Inverter cable and breaker sizing

Re: Inverter cable and breaker sizing

ABYC (American boating and yachting council) only allows around 130-160 Amps maximum for a 4 AWG cable (they allow 30-35 amps for a 14 AWG cable where NEC allows 15 amps--really 20 amps, another issue--maximum).

And 2/0 for 300 amp fuse.

If you want to run the inverter at 2,100 watts:

• 2,100 watts * 1/0.85 typical inverter eff * 1/10.5 volt cutoff * 1.25 NEC derating = 294 amp minimum branch circuit rating
• 2,100 watts * 1/0.85 typical inverter eff * 1/10.5 volt cutoff = 235 amp minimum branch circuit rating

And then there is voltage drop (simple calculator). For 235 amps and 0.5 volt maximum drop at 6 feet round trip for cable: 2 AWG for 0.5 volt drop.

Using the ABYC table, a minimum of 1 AWG cable.

Using NEC table 310-16 would be 300-350 circular mill cable minimum (plus even more deratings for temperature, conduit fill, etc.).

The reality is, most people will not run their inverter more than ~50% of capacity and only near rated capacity for a few seconds when starting motors (saws, well pump, etc.).

For a 600 AH flooded cell battery bank--The typical rule of thumb power support would be:

• 600 AH * 12 volts * 0.85 inverter eff * 1/20 Hour discharge rate= 306 Watts long term average power for cabin (2 days storage, 50% max discharge)
• 600 AH * 12 volts * 0.85 * 1/8 Hour = 765 Watts maximum continuous discharge
• 600 AH * 12 volts * 0.85 * 1/5 Hour = 1,224 Watts short term maximum discharge
• 600 AH * 12 volts * 0.85 * 1/2.5 Hour = 2,448 Watts maximum starting power

You could easily "dial back" the current rating to ~1,224 Watts as the maximum continuous power you would pull from that size battery bank--Save yourself lots of copper and expensive fuses/switches.

-Bill