The correct conductor size in a cable assembly is determined by the current carrying capacity of the wire and the voltage drop allowable in the circuit.
The wire gauge can be calculated using the following equation:
Wire Gauge = (Current Rating) / (Voltage Drop)
For example, if you are using a 12-gauge wire with a 5-amp current rating and you want to allow for a voltage drop of no more than 0.5 volts, then the equation would look like this:
Wire Gauge = (5 amps) / (0.5 volts) = 10