Breaker ? Amp

System
System Posts: 2,511 admin
in Solar Beginners Corner #1
Hello. This is my first post and i found this forum very helpful for those that are not expert on electrical things. I understand that i need a breaker(Vdc) between the battery and the inverter. I have 4 batterys 6v 185 amphrs eachone, all the battery are configurated as serial. Now what is the correct amp breaker that i need on the positive cable? The inverter is a 4500 watts 24v inverter/charger. Thanks.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Breaker ? Amp

    Breaker size is dependent on current draw of the inverter at minimum voltage and maximum wattage; that is, the most current you'd expect to see through the wires under 'normal' operating conditions. So:

    4500 Watts / 21 V (standard low voltage limit for a 24 V system) = 214 Amps * 1.25 ('safety margin') = 267 Amps.

    Obviously there's no such thing as a 267 Amp fuse or breaker, so go with the next size up: 300. You can go with the next size down (250) if you know your loads won't approach maximum. Or if you don't mind having it trip every time they do. :p

    If you look in the owner's manual that came with the inverter I bet someplace in there is information giving wire sizes and breaker/fuse ratings. Unless you didn't get a manual: that was the thing missing out of the box when I got my Outback! Fortunately you can usually find it on-line for most manufacturers these days. Can't beat the company's own recommendations for getting it right!

    In any case, be sure the wire is able to handle the load: the fuse/breaker must be the 'weakest point' in the circuit.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #3
    Re: Breaker ? Amp

    Fuses and breakers are not cheap... From our host's website, a 300 amp fuse and holder will cost you around $65. A 350 amp switch will cost another $30.

    A 300 amp DC breaker and box will cost many times that amount (and be hard to find too).

    Also, a 4,500 watt inverter on 185 amp*hours of batteries is pretty big... If you expect to draw anywhere near 4,500 watts for any length of time (more than just starting a motor)--that is probably not enough batteries to really do the job.

    You probably would need a minimum of ~600 amp*hours (at 24 VDC) worth of batteries to drive the inverter for any length of time.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
    Re: Breaker ? Amp

    Remember, that fuses/breakers are installed to protect the SMALLEST wire in the circuit, no the appliances or the loads. So for a 300 amp continuous load you would need a wire size about 3-4 sizes bigger than 4/0!

    Tony
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #5
    Re: Breaker ? Amp

    4/0 might be a little excessive--1/0 can be OK in open air for short runs (certainly not NEC table approved for conduit)...

    But even a 5' distance from battery to inverter of 1/0 cable at 285 amps will give almost a 0.3 volt drop.

    The big issue with large inverters and lower voltage battery banks. Anything much over 100-175 amps is just a pain to deal with (and expen$ive).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #6
    Re: Breaker ? Amp

    Assuming you want to be able to add battery capacity at a later date, it would be best to buy a DC breaker to suit the capability of the inverter. Get a 300 or 350 amp breaker. It won't be cheap. You can go with a fuse to save cost but I prefer the comfort of having a 'panic' cutoff switch capability if loud humming and smoke start coming out of inverter.

    DC breakers have a more powerful opening (and closing) mechanisim and better contacts then AC breakers because of the greater contact welding possibility of high current DC. You definately don't want to go light on the breaker and have it popping during surge loads. They are too expensive to have them destroy themselves.
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