# Fuse sizing on solar system

Registered Users Posts: 12
HI folks,

Can anyone help me I am a bit confused about the size of fuse I should be using between my batteries and inverter?

I have 4 x 535 amp 6 volt batteries ( 24 V) and I am about to hook them up to a Studer C 2600 inverter can anyone explain how I can calulate the fuse size I need?

Also can I use a normal VAC fuse for this?

David

Re: Fuse sizing on solar system

If you don't have the Inverter's manual, you should be OK with:
• Vmin=minimum voltage inverter will run out=21 volts assumed
• Eff=Inverter's efficiency at maximum load=85% estimated
• Pinv=Maximum Inverter Power Output=2,600 watts? (can use max needed load instead)
• Derating=Wiring derating (bundles, conduit, hot temperatures)=assume 80% max current of branch rating or 1.25x max load current from NEC
Estimated maximum current = 2,600 watts * 1/21 volts * 1/0.85 = 146 amps max inverter input current

Branch Circuit/Min fuse rating = 146 amps * 1.25 = 183 amps min wire/breaker/fuse

Round up to next standard wire rating/fuse rating... (again, if you only plan on a 1,000 watt load maximum, you can back down on the fuse/wire ratings to the lower load). You don't want to run the fuses/breakers at 100% load--will will get a lot of nuisance tripping (Outback sells breakers that are supposed to be rated at 100% load--so they may be an exception).

Note, AC current is easier to interrupt vs DC. Fuses, Switches, etc. are actually larger to handle DC interrupt current vs the same rating (and higher voltages) of AC circuits.

Lastly, with fuses and breakers, there is the Maximum Interrupt current... In theory, the maximum interrupt current rating should be larger than the current you will get with a dead short on your battery bank... However, large battery banks can supply tremendous amount of energy. Normally, shorts are not "dead shorts" but failures in wiring or equipment and the fuses/breakers (proper working voltage and DC rated) will work fine.

You can look through here for a selection of fuses/breakers that may meet your needs.

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