How many amps to relay? Anyone Know?

It is not as easy as matching amp output of alternator. The current interrupter has to be able handle a short duration over load without blowing. If you put install a 60 amp fuse it will interrupt the current flow when it reaches 60 amps, bang is opens.

But what if a series of loads total 70 amps for whatever reason for a second or so you ask. Don’t forget the battery is capable of supplying 600 to 900 DC amps or so to a dead short, so alternator output is not a reliable target for fusing DC.

This is why Chrysler, and other manufactures back in the day designed circuit protection with fusible links, a short length of smaller gage conductor covered in special nonflammable insulation placed somewhere between battery, and bulkhead connector to protect under wire harness on dash side of fire wall. A fusible link has the ability to handle a short duration current overload by dissipating excess heat before self-destructs.

One should use a rather large slow blow fuse of an undetermined rating probably in the 100 to 150 Dc amp range. This will protect harness from a dead short, but carry enough juice to sub circuits, allowing the fused sub circuits to do their job. The problem with our cars is there are several devices and circuits that are not over current protected, and are directly hot-wired to the battery with only the main fusible link for protection. Sometimes, one of these circuits will short to ground, and because they are of smaller gage wire than the fusible link, those circuits burn up before the fusible link can melt.