Electrical 3g Alt Voltage

You have to fuse the system based on the lowest rated component. In this situation, it's the wire. Depending on the wire quality, and the lenght of the run and heat the system sees, a 4G wire is really only rated to around 100A. Shorter runs in cooler locations can take more amps than long runs that pass by large sources of heat.

So your fuse should really be based on what the wire can handle. You rarely will see over 100A of current draw on your alternator unless you are powering some serious electrical components. But for your average Mustang even with an E-fan, you will probably be in the range of 75-85A, for which the OEM alt was slightly undersized for. So even fuse protection for 100A should be enough.

Your actual voltage output should be around 12.5V at idle, and 14.4V above idle (while driving) a voltage regulator will maintain this voltage, so no need to worry about overcharging. You just need to be sure the alternator pulley is such that you maintain at least 12V at idle, and a small blip of the throttle over idle creates 14.4V.