Circuit protection requirements?

PeterN Registered Users Posts: 7
I'm finally building my RV with solar, and I have a question about fusing. I have 4 x 4D AGM batteries at 12 V with 600 W solar.

My contractor is using ATC fuses for most of the loads. Self-resetting circuit breakers protect wiring to the fuse panels and to the solar charge controller from the main bus bar. The inverter (SureSine 300) and charger (TrueCharge 2) have internal fuses but no protection on the wiring (about 6 inches on the charger and 18 inches on the inverter) from the main bus bar. There is no main battery fuse.

I believe there are two concerns:

(1) There's no protection on the wiring from the bus bar to the inverter and charger. My contractor states this is okay because there are internal fuses on the equipment.

(2) ATC fuses have an interrupt rating (AIC) of 1000 A. The self-resetting breakers have an interrupt rating of 1500 A. My understanding is that this is okay for a branch, but I need a main fuse as close to the battery as possible with at least a 5000 A AIC.

Do my concerns have merit? Either way, I'd appreciate a reference to a good book or website to help me understand the requirements.

Thank you,


  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Circuit protection requirements?

    normally i'd have said you have a valid concern, but with the wiring that short, why bother? if you do go with something then find a dc breaker to act as a disconnect and not so much short circuit protection for the wiring to the breaker or fuse can be almost that long.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Re: Circuit protection requirements?

    It is a tough call... But fuses are rated to protect the wire leaving them. If you have a short inside the case of the Inverter--Will it, for sure, pop the internal fuses or not? I don't know--but it is usually a good idea to protect the best you can.

    Fuses and holders are expensive. Plus they usually take up quite a bit of space (let alone the cost and size of breakers).

    BlueSea has some very nice one or dual fuse assemblies that are intended to use on battery terminals--but should work fine on bus bars too (if you have the right size hardware):

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    Also, I kind of like to use some sort of large breaker or switch to allow disconnecting the battery from the loads or the charger (turning off system for storage, emergency power down):

    wind-sun_2197_959424Blue Sea Battery Switch 1-2-OFF 350 Amp

    I have seen batteries smoke cables before--and it is very scary to watch the wiring turning red hot and not having anyway of killing the power. You are correct--A lead acid battery bank can output 1,000's of amps into a dead short.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    Re: Circuit protection requirements?

    Don't know with your specific hardware, but the inverters I've opened up, changing a fuse would have been a pain. Would be much easier to put a slightly smaller external fuse, than what's inside the case, so swapping a fuse in the field would be a viable option.

    A Xantrex Prosine 3.0 has dozens of screws, and knowing which ones need to be removed to get the case open, took a call to their service dept. I didn't want all the guts coming loose because the wrong set were removed!

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    The Mega fuse is one of two internal fuses.

    Here's a 400W Vector inverter. To the right of the transformer are a couple of fuses:

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    I avoid relying on internal fuses if at all possible!