What extension cord should I use for a window air conditioner?

Get a 12-gauge cord as short as possible — no longer than 6 feet. Window air conditioners pull a lot of current, especially on startup, and a thin or long cord can overheat, cause voltage drop, or start a fire. Use a cord rated for at least 15 amps; 14-gauge might work for small 5,000 BTU models, but 12-gauge is safest. Never use a 16-gauge light-duty cord.