The basic answer is use 12/3 for 20 amp (2400 watt) and 14/3 for 15 amp (1800 watt). However the temperature rating of the wire is the important thing. If you use high temp cord rated at 90 degrees C you can get more amps through it safely. Another consideration is you should go up a wire size when going more than 50 feet.
Simple answer 12/3.
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