Max wrote:

The plate on my 3HP Jet planer says 15 amps.
It's plugged into a 20 amp outlet. No problems.

You just made it by the skin of your chiny chin chin.

A thermal-magnetic c'bkr is rated at 80% of nameplate when installed
in a panel board or load center.

Thus a 20A c'bkr connected to a #12AWG conductor will handle 16A on a
continuous basis.

Above 16A, you begin to operate on the time/current portion of the
c'bkr which is less than continuous duty.

BTW, just for reference, a 15A c'bkr connected to a #14AWG conductor
will only handle 12A on a continuous basis.

Case you are curious, it's all defined in your handy dandy NEC, a copy
of which should be on file at your local library.

Lew