Help me guess battery breaker amps

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sunbunny
sunbunny Solar Expert Posts: 59 ✭✭✭✭
in Off Grid Solar & Battery Systems #1
For 12 volt Sun-Extender 1040T. 24 volt system. Batteries rated at 104 amps/24 hour rate.
I have an 1800 watt inverter that I don't expect to test the limits of...
Outback FM80 input with 2000 watts of panels.

My guess: somewhere between 50 amps and 70 amps for each pair of batteries.

Thanks for your help.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Help me guess battery breaker amps

    Don't guess: measure. :D Or at least calculate.

    The only thing that matters for wiring between batteries and inverter is how much the inverter is going to draw. 1800 Watts @ 24 Volts is roughly 75 Amps. If I were you I'd wire it up for full power capacity just in case, as that is not far off from what you were expecting. Better safe than sitting in the dark feeling sorry. The manual for the inverter should have manufacturer's recommendations for both wiring and circuit protection sizing.
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  • sunbunny
    sunbunny Solar Expert Posts: 59 ✭✭✭✭
    #3
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    Re: Help me guess battery breaker amps

    Thanks !

    I kept hearing how hard it was to decide on the battery breaker size.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Help me guess battery breaker amps

    Probably due to the NEC multiplier. If you don't have to meet inspection common sense will do just fine.

    In my opinion it is worse to have too large a breaker than too small. Too small will just "nuisance trip". Too large can let wire get too hot.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
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    Re: Help me guess battery breaker amps

    one can look upon this in another way here. you won't be fully drawing on your inverter and you don't want to tax those batteries so one can use a smaller breaker providing there are no surges that would trip a smaller breaker. this assumes your wiring will be sufficient too as one would not want to pass 30a on say #12 wire. all somewhat common sense once one has a clear vision of what they are dealing with overall.
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  • sunbunny
    sunbunny Solar Expert Posts: 59 ✭✭✭✭
    #6
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    Re: Help me guess battery breaker amps

    Thanks, Niel. I bought 70 amp breakers for 2 reasons. They only come in round numbers and I found 5 used breakers for half price of new. They are 80 volt DC ones but that ought to cover it ;)
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
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    Re: Help me guess battery breaker amps

    very good if you won't need more than that 70a for normal operation. 70a is roughly a 1600w draw at 24v. giving yourself a bit of headroom may mean keeping it about 1400w or so max so the breaker doesn't accidentally trip. of course you are only using 2 sunxtenders in a 24v configuration and 70a represents about 70% of the total capacity. your batteries will have battery voltage sag and the inverter itself may drop out before the breaker could trip.
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  • sunbunny
    sunbunny Solar Expert Posts: 59 ✭✭✭✭
    #8
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    Re: Help me guess battery breaker amps

    Niel, I have 6 pairs of the batteries and expect to use 2 pair at a time.
    Also have a 1000 watt generator and a 5000 watt one that I already built one house with....
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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,144 ✭✭✭✭
    #9
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    Re: Help me guess battery breaker amps
    Cariboocoot wrote: »
    Probably due to the NEC multiplier. If you don't have to meet inspection common sense will do just fine.

    In my opinion it is worse to have too large a breaker than too small. Too small will just "nuisance trip". Too large can let wire get too hot.

    Wisdom is easy to recognize! :D

    FWIW, a healthy PVX-1040T will sustain about 3200 amps on a dead short for a full 3 seconds.

    Marc
    I always have more questions than answers. That's the nature of life.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #10
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    Re: Help me guess battery breaker amps
    sunbunny wrote: »
    Niel, I have 6 pairs of the batteries and expect to use 2 pair at a time.
    Also have a 1000 watt generator and a 5000 watt one that I already built one house with....

    ok then, if you want to have switched battery banks then try 3 separate groups of 2 in series and paralleled to another 2 in series. even though the battery can dead short quite a few amps it would be at a very low voltage so useful amps should be with a bit more capacity for each bank. 10% on 104ah is about 10.4a, but a bit lower due to the peukert effect, but parallel another approximate 10.4a and it will last longer with not as much voltage sag. i had a 1040t for about 8 and 1/2 years before i retired it and i got some more to replace it so that i can take my own advice.;)
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