How to size my breakers?

bc buck
bc buck Registered Users Posts: 59 ✭✭
in General Solar Power Topics #1
I need to know what size breakers to order. I will be running 10 ga solar tray conductor from panels to CC. Then 6 ga THHN from 40 amp CC to battery bank. Was thinking 30a from panels and 50a from CC. Does this sound about right?
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Can't answer your questions without more info... size of array and what controller?  Also, how long are the runs from array to CC and from CC to battery?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    What ever you think the max continuous current is, then multiply by 1.25x for breaker and wiring size, as a start.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bc buck
    bc buck Registered Users Posts: 59 ✭✭
    #4
    3 panels in series=450 w, 68 volts, 8.33 amps.
    10 ga run to CC 25'
    CC=Morningstar PS-MPPT-40 amp
    6 ga out of CC 4' to battery . This is where I was thinking a 50 amp breaker.
    A 30 amp breaker would protect the 10 ga conductor going to the CC but on the panel label says 15 A breaker. If you where to run these panels parallel you would need a fuse or breaker for each panel. 

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    edited July 2016 #5
    For you solar panels--You don't need a circuit breaker (only need breakers when you have 3 or more panel/panel strings in parallel, one 15 amp breaker per parallel string). In your case, a single 15 amp breaker is nice for a on/off switch (when you want to service the array/charge controller).

    What battery bank voltage are you running (12/24/48 volts)?

    What size Amp*Hour battery bank do you have?

    Do you plan on adding more solar panels in the future?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
    bc buck said:
    3 panels in series=450 w, 68 volts, 8.33 amps.
    Those numbers are not correct... volts X amps = watts.  68 X 8.33 = 566 watts.

    I think the 68 volts is Voc.  You should use Vmp for your calculations.  Your strings probably have a Vmp = 54 (3 panels in series).  

    You do not need a circuit breaker for safety between the array and the controller, but I recommend one because it will serve as the switch that you do need.  A 15 amp breaker will serve your needs.  10 gauge is adequate.

    Between the controller and the battery you must have a breaker for safety.  You didn't mention your battery voltage, so I cannot do the calculations for that wire or breaker.  A 12 volt system would need a larger breaker than a 24 volt system.  4 ft of 6 gauge wire and a 50 amp breaker will be adequate for a 12 volt system.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • bc buck
    bc buck Registered Users Posts: 59 ✭✭
    #7
    Battery bank is 260 AH at 12 volts and with this application I see no more panels added.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #8
    bc buck said:
    Battery bank is 260 AH at 12 volts and with this application I see no more panels added.
    You will have a charge rate of almost 10%.  Sounds like a well balanced system.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • bc buck
    bc buck Registered Users Posts: 59 ✭✭
    #9
    vtmaps Posts  You will have a charge rate of almost 10%.  Sounds like a well balanced system.


    Thanks to every one on this forum.
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