Battery to Inverter fuse & Charge Controller to battery fuse size

klane

I have a small 12v 300w power inverter and 3 35ah 12v battery bank. I want to get a larger inverter and add more batteries as well eventually.

What is the calculation for my fuse size for battery to Inverter connection and charge controller to Battery connection?

Thanks in advance!

Estragon

Fuse/breaker and wire sizes should be in manuals for inverter and controller. I would follow their recommendations.

mike95490

your small batteries cannot support a much larger inverter, the more load you add, the deeper the voltage sag of the batteries,

BB.

For a 12 volt system, you need a minimum of ~400 AH (at 12 volts) per 1,000 Watt of AC inverter for reliable and long term operations.

In general, getting a bunch of smaller batteries and placing them in parallel is not a good thing. The small batteries will not last very long and the wiring/proper fusing will be a mess.

A pretty good way to start is using 6 volt @ ~200 AH "golf cart" batteries. They are pretty cheap (~$80-$100 or so) each, and are pretty rugged/forgiving/reliable. And with good usage/maintenance, they should last ~3-5 years pretty easily.

A generic answer to your fusing/breaker question:

• 300 Watt AC inverter * 1/0.85 inverter eff * 1/10.5  Volts low battery cutoff voltage = 33.6 Amps max continuous operation
• 33.6 amps * 1.25 NEC derating for fuses and wiring = 42 Amps minimum rated wiring+fuses

Round up wiring+circuit protection to next standard size.

Note--Larger fuses and fuse holders can be a bit on the expensive side and you may still want to add an on/off switch for the DC power to your inverter--Think about using Circuit Breakers instead. Besides the fact you don't need spare fuses anymore, the breakers also offer a convenient on/off switch function too.

Also--You need to look at the ratings of Fuses/Breakers/Switches--When used on DC voltages, they need to be "heavier" than their AC counterparts. DC current is much harder to stop than AC current (DC current sustains arcs across switch contacts very nicely vs AC current).
https://www.youtube.com/watch?v=Zez2r1RPpWY
12 VDC is at the very minimum that electricity will sustain arcs... Higher voltages than 12 VDC will sustain arcs very nicely (i.e., DC arc welders).

-Bill

Estragon

@klane

With all due respect to the perfectly good comments above, we know nothing about your loads or objectives for bigger inverter or bank.

The question was about sizing fuses/breakers from CC and inverter to batteries. They should be sized to protect the wire. The wire should be sized (or IMHO oversized) to minimize voltage drop from device to battery.

Manuals for the devices should recommend appropriate wire sizes and overcurrent protection. Failing that, decide what size wire you want to protect, then determine overcurrent device size to protect it.