Splice or use Multibranch connectors

gkerlin
gkerlin Solar Expert Posts: 27 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
Hello,

I'm installing 6 panels on the roof of my RV.

There will be 3 parallel connected strings each string consisting of 2 series wired panels. (All MC4 connectors)

My question regards the best way to connect the strings together. My plan is to use Multi branch connectors to connect the 3 resulting positive and 3 negative leads in parallel. That would mean 2 multi branch on each leg (4 total)

I would then run my positive and negative leads down a vent to my controller.

Another option is to use a combiner box on the roof and connect everything on 2 pole bus bar. I'd rather go with option one and use the Multibranch connectors and not have another box mounted (drilled & screwed) up there.

My question is simply other than cost - is there any reason not to use the multi branch connectors? I'll be well within any voltage or amp limitations (will be about 50v 4.3a per string)

Thanks for the advice.
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #2
    Re: Splice or use Multibranch connectors

    Typically, with 3 or more parallel connected solar strings, you need a series protection fuse (or breaker). This protects against one panel getting shorted and the other two panels feeding current to the shorted panel. Check the data sheet for series protection fuse rating.

    You could connect everything without the fuses/breakers and all will otherwise work OK. (solar panels are, more or less, current sources and share current just fine when connected in parallel. Batteries are voltage sources and very sensitive to battery differences and wiring resistance to properly share current with paralleled paths).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • gkerlin
    gkerlin Solar Expert Posts: 27 ✭
    #3
    Re: Splice or use Multibranch connectors

    the spec sheet calls for "Max Series Fuse Rating 10amp). Short circuit current is 5.3amp

    so - do you suggest a 10a fuse (or even 8amp) in each of the 3 positive leads before they get combined?

    If so - what would be the best way/product to accomplish that? A combiner box with fused buses, - waterproof inline fuses - something else?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #4
    Re: Splice or use Multibranch connectors

    It is really a close call for 3 parallel strings here... Isc of two arrays is around 10.6 amps. And I would use a 10 amp fuse (no reason to use 8 amp if 10 amp is "legal").

    The only thing to recommend against--Standard (inexpensive) automotive fuse holders have melted when operating even under rated current inside a box (Wayne from NS Canada said that this could have easily started a fire). I think this is similar to the ones that melted:

    UuZltQin-tfe3OKxONGpXRxRNTD49KL2vnHpQfpuAmyg85GsOopGRd2FXfvVA4yVCRTztxWa5AfDfgdtUqbiussU5EPUVZ3zFi5uWsBaJLrI9RkxUc7kMWx0aw6Bmxyl1huEObXGgG70hKqNbK7wNWzgwmEYJge7-r6xb7kHavQ

    Otherwise, you can get standard panels mount glass fuse holders, panel mount circuit breakers, or (fairly large) fuse/breaker boxes that use UL/NRTL rated components (for an RV--these may be too large to justify).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • gkerlin
    gkerlin Solar Expert Posts: 27 ✭
    #5
    Re: Splice or use Multibranch connectors

    OK then how about this:
    I've got a 4 breaker panel mount circuit breaker box already mounted (inside the rv basement) I was going to use 2 breakers in it. One 30a from the panels.. then out to the controller, and then from the controller to the second breaker - an 80a breaker from the output of the controller to the batteries. This allowed me to isolate the MPPT from the panels and the batteries.

    Instead of the 30amp from the panels I could put 3 10 amp breakers, and bring all three positives down to the box and put a breaker on each one. Then spice them together after the breakers. I'd save a bit on the cost of the MC4 connectors, spend a bit on 2 additional pv wires down to the controller, but overall have more copper and less voltage loss.

    Does that sound reasonable?

    If so - there is still no reason for me not to combine my negatives up on the roof with combiners right? Then have 4 wires coming down the vent. (3 positive and one negative)

    sound reasonable?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #6
    Re: Splice or use Multibranch connectors

    That would be fine...

    By the way, typically for small jobs, buy some male/female MC4 cables. Just cut them in half and wire the cut ends into your system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • gkerlin
    gkerlin Solar Expert Posts: 27 ✭
    #7
    Re: Splice or use Multibranch connectors

    Gotcha. Thanks for the help.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #8
    Re: Splice or use Multibranch connectors
    gkerlin wrote: »
    OK then how about this:
    <snip>
    Does that sound reasonable?
    Make sure the breakers you use are rated for DC. You may need to order something from the link that Bill provided earlier.

    One of our forum members "ywhic" built an inexpensive combiner box from QO breakers which are rated for DC. You might be able to find his posts which are heavily illustrated and descriptive.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • gkerlin
    gkerlin Solar Expert Posts: 27 ✭
    #9
    Re: Splice or use Multibranch connectors

    Thanks... I do have DC breakers
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