Should I increase wire size when extending a 75 foot circuit to reduce voltage drop?
You could use #12 wire. Your voltage drop will be slightly above 3%, but at the end of a 170' run with 1200w load the voltage drop would be 5%. Code recommends 3% for the feeder and 5% max at the end of the line, but both are recommendations and not hard rules, so 12 will be fine. Since you are only going 120' the actual voltage drop will be 3.5%