What fuses and wiring do I need?

AlaskaJudeAlaskaJude Registered Users Posts: 1
I have a 20'X24' 1-room cabin in Hawaii, 3 100 watt, 6 amp, 12 volt solar panels, 4 6 volt Costco golf cart batteries set up as 2 12 volts, Renogy Tracer 4210 40 Amp MPPT Charge Controller 12/24V 100VDC Input. 4 gauge copper wiring between batteries, 10 gauge copper from panels to main wiring, which is 4 gauge aluminum, to the cut off box, then 10 gauge copper to charge controller and from controller to batteries. 300 watt inverter.

I plan to expand to 6 100 watt panels, 6 batteries, get a 1000 watt pure sine wave inverter. I just installed the charge controller (last, little 30 amp $30 one had a meltdown after long healthy life). What fuses should I use? The manuals do not say, just say add fuses. Also, do I need to change any of the wiring? I do plan to replace the cut off between the panels and charge controller - it's old and rusty and uses fuses. What would be the best kind of circuit breaker set up? I'm handy but out of practice where electrical is concerned... so please - layman's terms. Thanks!


  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Welcome to the forum Jude.

    Basic safety standards:
    • 40 amp charge controller * 1.25 NEC derating = 50 amp minimum rated wiring/fuse/breaker branch circuit
    Note that there are different types of charge controllers (PWM, MPPT) that operate differently. And, if you have a small solar array, you don't need as large of fusing/breakers.

    For a 12 volt 1,000 Watt inverter:
    • 1,000 Watts * 1/0.85 inverter eff * 1/10.5 volt battery cut off * 1.25 NEC derating = 140 Amp minimum rated branch wiring/breaker
    For the solar array--Basically, when you connect three or more panels in parallel, there should be a series protection fuse for each paralleled panel string (note: many newer solar panels should have a series fuse rating in their documentation).:
    • 100 watt / 17.5 volts Vmp = ~5.7 amp panel
    • typical series fuse rating = 5.7 amps * 1.25 NEC solar derating * 1.25 NEC wiring derating = 8.9 amp ~ 10 amp rated fuse/breaker
    Note, for many people, doing circuit breakers is a better solution. No fuses to keep on hand. And the breaker is also a handy on/off switch for diagnosis / turning of power when going on vacation, etc.

    I would suggest doing some paper designs and have a goal in mind for the loads you want to power. It can be frustrating to try and by piece parts and slowly "grow out" the off grid power system. Many times, as systems grow, parts are incompatible, larger panels are cheaper per watt vs a bunch of smaller panels, etc. And once your loads exceed ~1,200 to 2,000 Watts, you need to go from 12 to 24/48 volt system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • scrubjaysnestscrubjaysnest Solar Expert Posts: 175 ✭✭✭
    The 10 awg from your cut off box to CC and CC to batteries is likely to be on the small side for ~15 amps. I don't like aluminum wiring but that said go up the next size from what you would normally use for copper.
    For example 4 awg copper go up to 2 awg to use aluminum. You will also need to use an anti corrosion grease with the aluminum.

    And welcome to the forums.
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