Here is a basic diagram of what a Big 3 upgrade entails.
The connections are basically:
Alternator >>> PDC
PDC >>> Battery +
Battery + >>> Starter
Battery - >>> Engine block
Battery - >>> Firewall 1
Valve cover >>> Firewall 2
There is some debate about whether to use a fuse or a fusible link. Basically, a fuse, in this case, a high-capacity ANL fuse, works just like the one in your glove box. If the current gets too high it blows, thus protecting your jeep from catching on fire.
A fusible link is simply another piece of wire that is 4 AWG sizes smaller than the main conductor. (i.e., if you have 2AWG wire, your fusible link would be 6AWG) that is in-line with another conductor. The idea is that, if the current gets too high, the smaller conductor in the link will heat up, destroy itself, and break the circuit before the rest of the wiring is damaged.
Some people argue that having a fuse alone for this purpose is unwise because there will occasionally be spikes in amperage, known as in-rush current, that can cause the fuse to blow, but would not damage your wiring otherwise. The advantage of a fusible link is that it can absorb those brief spikes without melting. The OEM system uses a 10AWG fusible link. If you look closely at your PDC, you will see a green wire close to the termination. That's it. It connects to the wire coming off the alternator with a butt splice.
If you choose a fusible link, you MUST use flame-retardant connections and cable, the idea being that the link will melt, but won't catch fire, preventing a fire in your engine bay. I've also wondered if it would be feasible to "make" a fusible link with cable and ring connectors, and then just install it in an ANL fuse holder. Sounds good on paper.
I chose to go with a fuse simply for replaceability's sake. If something happens in BFE, I'd rather be able to replace a fuse than have to replace a fusible link which, while not difficult, would probably be herder than the fuse. Time will tell if that was a smart decision.
Now, about fuse size. I had a hard time finding information that was specific to automotive uses, but I found a lot from boating web sites. I figure that boaters might have a higher margin of safety since, you know, you're gonna have a bad time if your boat catches on fire in the middle of the ocean. Here is some stuff from
one of the sources I used:
According to ABYC tables, 4 gauge AWG wire will safely carry 160A. Furthermore, this wire will safely tolerate 185A, approximately 115% of its rated value. Since most fuses and circuit breakers have a blow point or trip value that is 130% of their nominal rating, choose a fuse that is rated at 80% of 160A. A 125A nominal rating SEA fuse is a good choice-it has a blow point of 130% of 125A=162A.
BUT
Because ANL fuses behave differently than all other circuit protection devices, that is, their blow point value ranges from 140 to 266% of nominal value, the 80% rule doesn't work. It is necessary to use a different procedure to select a suitable ANL fuse. To choose a suitable ANL fuse, refer to the ANL Fuse Blow Point table above. According to the table, a 100A ANL fuse has a blow point of 175A. Therefore, a 100A ANL provides suitable protection for the 4 gauge AWG circuit in this example.
I chose to use 2AWG wire for it's higher load capacity.
According to this table, 2AWG can handle 210A @105C. Looking at the ANL fuse table, a 130A fuse will typically
actually blow at 220A, so that's what I chose. Stock alternators are listed at 117A, so I think I'll be OK.