Breakers and Fuses?

Azel Registered Users Posts: 9
I’ve been doing a lot of reading on solar and I understand pretty well the components I will need to build my system: Solar panels, panel stand, charge controller, batteries, DC AC Inverters. (For a small two panel 500 watt system.)

What I don’t understand and can’t seem to find much information on is Breakers and Fuses. (I guess breakers and fuses are not very sexy to talk about.) I know I need them but I don’t know if I need to use breakers or fuses, and I don’t know where in the circuit to place them. I suppose some people don’t use breakers and fuses at all on smaller solar energy systems, but I don’t think it’s a good idea. Is there layout information somewhere I can find?



  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
    Re: Breakers and Fuses?

    I don't have a drawing--But breakers and fuses are very important for safety. You can use either--Breakers are more expensive and very handy to use as an on/off switch. Fuses are fine when you just want to install and forget--And only change when something goes terribly wrong.

    I like to describe where to install breakers/fuses this way:

    Think of your battery bank as the "heart" of your system... It is a fixed voltage (12/24/48 volts--technically ~16/32/64 volts when charging/equalizing maximum) with potenitially 1,000 amps or more into a dead short (if the battery bank is Lead Acid and >~100 AH).

    Every wire leaving the main battery bus should have a fuse/breaker at the Amp Rating of the wiring (and attached devices may have a maximum input rating too).

    Note that for fuses/breakers, they should be ~1.25x the maximum continuous expected current (i.e., 100 amp load should have fuses/wiring rated to 125 amps minimum).

    There are different ways of rating the maximum current through wiring. The NEC (US National Electric Code) has one based on insulation type, conduit fill, etc... A sample chart:

    Wire Current
    Ampacities NEC Table 310-16

    And the boating organizations have another (typically less conservative than NEC):

    ABYC Wiring Standards

    So, it works this way... First calculate the maximum continuous load. For example a 1,200 watt inverter with 85% efficiency running on a 12 volt battery bank with minimum of 10.5 battery cut-off:
    • 1,200 watts * 1/0.85 inverter eff * 1/10.5 volts cut-off * 1.25 NEC safety factor = 168 Amp minimum ~ round up to next standard size

    Then look up the minimum wiring size from the NEC/ABYC/your standards.

    Next, for low voltage DC especially, you should use a voltage drop calculator to see that the AWG and length of cable will work with your load (for example, you may want a maximum of 1/2 volt drop for your 12 volt inverter wiring.

    Note, typically the battery negative is tied to local ground (metal frame, water pipes, earth, conduit, fuse boxes, etc.)--So no fuses/breakers in the "return" wiring. Only place fuses in the positive cables "leaving" the battery bank.

    In some cases, there are special requirements. For example, if you place 3 or more strings of solar panels in parallel--each string should have its own fuse/breaker, because the two (or more) other strings can feed a short in the one panel/string and risk a fire (check "series fuse rating" on spec. sheets--not everyone lists them).

    Another is if you have three or more parallel battery strings, each string should also have its own fuse/breaker to prevent a short in one string being fed by the other strings.

    There are pre-wired "E-Panels" and solar panel combiner boxes from different vendors (Midnite is one). These can make your first major solar power system install a lot easier (and safer). You can also buy pre-wired/pre-tested systems too.

    It is not that difficult--It is just a bit of understanding and ensuring you protect against major failures.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Azel
    Azel Registered Users Posts: 9
    Re: Breakers and Fuses?

    Thank you Bill for the lengthy explanation concerning breakers and fuses. I think I have a good picture of what I need to do now. Thanks.
  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    Re: Breakers and Fuses?

    If your going to be solely off grid without charging from the grid or a genny a DC disconnect box might be all you need at your inverter. It has a DIN rail for incoming from combiner box and can also mount an AC breaker for going from the inverter to the load or fusebox.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.