A fusible link is a durable, slow-blow fuse carefully matched to the wiring it protects. Hence, the formula for safely selecting fusible links is very exact and unforgiving.
The size of fusible wire is described by its cross-sectional area in (mm)
2. Typically, fusible wire is imprinted with its metric size in (mm)
2, which is then converted into the equivalent wire gauge using a chart.
Regarding application criteria, a fusible link
must measure 6" long and
must be 4 wire gauges
physically smaller in diameter (which corresponds to 4 wire gauges
numerically greater) than the vehicle wire to which it attaches and protects. So, a 16-gauge (or 1.0 (mm)
2) fusible link is used to protect a 12-gauge wire. Additional considerations are that the eye terminal crimped onto the end of the link has the correct inside diameter for the stud to which it attaches, and the splice crimped onto the opposite end of the link is properly sized to fit the gauge of the wire in the vehicle to which the link will connect. With all of that stated prefab fusible links work just fine, though, making your own is not at all difficult and can even be fun.
For your benefit, application charts and discussions are provided by VileZambonie in
How to make a Fusible Link and by yours truly in
Fuses and Circuit Protection to make fabricating a suitable fusible link virtually painless.