12-14 ga wire is fine.. You may even find a pre-made power cord with a molded plug at HD or Lowes.. Otherwise, standard SJO cord is fine..
12 gauge is over-kill, original equipment was 16 gauge, but I always use 14 gauge because it allows for a longer cord, Lowes hardware has a nice 10-foot black 14-gauge extension cord for about $9.95, just perfect for this application, just cut off the end with the three-sockets.