Fused in Battery Box

WilisWilis Solar Expert Posts: 84 ✭✭✭✭

Still a little confusion about the fuses I am using in my battery box. I have searched the forum but didn’t seem to answer this question.
I have a 200 amp fuse in each string. I have 24v/900 AH of batteries, 4 stings of 4 T105’s. I am using 2/0 12 inch connecting wire between the batteries and 2/0 6 foot connecting wire to the E-panel and my larger inverters is a MS-AE. I have an AC well pump that draws 9 amps running and about 21 amps start. I never have any other load when I run the well pump. This load is not my main load and I will only pump if the grid is down. At 240v/80amps the inverter is capable of drawing larger loads. In the user manual it shows I could draw 5800 watts-5 sec. surge power (real watts).
When placing fuses in your strings of batteries do you fuse for the max amperage your system will draw or the max amperage you could draw?
How do you figure the max size fuse for the system looking at the wire used and the amount of amps the inverter could draw?
In the last 7 months everything has worked fine, but I am interested in the formula I might use, if I ever have a larger load and want to set my system up for that?

I am using the JJN-200 200 Amp Fuses I purchased from NAWS.


  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Fused in Battery Box

    Your 200 amp fuse should work. The main issue is balance on the four series battery banks as it relates to how the inverter load will be distributed.

    If you can get a 5800 watt surge load, with efficiency loss through inverter, batteries at mid to low SOC, you could get over 350 amps inverter surge current.

    Ideally that would distribute to less then 100 amps per battery string. It is not uncommon to get 50% variation in balancing some point thoughout a discharge cycle. Then there is the case when one battery starts to go bad and causes the string not to contribute much to load. You don't want to start blowing fuses in good legs when a string starts going to high internal resistance because of one battery.

    If you had it to do over again, I would bump the fuses up to a bit higher amperage.

    Other factors is balance in wire length matching and terminal lugs. Crimp on lugs have more IR drop then a 6" 2/0 jumper wire. It is the neck down barrel area between the crimp and the flattened terminal. It is approximately equivalent to a half inch length of half inch diameter 'M' class copper water pipe in resistance.

    It is a good idea to invest in a clip on AC/DC amp meter that you can quickly check the current in each of the four strings of batteries. It can give you early warning on a battery going bad before it damages other batteries in the string.
  • mike95490mike95490 Solar Expert Posts: 8,766 ✭✭✭✭✭
    Re: Fused in Battery Box
    Wilis wrote: »

    How do you figure the max size fuse for the system looking at the wire used and the amount of amps the inverter could draw? .

    You first figure the inverter draw.
    Chose wire gauge
    size fuse to protect wire if it shorts.
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  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: Fused in Battery Box

    The battery string fuses should be sized to protect the wiring (and batteries) for maximum supported current by the wiring...

    The wiring, at a minimum, should support the maximum current o the loads/charging (plus 25% or x1.25 using NEC safety formula... Fuses/breakers can blow at 100% rated load--So you take the load current you need and multiply by 1.25 and round up to the next standard fuse/wire ampacity).

    If you have 4 strings of batteries--I don't think they share 1/4 of the load predictably...

    My own recommendation--assume a 4 parallel string battery bank--each string may be called on to supply upwards of 50% of the total load.

    If you have a 400 amp load--I would size the string fuse/wiring for 200 amps * 1.25 = ~250 amps minimum.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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