Fuse Size Question

I have a Ford 302 in my MX. I put in a one wire alternator (long story) I put an inline fuse from the alternator to the battery. So I had a slight grounding accident.  Bottom line it started to melt the wire to the alternator but didn’t blow the fuse. I don’t think that fuse is protecting the alternator.

It’s a 100A fuse.

My question is, if I go with a smaller fuse. What Amperage would be right?

Charging circuits are generally protected by fuseable links.  They are a short section of small diameter wire with special insulation that will not burn.  The link is sacrificial and designed to burn in two if you get a short.  They are sized 4 numbers smaller than the wire they protect.  Example:  A 10 gauge wire gets a 14 gauge fuse link.  You position the fuse link where if it melts it will not burn anything else since they do get hot. Think of them as a very slow blowing fuse.

Common practice is to put them next to the alternator but being a belt and suspenders kind of guy when it comes to electrical protection I usually put one at both ends of the main charging cable.  This is because that is generally the only circuit that could be fed power from both ends.

They can be a pain in the butt to change but they are the absolute last line of defense against a torched car. And fiberglass cars don't end up well in a fire.

You need to run a fuse that's sized the same or slightly more amps than what your alternator is rated to put out. You also need to use the appropriate size wires for the system. It sounds like your wiring to the alternator was too small if it was melting before the fuse blew.

AHSebring64 wrote:You need to run a fuse that's sized the same or slightly more amps than what your alternator is rated to put out. You also need to use the appropriate size wires for the system. It sounds like your wiring to the alternator was too small if it was melting before the fuse blew.

I’m not sure of the amp output. But the wire is 8ga. Fuse is 100A

If it is a one-wire alternator then it is most likely an aftermarket alternator and I'd be surprised if it puts out less than 100 amps. You should be able to see a Mfg name or model number that you can Google to see what size alternator it is. An 8 ga wire is probably too small; see chart:
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AHSebring64 wrote:If it is a one-wire alternator then it is most likely an aftermarket alternator and I'd be surprised if it puts out less than 100 amps. You should be able to see a Mfg name or model number that you can Google to see what size alternator it is. An 8 ga wire is probably too small; see chart:
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I looked up the number on the alternator. It’s 63Amps

I'd put in a 75amp fuse (and replace the melted wire) and that's about as far as my electrical knowledge goes.

AHSebring64 wrote:I'd put in a 75amp fuse (and replace the melted wire) and that's about as far as my electrical knowledge goes.

I’ve replaced the wire with slightly melted insulation and the 75A is what I was thinking as well. Thanks