I use wires from an electric cable. Their advantage is the insulation, which prevents the wire from forming a cold bridge. As to the gauge, it's difficult to give any advise for two reasons: I don't know how much weight they will have to carry (depends on the height of the tunnel and cover material used) and I don't know your American standards for producing cables. Mine are 25 or 35 square millimeters (35 = 0,0542 square inches) in cross section. Material either aluminium or copper. I was lucky to find a lengthy piece left behind by sloppy electricians.
I guess the gauge doesn't matter much as you can always adjust the span.
Milan HP