Fusible link theory

Many automobiles use fusible links instead of fuses in their 12v power distribution. These links are of certain area and certain length. I have repeatedly read that the link should be 4 wire sizes smaller than the wire it is connected to (a #14 link will protect #8 wiring).
How is this 4 size rule determined and how is the length of the link determined?

Fusible link is nothing more than a different way of saying inline fuse. The wire size and length are figured based on the amount of energy it will require to melt the link. The smaller wire means higher resistance so the link will burn before the higher gauge main wire, the length adjusts the total thermal mass of the element and it's total resistance, so it can be tailored to slowblow/fastblow based on length. It's no different than any other fuse, simply it's location is different.

Keep in mind you can't just use normal wire for the fusible portion of the link, if it ever actually trips that means it will have reached the metals melting point and one end will still be connected to 12V, which will have a piece of copper at it's melting point which can easily fuse to the chassis of the car causing a true dead short condition which would vaporize the rest of the wire harness to the battery.

The two Toyota'a that I have owned used short length of wires with Fast-on terminals on them within their "engine fuse compartment". The ends won't go anywhere and do any damage if they melted. Way back when, like the 60's American cars may have had one fusable link in the engine compartment. They both were nothing but wire. In the latter, they were moulded into the wiring harness. I suspect, there they would have used a much higher temperature insulation. I never had to replace one either.

Generally they allow the wire to "break" at a nice place. You'll probably find one from the + of the battery and the alternator, for example.

I worked for GM/Packard Delphi and the fuse-able link wire was just a normal copper core of the correct size but the insulation was a type of teflon compound that won't burn. The wire mill hated to run that stuff