I found a nice wire size calculator at:
http://www.powerstream.com/Wire_Size.htmFill in the gauge, select 12 VDC, fill in the feet distance, enter the load in Amps and press the calculate button. Remember, Amps = Watts / Volts (12). You get the voltage drop as a result of the wire run.
Typically for automotive work, 18 gauge is the smallest wire to be used. For lighting purposes in four wheel vehicles, I have used as large as 10 gauge.
BTW, here is an alternative location for your PC-8...
