KC8QVO, actually the starter solenoid IS a relay itself for the starter. That is the function it performs. If you can read the resistance of the coil, you can use Ohm's Law to figure out the current draw of the coil. I=E/R So if your solenoid reads 40 ohms and you use 12 volts, then the coil is drawing 300 milliamps, or .3 amps. So most any wire size will handle that amount. I'd use 16 gauge myself. Hope that helps you out some...