I am about to do the Big 3, and needed to ask one quick question first.
What size fuse should I use from the +battery to the +alternator? I have heard to use the maximum your alt. puts out, and the maximum your wire can handle. My alt. is rated at 120amps.
If you have no plans to upgrade your alternator, you could get away with 4AWG, as it's rated for 125A max, and it's a short run of under 5 feet. Use OFC if you go 4AWG (oxygen free copper) not the cheap crap. OFC has a higher peak current rating. If you want to play it safe, just run 1/0 for everything under the hood.
You can technically draw more than the output of the alternator, as the bettery will supply additional current before it discharges, so you really should go larger with the 1/0 since your car will likely use about 50+A itself, and the audio system will depend on the amplifiers additionally.
also fuse the line between alternator and battery, and fuce between battery and amplifiers, with fuses rated for the peak current draw of the wire. This will prevent engine fires if the wire casing melts from touching something hot and metal. Also zip tie the wiring in place so as teh compartment heats upp, the wiring doesn't sag and again, touch something hot.
Thanks for the info. I'll just use 1/0 for everything, because its not much more in cost and I don't want to have to upgrade again in the future. Thanks again for the help.
300 amp is the fuse you'd need for 0 gauge wire. But just a tip, it's pretty useless to fuse the alt to batt wire... that wire has power sources at both ends, think about it, it's gonna burn no matter what if it shorts.
The wires from battery to amplifier or other devices DO need to be fused.