Skip the extension cord for window ACs.

Yes, there is a maximum recommended length, but the real answer is don’t use an extension cord at all for a high-draw appliance like a window AC unit.

The problem is that extension cords create voltage drop and heat buildup. Most extension cords are not rated for continuous high current, and window ACs draw a lot, especially during startup. The longer the cord, the higher the resistance, and the more heat you generate. Even a heavy-duty 12-gauge cord longer than 25 feet is risky for a 15-amp AC unit.

Manufacturers explicitly say no extension cords for a reason. If you absolutely must, use a 12-gauge or thicker cord, as short as possible — 6 feet max, and only if the AC’s plug can reach it without daisy-chaining. But honestly, just get a dedicated outlet installed. Saving thirty bucks isn’t worth burning your house down.

Your AC deserves better.