Combiner Box

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Arrow
Arrow Registered Users Posts: 11 ✭
in Solar Beginners Corner #1
I've got a relatively small, low power solar setup. In all, I'll have two or maybe three strings generating up to 400W. Do I need a combiner box for such a small arrangement? Or can I simply use 2 to 1 MC4 connectors? What's the limiting factor: The ampacity? The potential rat's nest? Something else?

Thanks
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    Re: Combiner Box

    You need to look at the Series Fuse Rating for your solar panels...

    For example, you may have a panel with Isc~8 amps, and a series fuse rating of 15 amps.

    If you have a three parallel string array--If one panel is shorted, then the other two panels can backfeed ~16 amps into the one shorted panel/wiring harness.

    Typically, you can have one (no parallel) string or two parallel strings, no combiner box/fusing is required. And if you have three or more parallel strings, then a fuse/breaker per string is required.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Arrow
    Arrow Registered Users Posts: 11 ✭
    #3
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    Re: Combiner Box

    I see what you mean. All the combiner boxes I see online are way over spec'ed for my purposes -- 20 inputs rated to 1000V; that sort of thing. Is there a smaller, cheaper but still safe way to combine power from my various PV strings?

    PV specs: STC/NOTC
    Pmax 205W/148W
    Vmp 37.7V/34.4V
    Imp 5.43A/4.30A
    Voc 45.3V/41.6V
    Isc 5.81A/4.70A
    Maximum series fuse rating 10A
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #4
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    Re: Combiner Box
    Arrow wrote: »
    I see what you mean. All the combiner boxes I see online are way over spec'ed for my purposes -- 20 inputs rated to 1000V; that sort of thing. Is there a smaller, cheaper but still safe way to combine power from my various PV strings?

    I suggest: http://www.solar-electric.com/mnpv3.html

    Its weatherproof so you can set it up outdoors near the panels. It has room for a Midnite surge protector. The best place for the lightning arrestor is outdoors by the panels... you don't want lightning coming into your house on the PV wires.

    Are your panels on the roof? If so, run the ground cable down the outside of the house to its ground rod.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Arrow
    Arrow Registered Users Posts: 11 ✭
    #5
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    Re: Combiner Box
    vtmaps wrote: »
    I suggest: http://www.solar-electric.com/mnpv3.html

    Its weatherproof so you can set it up outdoors near the panels. It has room for a Midnite surge protector. The best place for the lightning arrestor is outdoors by the panels... you don't want lightning coming into your house on the PV wires.

    Are your panels on the roof? If so, run the ground cable down the outside of the house to its ground rod.

    --vtMaps

    They're on an outdoor pagoda in a school's courtyard. I'm planning on putting as many components as possible in a glass-roofed underground container, guts open if possible as an educational tool.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
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    Re: Combiner Box
    Arrow wrote: »
    They're on an outdoor pagoda in a school's courtyard. I'm planning on putting as many components as possible in a glass-roofed underground container, guts open if possible as an educational tool.

    All the more reason to do it right. The midnite solution is listed and meets code. If you are trying to educate students you should show them how to build a safe legal system.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #7
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    Re: Combiner Box

    Since you'll be in a weather proof area, you could save a bit by using one of the Midnite Baby Boxes. You will also need din rail breakers for each string(up to 4 strings). You can combine the negative side of the solar panel feeds with a suggested Bus Bar, or you can now find push-in multi line wire connectors, note that these are connectors and must be done in an accessible manner, like inside the baby box.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #8
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    Re: Combiner Box
    Arrow wrote: »
    I've got a relatively small, low power solar setup. In all, I'll have two or maybe three strings generating up to 400W. Do I need a combiner box for such a small arrangement? Or can I simply use 2 to 1 MC4 connectors? What's the limiting factor: The ampacity? The potential rat's nest? Something else?

    Thanks
    You can use Y connectors if you only have two strings, Any more than that and you need a combiner box.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #9
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    Re: Combiner Box

    Be careful if below grade--You want to make sure that it does not get flooded in winter/from sprinklers.

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