Daniel Who Wants to Know wrote:

Also I am
no expert here but I think intermittent loads can exceed the 80% rule hence
the 14 gauge cord which would normally only be good for 15 amps but is
protected by a 20 amp fuse inside the oven and a 20 amp double pole circuit
breaker in the service panel.
..
The US NEC allows about any cord of 2 conductors (not including ground)
to be used at 18A. Most (all?) cords with type starting H (hard use) can
be used at 20A.

The 80% rule is for continuous loads - over 3 hours.

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