What gauge extension cord do you need for your generator?

This checks two things, not one. A cord fails if it's too thin to carry the current safely (it overheats), or if it's technically safe but so long and thin that voltage drops enough to stress your equipment or dim/slow it down. The recommendation is the thinnest, and therefore cheapest, gauge that clears both bars — sized for continuous use at 125% of your load's current, the standard practice for loads running more than 3 hours, which describes most generator backup situations. This is a general estimate, not a substitute for checking your specific cord's rating or consulting a licensed electrician, especially for 240V loads or anything permanently wired.