Here's the whole 3g write up - most of the prices are not current.
For a 120 amp circuit breaker see
https://www.delcity.net/store/Hi!Amp-Circuit-Breakers/p_167861
Fuse holder from local NAPA dealer - Item#: BK 7821143 Price: $10.49
Product Features: Thermal Plastic Holder For AMG Type Fuse Rated From 100 To 300 amp
Check on the wire prices, they tend to change often. Copper is an expensive commodity these days.
4 gauge black wire @ about $1.95 a foot (use string to lay out routing & determine length).
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=100-196
4 gauge red wire @ about $1.95 a foot (use string to lay out routing & determine length).
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=100-194
4 gauge ring crimp terminals (package of 5) $3.25.
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=095-584
3/4 “ Black heat shrink tubing, 4ft length, $3.56
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=082-058
3/4 “ Red heat shrink tubing, 4ft length, $3.56
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=082-064
Pre-fab 4 gauge cables with lugs already on the ends are available in most auto parts stores. Look for the starter switch to starter cables.
I am very careful to maintain backwards compatibility, so I did things a little different. The white/black stator wire gets the insulation stripped back about 1 1/2" in the middle of the wire & cut in the middle of the stripped area. Then a short length of white wire with a 1/4" slip on female spade connector gets spliced on to the white/black wire. Slide on enough 1/4" heat shrink tubing on the white wire to cover the solder splice you are going to make. Next all 3 wires get soldered together & the heat shrink tubing gets shrunk. When you finish, the white/black wire looks like a "Y" with 1 white arm and 1 white/black arm. I left the black/orange wires connected to the original plug and did not do anything to them. When you are done, the original plug still has all the wires connected to it and they are still functional. The extra white pigtail wire that you spliced, soldered in & covered with heat shrink tubing is just long enough to plug into the 3G without much left over.
I ran the 4 gauge wire under the front of the engine next to the 4 gauge wire for the starter power feed.
It came up the same path as the fuel injector supply lines, and gets bolted to the power output lug of the 3G alternator. The 125 amp fuse is mounted on a plastic panel bolted to the stock ignition coil mounts. One of side of the fuse has a 4 gauge wire connected to the battery side of the starter solenoid & the other to the 4 gauge power feed wire for the alternator.
I had some 1" silicone aircraft heat shield tubing that I fed the 4 gauge alternator power feed wire through and tie wrapped & clamped it in place with some aircraft cushion clamps. That provided the wire extra protection from road debris and rocks. Some heater hose could be used to do the same thing.
I have an additional 4 gauge ground running from the power steering pump mount to the common chassis ground being pointed to in the photo.
Apart from the grinding I did on the mount bracket, there wasn't much to it. Rather than just grind a notch, I ground the whole web back to the thick part of the bracket. It looks much more factory that way.
Here is the reasoning behind using only a single 4 gauge fused power feed to the alternator. If you use the two 10 gauge black/orange wires in addition to the 4 gauge wire, you have two fused power feed paths. The total current capacity of the wiring is the sum of the fused paths. The 4 gauge path is fused for 125 amps, and the two 10 gages wires are fused for 60 amps. That is a total of 185 amps, which exceeds the capacity of the alternator. Overload can occur without the fuses blowing, damaging the alternator.
The worst case scenario is that the alternator develops an internal short to ground resulting in a catastrophic failure. The initial short circuit surge current is limited by the resistance of the wiring. The current in a parallel circuit divides up according to the resistance of the branches. If the 4 gauge fuse opens up first, the two 10 gauge black/orange wires will be carrying the short circuit surge current. Depending on the time lag of the fuse links, they may open up before a fire starts or they may not.
http://www.stangnet.com/mustang-forums/threads/3g-alternator-install-a-how-to.646825/#post-6673702
Alternator wiring.
If you have a 3G alternator, the white/ yellow wire is critical to proper operation. It is the voltage sense and regulator power lead that picks up the difference in voltage at the alternator output stud and the connection point at the starter solenoid. If you cheat and run it directly to the alternator output, it sees the voltage at the alternator output stud. It does not see the voltage at the starter solenoid connection point where it feeds power to everything else. You may have a voltage drop in the wiring between the alternator output stud and the connection to the starter solenoid. Thus you may have low voltage or less than the standard regulated voltage at the starter solenoid connection point. This makes for low voltage throughout the rest of the car: everything operates at less than full efficiency.
Starter solenoid wiring 86-91 model cars.
Connect the fused 4 gauge wire to the alternator and the battery side of the starter solenoid.
Starter solenoid wiring 92-93 model cars.
