there are a lot of articles on this on the web- first, 4 0hm or 8 ohm matters as stated above--in general 14 gauge or better for that length - can't go wrong with thicker-i have a 40- 60 ft run with 4 ohm-- i ordered 10 gauge at first but it had to go under the carpet( not ideal) and was too thick and inflexible so i switched to 12 gauge and its perfectly fine.
