Fasian

Im making a wiring harness. I have a diagram that tell me everything suck as wire color and size. The only thing it doesnt tell me is what amp fuses to use in the fuse box. Is there a formula or some other way to figure this out.

Fabrik8

You can start with the factory fuse ratings if the loads on each of the circuits matches up to what the factory circuits were. Otherwise, you'll need to find out what the steady state current draw (not the transient current draw) is of each of the loads and then add a little headroom to round up to the most logical fuse ratings. So if you have a 7 amp load you can go with a 10A fuse, etc. You don't want to cut things really close though, so don't put a 10A fuse on a 9A load because that doesn't leave any margin for things like temperature-based load changes, higher current at lower system voltage, etc.
You can measure current draw fairly easily too, but you'll need a decent multimeter with an internal current shunt capable of measuring the current that you need (all of the current travels through the multimeter), or you'll need a current probe that is capable of measuring at DC instead of just AC.

It's actually a very easy process, you just have to follow a few simple rules of common sense.