mc branch couplers ok?

lamplight
lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
in General Solar Power Topics #1
I have 3 paralleled sharp 175's and am adding a fourth soon hopefully. i just planned to do another branch coupler. i am still well within their rated amp capacity (30 amps). my local installer suggested i setup a separate combiner box with breaker for each panel.

any thoughts on this? obviously the couplers are easy and cheap. ive been using them on the 3 panels for a couple years, seems fine.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: mc branch couplers ok?

    From what little I have seen, the series fuse ratings of most solar panels seem to limit their total short circuit current limits to a maximum of two parallel strings without fusing/circuit breaker on each string...

    So for three more more strings, for safety, you should add something (fusing, breakers, combiner box) in each parallel connection to prevent a wire/panel short circuit from fire in the case of a short/panel failure in any once string.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lamplight
    lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
    #3
    Re: mc branch couplers ok?

    thanks Bill

    as i was writing my installer emailed me and said there can be voltage drop as well. (im paralleling all 4 panels) i figure i might as well get the combiner box and make it legal/up to code , especially if i may be having some voltage drop, want to get every bit of energy out of these that i can.

    sometimes i think the 3 panels should put out more than they do , and i wonder if this has something to do with it. soon will find out.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: mc branch couplers ok?

    There is always the solar voltage drop calculator to double check your voltage drops.

    My two cents--unless you have a really big voltage drop issue or other "design" problem (such as Vmp is too low for battery bank + solar charge controller voltages)--I would look at other issues for low output (bad connection somewhere--battery post, solar panel string connection bad, open panel, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lamplight
    lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
    #5
    Re: mc branch couplers ok?

    was thinking of getting this midnight solar:
    http://store.solar-electric.com/mnpv3.html

    my panels output about 5 amps each, do i go with the 6amp breaker for each panel? http://store.solar-electric.com/mnepv.html

    (my solar panel specs:
    Short circuit current Isc - 5.55
    Maximum power current Ipm - 4.95 )

    also, if i run it this way i need to run about 20' of EIGHT MC cable extensions to this box, what do people run that in and how to connect it to the box? a big 1.5" pvc pipe? maybe i should get an electrician. or maybe i should just use the MC3 couplers. :confused::cry:
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #6
    Re: mc branch couplers ok?

    Why not run in series? solves all your issues and no combiner or new wire needed.

    If its the Charge Controller, it would be better to get a new Mppt unit than go down the path your considering
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: mc branch couplers ok?

    At the very least:

    Isc * 1.25 = 5.55a * 1.25 = 6.9a = 7-10 amp fuse minimum

    Remember the fuse is to protect the "wiring", not the device (with solar panels, it is probably to "protect" the "internal wiring").

    With any of the larger panels (typically used in grid tied applications), the spec. sheets call out a series fuse that is typically a bit larger than 2x the Imp or Isc, and less than 3x Isc.

    And what about the MPPT controller--takes care of a few issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lamplight
    lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
    #8
    Re: mc branch couplers ok?

    thanks guys, well i do have an mx60 , but i have a couple reasons for parallel:

    last i read a couple years ago, when i went from 2 to 3 panels , i was getting near the upper limit (forget which spec) of the mx60 if run in series. plus:

    the main reason is shading in winter, they are oriented in the best possible place but still get shading on the bottom panels in winter. with the parallel arrangement i only get knocked out 1 panel at a time as the shadows spread. i might try doing 2 strings of 48v though now that i think about it, less wiring issues.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #9
    Re: mc branch couplers ok?

    Shading is NOT an issue when its a single series string, the controller will simply adjust to the reduced voltage as the panels get shaded. A single series string with Mppt is actually better than all in parallel as long as the string is above the battery voltage when shaded and would be in your 12V configuration.

    The Sharps are 24V panels, so that will have limits ..
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  • lamplight
    lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
    #10
    Re: mc branch couplers ok?

    i did experiments and we discovered that partial shading on one of the panels in a 48v 2 panel array had a much larger drop in power production that if i did the same shading in a parallel config.

    thanks bill, i dont need anything else anyone.
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