Fuse size

GrandpaGrandpa Registered Users Posts: 7
I have 1 kw of panels....2 strings of 5 100 watt panels, each vmp 18 times 5...oh and each positive lead protected by a 10 amp fuse. After that a combiner of 2 15 amp breakers. Then into the Outback FM80 cc. Next a 100 amp fuse. Onto a bank of 4 GC2 bats in series....total 24 volts. From that bank to a 400 amp fuse then onto a 2500 to 5000 amp pure sine wave inverter.

My question....

Is the 400 amp fuse between the bank and the inverter too high?

Thank you, Grandpa.

Comments

  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    What size fuse does the manual for that inverter call for? My 1500 Watt inverter calls for a 100A fuse and I have a 125A in it

    ps second post as first doesn't show???
     
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  • Ethan BrushEthan Brush Solar Expert Posts: 235 ✭✭
    Grandpa wrote: »
    I have 1 kw of panels....2 strings of 5 100 watt panels, each vmp 18 times 5...oh and each positive lead protected by a 10 amp fuse. After that a combiner of 2 15 amp breakers. Then into the Outback FM80 cc. Next a 100 amp fuse. Onto a bank of 4 GC2 bats in series....total 24 volts. From that bank to a 400 amp fuse then onto a 2500 to 5000 amp pure sine wave inverter.

    My question....

    Is the 400 amp fuse between the bank and the inverter too high?

    Thank you, Grandpa.

    It does seem a little high......If you want to figure out what your absolute worst case current draw would be you would use the highest surge watts of your inverter and calculate the current at the low battery shut off voltage of the inverter. Then you would multiply that current times 125% if it was continuous for more than 3 hours. being that conservative is probably un-necessary but up to you. The 125% is probably not necessary as I cant imagine most off griders running their inverters at max output for more than 3 hours unless you were using it to run an opportunity load or something. Keeping it sized more realistically (smaller) gives you a little more safety in case something really strange happens.....
  • petertearaipetertearai Solar Expert Posts: 463 ✭✭✭✭
    Hi . this fuse is also protecting the cable . Make sure the cable is rated for 400 amps or more .
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • PhotowhitPhotowhit Solar Expert Posts: 5,650 ✭✭✭✭✭
    I think what petertearai is saying needs to be respected, I suspect you need a much smaller fuse/breaker likely in the 250amp range, a 175 amp was recommended for my 1800 watt inverter. As someone else said check with the manufactures manual for the inverter.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • GrandpaGrandpa Registered Users Posts: 7
    Thank you all.

    The inverter didn't come with a manual. My thoughts were that the inverter will surge to 5000 divided by 24 volts equals 208 amps. I did find an old post by you Photowhit in which you say.....quote "A couple things, First the fusing is a straight calculation;

    Peak wattage of the inverter / minimum voltage to run = Amps to protect

    So for you 3000 watts / 11.5 Min voltage = @260 Amps"

    I also think my fuse is too large. I wanted to check with people here to see if I was missing something before I contact the vendor who sold it to me.

    Grandpa.




  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    To run a 2,500 Watt AC inverter to its rated load over all conditions and for extended periods of time:

    2,500 Watts * 1/0.85 inverter * 1/21.0 volt battery cutoff * 1.25 NEC cable/breaker/fuse deratings = 175 Amp branch circuit rating

    If the inverter is rated for 2,500 Watts, usually its surge load rating is ~2x that amount, you usually do not size the fuse/breaker for that 2x DC load current (fuses and breakers should not open for a 1-2 second surge load).

    However, make sure your voltage drop calculations are correct. That 2x DC current can drop the voltage to the inverter to below the battery cutoff voltage of (typically) 21.0 volts.

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