You don't say if it is the power line into the house or if it an internal circuit. On interenal circuits use #12 wire with ground. for 20 amp circuits and you can use #18 for 15 amp cuircuits (lights only). I would never use this as #12 isn't that much more costly and all the circuits can run 20 amp. For 30 amp, (water heaters, stove top and ovens use #10 with ground. For power lines into the house the power company will install it and decider the proper wire size. Leonard