfed breaker can now be. Article 220 confuses it more for me.
Also, is it common to tap the supply side before the main breaker and can you do it in the main panel itself?
I am looking at putting on about a 10 kW system. There would be 2 arrays, the larger one going into an existing 100A subpanel and the rest in to 200A main. Obviously, it all combines in the main.
Any thoughts?

Thanks

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  • Ecnerwal
    Ecnerwal Solar Expert Posts: 101 ✭✭
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    Re: Inverter Output Connection

    No idea about prognosticating the intent of the NEC2008, not having bought a copy, and not finding it to be the best written, clearest document on the planet in prior versions.

    However, 10KW @ 240 V is all of 42 amps, so feeding it all to the 200 amp main would appear to make sense. Not sure that you gain a thing by splitting half of it to the 100 amp sub, other than confusion.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,501 admin
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    Re: Inverter Output Connection

    And, as far as I recall, the 120% only applies to residential connections. For commercial the limit is 100% (no overage allowed).

    I am not an expert on this part of the code... But, basically, you have to look at how your AC comes in and is routed through the home.

    If the 100 sub-panel connects to the 200 amp panel where the other 1/2 of your array will be connected--there is no "code" advantage to do this...

    So, for example, if you have a 200 amp panel with a 200 amp breaker/protected service inlet, then you would be allowed 20% of 200 amps or ~40 amp maximum branch circuit for solar PV power.

    40 amps * 240 volts * 80% (max branch load current per NEC) = 7,680 watts of PTC Rated power (~8,960 watts of STC rated panels).

    If you want 10kW or more--it sounds like you need to rip out (or add) another panel that is rated more than 200 amps for your system.

    I wonder if this would work... Say you currently have a 200 amp service/box, but really don't use that much power (no A/C or electric range/hot water heater). Could you, back the main breaker down to, for example, 125 amps (still 200 amp box) and now:

    200 amps * 120% = 240 amp limit for breaker box

    240 amp - 125 amp main breaker = 115 amp available for solar installation circuit(s)?

    115 amps * 240 volts * 80% = 22kW worth of solar power (max).

    Again, this just how I, roughly, understand the issue.

    Jim/Crewzer, Solar Guppy, or somebody else here can give far better information than I.

    After having to live with the NEC when I used to design stuff--I kind of refuse to now as a hobby. :p

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolMan
    SolMan Registered Users Posts: 15
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    Re: Inverter Output Connection

    Thanks guys.
    The reason for using the sub is that most of the available space for mounting the panels is on the garage roof where the sub is. There is somewhat less viable space on the main house where the main panel is.
    I'm looking at Sharp 198 Watt modules right now, Isc 8.23A. 2 strings into each inverter, each inverter putting out 16.5A (25.7A NEC). So I would need (2) 30A breakers in the main panel and that is more than allowed.
    I am waiting to talk to my electrician and see what he recommends, but am trying to learn something in the process.
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: Inverter Output Connection

    The "120% residential exception" to NEC 690.64(B)(2) has been changed in the 2008 NEC. In general, the "120% rule" now applies to both residential and commercial installations.

    HTH,
    Jim / crewzer
  • BB.
    BB. Super Moderators, Administrators Posts: 33,501 admin
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    Re: Inverter Output Connection

    Thank you Jim for the update...

    By the way, did I understand the intent behind the 120% rule?

    Or should I stop typing about it because I obviously have no clue how it works?

    -Bill :confused:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: Inverter Output Connection

    Bill,

    I believe you've got it... the intent was for the sum of the "source" breakers to not exceed 120% of the bar rating. Wiles addresses this very issue near the end of this Code Corner article.

    My understanding of the old NEC was that the 120% residential exemption took into account that home load centers rarely operate at full capacity.

    This separate CC article by Wiles specifically addresses this change in the NEC.

    HTH,
    Jim / crewzer