Does the 80% of peak rule apply to extension cords?

I know you're not supposed to have a continuous load on a circuit for more than 80% of its peak capacity. Does this apply to extension cords too?

yes sir...... stupid char minimum

Your extension cord has a resistance, therefore a maximum rated load before it gets too hot which can lead to all sorts of problems (melting, failure, fire).

So, yes, I would apply the 80% rule to extension cords for safety's sake.

But this is just a guess, right? What really determines if we can go up to 99% continuously or not?

80% rule refers to startup currents. It makes sure you do not exceed max on startup of devices. Wire has a certain current carrying ability based on size; larger size wire more current it can carry. If you use a undersized wire it has more resistance as the amperage(current) flows through it creating heat and can case failure.

EDIT: after thought ; It is always best or it cant hurt if you use 12ga. ext cords they will be good for 20amp. Going heavy is always better.
14awg=15amp
12awg=20amp
10awg=30amp