As Dave M mentions there are two figures at play - ampacity is the ability of a certain size wire to carry a current without melting, and resistance is the resistance the wire adds to the circuit based on its gauge and length. The two figures are somewhat related. And you don't want to design to be running right at the edge.Originally Posted by Ron
The fun part about this is "total length of circuit". This means the length both wires + and - added together. It gets a bit more complicated since the return path in most cars is the frame, and in this car it's often a black wire that is shared among many circuits. The return path on the fuel pump goes through the inertia switch (often resistive in itself) and then into the harness where it is shared with some other circuits on its way to the back of the car. This sharing does not effect the voltage drop (other than via the added tie points) but it does impact the ampacity.
In DeLorean03's case he is planning to run a hot wire from the RPM relay to the pump. That is probably 10-12 feet of wire. If he were to run a dedicated neg wire back to the bulkhead, that would be another 10-12 feet so he could calculate based on about 25 feet total length of circuit. Easy to figure out.
His ground path will probably be another wire connected to the front frame at one of the ground points (bypassing the inertia switch - save that for another discussion). The ampacity (current carrying capacity) of the return path (the frame) is not an issue at all, but the resistance of that ground connection and the connection from the frame to the battery are a bit more difficult to determine.
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