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I dont know anything about electrical but this seems like it could just be a V=I*R and P=V*I solution here.

By code, 20 Amps x 80% = 16 Amps max allowable.
16 Amps x 240 Volts = 3840 Watts max.
If you use a 100% rated breaker the max Watts is 4800 Watts.
Good luck finding a 20 Amp breaker rated for 100%.

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Ohm's law
Not just a good idea;
It's the LAW!