# fuses what amp

Registered Users Posts: 3
fuses , have 2 separete battery banks 2-6 v -220 amp wired 12 volt , size fuse , from batt bank to controllers , from controllers to panels , controller to output ie fans etc , 1 controller running 2 100 watt panels , other controller running 2 130 watt panels . 10 amps from each controller . ps i have 1 more panel 125 watt ,may be added

• Registered Users Posts: 3
Re: fuses what amp

ps prob pulling [ up 2 ] 10 amps from each controller
Re: fuses what amp

Welcome to the forum "lcalarea".

First, understand that fuses are used to protect the wiring/cabling from excessive current (typically from the battery bank or larger solar arrays).

So, some ways of figuring out fuse sizes. You need to size the wire to the loads. For example, say you have a 300 watt AC inverter that runs form your 12 volt battery bank. The minimum wire size would be calculated as (assuming you are in the US):
• 300 Watts max average power * 1/0.85 typical inverter eff * 1/10.5 inverter cutoff * 1.25 NEC fuse/wiring derating = 42 amps minimum branch circuit wiring fusing

Next you go to the National Electric Code and look up the size of wire (and insulation type, and even conduit fill factor) to figure out the minimum size of wire you need. And you see somewhere between 8 awg and 6 awg minimum.

Or, if you want to be a little less conservative (wire is not in conduit, your installation does not need to comply with NEC), you can use other tables such as the one from the ABYC... Which would suggest somewhere around 10 SAE gauge (note that SAE wire is slightly smaller than AWG wire--So it is OK to "error" in using AWG wire sizes here).

Next, you want to look at voltage drop... For an AC inverter on a 12 volt circuit, you would want (probably) no more than a 0.5 volt drop from the battery bank to the Inverter. Using a generic voltage drop calculator, say you have a 5 foot (one way) wire run, 42 amps (worst case continuous) current, the calculator would say:
• 10 AWG @ 5 feet one way run @ 42 amps gives 0.5 volt drop

And then, need to pick a fuse or breaker for the level of protection. These breakers are available in:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 20, 30, 40, 50, 60 and 63 amps.

Fuses are available in similar (and larger breakers/fuses) sizes too.

So, from looking at all of the above, you would need a 50 Amp Breaker and a minimum of 10 SWG to 6 AWG (depending on your choice).

For a solar charge controller, they will generally list the maximum size fuse they will support (and minimum wire gauge). For a 12 volt solar charge controller, I would suggest your wire size give you a maximum voltage drop at rated current of 0.05 to 0.10 volts maximum for best battery charging performance.

To calculate the minimum fuse/breaker from a solar charge controller is a bit more work. The output current depends on the size of the array and the type of charge controller (PWM vs MPPT) and such... The two controller types really need two different methods to calculate the maximum continuous current. And if you want to size the wiring for future upgrades or not.

If you understand your maximum current is 10 amps and you have a 4 foot run from controller to battery bank. The minimum wire size would be around 8 AWG for a 0.6 volt drop at 10 amps and 4 foot run.

The fuse should be, roughly, 10 amps * 1.25 NEC wiring derating * 1.25 NEC solar charge controller derating = 15.6 amp minimum fuse/breaker (round up to 20 amps).

That would also work well with the 8 AWG cable.

If you have three or more solar panels/strings in parallel, there should be a fuse/breaker per panel string too (so a shorted panel does not get over current from the other 2 or more parallel arrays).

Sorry for the confusion--But trying to make sure your system is safe. And this can be a complex subject to just jump into with an "answer".

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 3
Re: fuses what amp

thx reply . i have 10 gauge from controller to bank [rv ] yes about 4 ft . think will change to 6 gauge . the 2 130 amp panels think get about 18 amps max . so schould go with a 20 amp or 25 amp ,,,,,two 100 watt panels think about 12 amps max sun . so 1 or 20 amp . my 2 volt banks i got a 100 amp breaker , is that to high ,,,,,lonnie .