Fuse question

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brownbear
brownbear Registered Users Posts: 30 ✭✭
in Solar Beginners Corner #1
I came home yesterday and saw that the solar system was not charging.  I have a small system. Two 100w panels, 20amp charge controller. So I went up to the roof to clean the panels and to inspect the wires.  I ended up taking off the solar pv fuse 10amp and now my system works.  My question is the solar pv fuse 10amp really necessary? Did it explode? Sometimes my system takes in more than 10amps could that have blown the 10amp pv fuse?  I have a 20amp charge controller so it should handle two 100watt panels right?  Should I replace the 10amp pv fuse with something bigger like a 20amp fuse or 30amp fuse?   Please help answer these questions.  
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  • brownbear
    brownbear Registered Users Posts: 30 ✭✭
    #2
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #3
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    The fuse protects the wire so unless you have pretty light wire going from where the panels are combined to the controller, you should be able to use a bigger fuse or breaker.  12awg would handle 20a, but usually bigger wire is used to minimize voltage drop.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #4
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    As Estragon says, the fuse is too small your your current flow.

    Fuses protect wiring (and solar panel arrays with 3 or more parallel strings).

    Check the AWG size of the wiring (typically 12 AWG is used on arrays) and many times heavier wiring from array to the controller (depends on design and installation of the system). If this is on your wiring between the array and the solar charger, and you only have two panels and properly sized wiring, the you really do not need a fuse as solar panels cannot produce more current than their rated output in normal conditions.

    Fuses and breakers are designed to be "unreliable"--I.e., trip when supplying >~80% of rated current for standard fuses/breakers.

    Say you get 12 amps of current... Then a proper derating would be:
    • 12 amps * 1.25 NEC derating factor for continuous current flow = 15 amp rated fuse (based on current flow)
    • 20 Amp rated 12 AWG wiring * 1/1.25 NEC derating for continuous current flow = 16 amp rated fuse/current flow for wiring on derated branch circuit for continuous current flow
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • brownbear
    brownbear Registered Users Posts: 30 ✭✭
    #5
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    I'm using 10awg wiring for the panels going to the charge controller.   So I'll be ok without a fuse?  
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #6
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    Fuse is not really needed on the pv side then, as the panels can't put out enough current to be a problem for 10awg.  A breaker does make for a handy pv disconnect though.

    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
    · Share on Twitter

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