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Which fuse for an inverter?

Dorothea

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Jan 24, 2020
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I try to figure out which fuse I need between battery and inverter. What is the best type? A MEGA fuse, or a terminal fuse like https://www.amazon.com/Single-Terminal-Fuse-Block-Battery/dp/B081ZJFQX2?th=1 ? I read somewhere that inverters need slow fuses. Are terminal fuses slow fuses? And which size fuse? Is 200A enough? I have a 1500W (3000W surge) inverter, and the inverter manual does not tell me what size fuse is recommended. Once I know the fuse size, I will size the cable from the battery to the inverter accordingly.
Thanks!
 
What voltage are you running?

If 12v...
3000/12 = 250amps
The fuse you list looks good.
Or get quality Bussman breaker or anl fuse in the 250 amp size.

If 24v, 125-150amp size.
 
Hi @Dorothea, I thought I'd chime in here since that's our MRBF listing that you linked to; @MisterSandals has it correct, though - for your application, you should have a 250A fuse if you're running a 12v nominal system. These MRBFs are perfectly appropriate for that. Please bear in mind, though, that the main purpose of the fuse is to protect the cable connecting the inverter to the batteries, not the inverter itself, so as long as you're using appropriate cable gauge for the length of your run you're good with the 250A MRBF. If, for one reason or another, you need to under-size your cabling then you should also under-size your fuse so that the cable stays protected at all times.

For instance (still assuming a 12v nominal system voltage): if your inverter is 4' or less away from your battery bank (so 8' round-trip), you should be using 1AWG or heavier to safely carry the maximum current rating of the inverter that distance while maintaining less than 3% voltage drop. In that case, a 250A MRBF (or Bussman ANL or breaker) is perfectly appropriate. But now let's say that for some reason you can only possibly use 4AWG cable... you shouldn't, and I hope you won't, but for the sake of argument let's say that you absolutely need to use 4AWG, you positively cannot go to a heavier gauge than that. Well, in that case, you would want to put a 150A fuse on the circuit, because the maximum continuous current rating of an 8' round-trip length of 4AWG wire is 150A... so you want the fuse to blow before your cable insulation starts to melt, if that makes sense.
 
Hi @Dorothea, I thought I'd chime in here since that's our MRBF listing that you linked to; @MisterSandals has it correct, though - for your application, you should have a 250A fuse if you're running a 12v nominal system. These MRBFs are perfectly appropriate for that. Please bear in mind, though, that the main purpose of the fuse is to protect the cable connecting the inverter to the batteries, not the inverter itself, so as long as you're using appropriate cable gauge for the length of your run you're good with the 250A MRBF. If, for one reason or another, you need to under-size your cabling then you should also under-size your fuse so that the cable stays protected at all times.

For instance (still assuming a 12v nominal system voltage): if your inverter is 4' or less away from your battery bank (so 8' round-trip), you should be using 1AWG or heavier to safely carry the maximum current rating of the inverter that distance while maintaining less than 3% voltage drop. In that case, a 250A MRBF (or Bussman ANL or breaker) is perfectly appropriate. But now let's say that for some reason you can only possibly use 4AWG cable... you shouldn't, and I hope you won't, but for the sake of argument let's say that you absolutely need to use 4AWG, you positively cannot go to a heavier gauge than that. Well, in that case, you would want to put a 150A fuse on the circuit, because the maximum continuous current rating of an 8' round-trip length of 4AWG wire is 150A... so you want the fuse to blow before your cable insulation starts to melt, if that makes sense.
Thanks! Yes, I will use a far heavier cable than comes with the inverter (the ones I looked at have 4AWG or thinner cables). I'm very puzzled why many inverters come with such thin cables, it is a total mismatch.
 
Please bear in mind, though, that the main purpose of the fuse is to protect the cable connecting the inverter to the batteries, not the inverter itself, so as long as you're using appropriate cable gauge for the length of your run you're good with the 250A MRBF. If, for one reason or another, you need to under-size your cabling then you should also under-size your fuse so that the cable stays protected at all times.

Is the converse true, if you over-size your cabling, then over-size your fuse? In other words, match the fuse to the cable, either way?
 
Is the converse true, if you over-size your cabling, then over-size your fuse? In other words, match the fuse to the cable, either way?
No. Just as you can use a heavy duty extra thick extension cord for your desk lamp, you can always use thicker wiring, and there is no point in putting in a stronger fuse than needed or required by the load. For example, my fuse box for the small stuff needs to be protected by a 125A per fuse box specification (nice the specification tells me, so I do not have to figure that one out). So I need a wire to the fuse box that can take at least 125A without too much voltage drop and of course without getting damaged. I'll use a 6" 4AWG. I do not go thinner because I have 4AWG cable anyway. I could go thicker, and I might if I have spare 2AWG and not enough 4AWG cable.
 
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