new to forum and solar.

woodvettewoodvette Registered Users Posts: 3
hi. i am trying to plan a grid tie solar array. need help figuring out how big of a system i can install based on my service panel rating. i am going to install the pv panels on my garage roof. the garage has a sub panel that i want to connect the solar system too which is a 100A. however the wire that that feeds it from the main panel is rated for 90A so i assume that is what i need to go by. also the breaker on the main panel for the garage sub panel is a 90A. so going by the 120% capacity rule, that is 120% x 90A = 108A. so my system amps can't go over 18A? i have read that i can reduce the breaker to allow for more amperage capacity. so if i reduce the breaker to a 70A for example, does this get me to 38A? am i on the right track with this? also how do i know how many amps an array will produce? i was planing a 7650W system.
thanks so much for any help. i will have more questions to follow.


  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: new to forum and solar.

    There are a bunch of considerations. Like what does your local AHJ require and what does your local utility require? They often have additional requirements than just what the NEC does. Is an AC disconnect required? Where does it have to be located (usually near the main panel not off by your garage subpanel). And some places still want a DC disconnect even though the inverter has an integrated one. What version of the NEC code does your AHJ use? What labeling do they require? Usually its pretty tough to backfeed solar power into a subpanel.
  • woodvettewoodvette Registered Users Posts: 3
    Re: new to forum and solar.

    i know i need an ac disconnect. they want it near the electric meter. if not they have to do a review of the plans to decide whether or not to approve it. thing is is that to run the wiring to the main panel in the basement, i would have to dig a trench which would be a pain. did it once for the garage panel. it would be so easy if i can connect to the sub panel righ tin the garage since the pv panels will be on this roof.
  • jcheiljcheil Solar Expert Posts: 722 ✭✭✭
    Re: new to forum and solar.
    woodvette wrote: »
    ... i would have to dig a trench which would be a pain. did it once for the garage panel.

    I've learned that the hard way too :) That's why ANY time I have to dig a trench now, I ALWAYS lay an extra PVC conduit in it. it's cheap and you never know when you are gonna need to pull something else thru it someday.

    I'm no expert on the subject but from what I have read here in the past, you always have to de-rate your main breaker to compensate for the solar. So if you had a 200a main breaker and 50a of solar, you have to change your main breaker out to a 150a breaker when you install the 50a breaker for the solar.

    I have a feeling you are not going to be able to "easily" do it in the sub-panel if you need to have 100a service in the garage. If you never "need" 100a in the garage then you would likely be able to get by with changing the main breaker in the sub-panel (assuming it has one - which would be required now with the solar) in the garage to a 50 and then the solar feeding into another 50 there, but then you have the issue of the wire only being rated for 90 which shouldn't have passed inspection the first time around.

    So like others have suggested, better off going to the AHJ and asking what THEY want you to do. Every AHJ is different and from what i hear almost NONE of them have a clue when it comes to this kind of stuff :)
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: new to forum and solar.

    I see some problems and confusion.

    First, if the wiring between the sub panel and main panel is only rated for 90 Amps (what size wire?) then the sub panel's main breaker should be less than 90 Amps. It is probably 75, but technically that would be too large.

    Second, the sub panel's bus bars are likely rated for 100 Amps. This means the total input to the bars from the main panel and the solar cannot exceed 120 Amps. If the sub panel's breaker is 75 Amps then the solar could be up to 45 Amps. If the loads on the sub were zero then the solar output of 45 would not exceed the sub panel's breaker rating nor the rating of the wire to the main service panel.

    Third, the sub panel should be connected to the main service panel with a breaker at the main service panel. An additional breaker on the sub panel that shuts everything off may be convenient, but it is redundant. At this point your average inspector is going to be totally confused about current ratings and will probably double derate things, which they're not supposed to do.

    Fourth, the need for an external AC disconnect (from an engineering POV it is not needed) will complicate the wiring as it means additional run will have to be made from the solar to the desired shut-off point. Longer wiring = more resistance = greater possibility of the GTI not working because its output Voltage may go too high trying to overcome all that wiring resistance. Thus a need for larger wire. Oh boy; nothing like rolling up the expenses to meet useless regulations.

    And then they wonder why people get fed up with the rules and do things illegally.
  • woodvettewoodvette Registered Users Posts: 3
    Re: new to forum and solar.

    i do have a breaker at the main panel for the sub panel. it is actually a 70A just because i had it laying around. no breaker on sub panel.
    for what i do in the garage i do not need a lot of power. mostly lighting and woodworking machine run only 1 at time which are on 240v to keep amps down. maybe 10-15 amps at time.
    i guess i should go to my county permit office and see what they want. i am trying to get registered so i can track and sell my srec's so i guess i have to do what they want.
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