Dave Platt...
Run 20 amps through a heavy-duty extension cord, lose a few volts due to
IR
drop, and you can end up with quite a few tens of watts of heat to be
lost. Under prolonged load this might, perhaps, be enough to affect
the integrity of the cord's insulation somewhat, or even cause a
thermal burn.

Somewhat more than somewhat! 13A through a typical coiled extension cable
(in a plastic drum) is enough to melt the drum and insulation - I've seen it
happen! My present 9m 13A extension cable is rated 13A fully unwound but
only 8A wound.

Excess extension cable should be neatly but loosely coiled to prevent a trip
hazard whilst maintaining adaquate cooling.

David