Another fuse size question

[blammo585]

I upgraded my charge controller today to an Epever 40 amp controller, and I thought I'd actually ask about proper fuse size because the more I read, the more confused I get. At first I was using a 10 amp PWM controller, 100 watt panel, and a 12 ah SLA battery. I am using 10 gauge wire. I used a 10A fuse on the PV cable and a 30A fuse at the battery. But after doing some searching tonight, I came across something from Renogy that said to use the same size fuse at the battery that the charge controller is rated at. So now that I have a 40A controller I should use a 40A fuse at the battery? Also, the article I read said when fusing from the solar panel to take the amps that the panel puts out, multiply by 1.25, and then round up. A 100 watt panel will output about 5 amps? So 5 x 1.25 = 6.25. Use a 10A fuse at the SCC?

To further complicate things, I came across a thread here where people suggested using the same size fuse from the panel as the SCC is rated for. And the fuse at the battery should be fused according to the wire size. Which I assume a 30A fuse is fine for the 10 gauge I have. But when I get my new 100ah LifePo battery, I plan on going to the max wire size the Epever takes, which is 6 gauge. Then I should step the fuse up?

 

[Bobert]

I’m gonna start with a short answer and then a long drawn out explanation. You need to put a 30 amp fuse at your battery for the charge controller. Once you upgrade the six gauge wire you can simply follow the instructions of your charge controller. you don’t actually need a fuse on your solar panel at all so just leave that alone for now.

The way I keep fusing straight in my head is to remember that as a general rule fuses protect wire not devices.

2 note worthy exceptions are batteries and solar panels they supply energy to a circuit.

Solar panels are self limiting providers of energy and usually don’t need to be fused unless more than 2 of them of them or more than 2 series strings are in parallel. Then they need to be fused between the panels because a shorted panel could potentially have more amperage sent to it than it could handle.

Another exception is batteries ideally they should be fused less than the continuous output rate of the battery or battery bank so that you are not exclusively relying on the bms to prevent it from overload. In this case it is not unusual for the size of the wire going from the batteries to be smaller amp rating than the bms of the batteries in which case you size the fuse at the recommended rating for your wire.

The scc should be fused as close to the battery as possible to protect the wire and the wire size needs to have an amp rating not less than the output of the scc. In this case the fuse shouldn’t be to much larger that the scc rated output in case of an internal short.