Hot cords carry less current.

Yes, ambient temperature matters—a lot. Extension cords have a rated ampacity based on a standard temperature (usually 30°C / 86°F). If it’s hotter than that, the cord can’t shed heat as well, so it has to carry less current to avoid overheating and melting its insulation.

The actual derating depends on the cord’s insulation type and the temperature, but the rule of thumb is: for every 10°C above 30°C, derate by about 10% for common cords (like those rated 60°C or 75°C). In direct sun or a hot attic, your 15-amp cord might only safely handle 12 amps. That’s not a theory problem—that’s a fire hazard.

Nobody remembers these charts. So just keep cords out of hot environments if you’re pushing them near their limit. If you must run a cord across a black roof in July, go up one gauge thicker than you think you need.

This is not the place to gamble on “probably fine.”