Another 12 vdc question

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I have read that in a 12 volt system every wire should be fused. If I cable from my battery bank to a service panel with a main breaker, won't that suffice? This is a total 12 VDC system without an inverter. I will use breakers for the two lighting circuits from the service panel.

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  • Brock
    Brock Solar Expert Posts: 639 ✭✭✭✭
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    Re: Another 12 vdc question

    I would recommend fusing them as close to the batteries as you can. Just imagine if for some reason the wire you ran to the panel shorted, there would be nothing to stop the full power of the batteries from melting and then likely starting a fire somewhere in the line. The larger the wire the bigger reason I would fuse it.

    I have in the past I have done bad things like used a 2 inch piece of #18 as my fuse that then lead to #8 runs. I figured if I ever did short the line the short piece of #18 would burn up, like a fuse.

    I have gone to all fuses now.

    What amperage are you pulling? You can get up to 60 amps in a maxi sized automotive fuse. I use some of those in like 20-30 amp sizes for lines off my main feed for 12v loads around the house.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
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    Re: Another 12 vdc question

    Ideally, you want every (positive) wire in the battery system to have a fuse that limits the maximum amount of current available such that if there was a short, the wire will not catch anything on fire. (generally, only fuse one side of any circuit--you should not be fusing ground wires except in very limited/specific circumstances).

    So, ideally, you would want a fuse on the positive bus bar to the breaker box based on the current carrying capacity of the wire and the capacity of the breaker box...

    If, the positive wire from the battery to the breaker box is short, well insulated and kept away from sharp edges/damage, many folks probably would not place a fuse on the + bus bar connection...

    I am sure that folks here can give you part numbers/suppliers of fuse/breaker blocks that they would recommend.

    Myself, I would fuse at the battery too, then run an appropriate size wire to the breaker/distribution box... Especially if this was in my home or somewhere a fire would be dangerous and/or expensive. And, each leg (assuming they are smaller gauge wires) should have an appropriate fuse/breaker protecting the downstream wiring and devices.

    Batteries themselves have massive amounts of current available and will fry most wires (an can even damage heavy gauge wires used for busing battery banks together). If you have never had a battery/DC wiring harness short--count yourself lucky. Large battery systems, in some ways, are actually more dangerous than the 240 VAC wires that come into your home... They are designed to carry the fault currents, and the pole transformers are designed to limit maximum inrush currents to "safe limits"... And DC arcs are much harder to stop than AC arcs--so larger AC fuses and breakers are not always appropriate to use in DC battery systems as safety devices (an AC breaker will arc and can fail when operated on low voltage/high current DC circuits).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
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    Re: Another 12 vdc question

    Thanks for the advice and answers. I agree that it's probably safer to put an inline fuse near the battery on the positive line to the panel. My two circuits will only be pulling about 5 amps max each at this time. What size breaker is recommended using 10 gauge copper wiring?
  • Brock
    Brock Solar Expert Posts: 639 ✭✭✭✭
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    Re: Another 12 vdc question

    For a # 10 the maximum fuse size would be rated at 30 amps. Honestly I would put a fuse in about 1.5 times your load so in your case I would fuse it at 15 amps, just to be safe. Fuses and holder in that size are very common and quite cheap.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI