What is the total load allowed on a 20 amp 120 volt circuit?

The total load in watts would be W = A x V. 20 x 120 = 2400 watts. Any wattage higher than this will trip the breaker and shut the circuit off.

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The theoretical resistive load is V/I = 120/20 = 6 Ohms. The lower the resistance the higher the current. Usually you don't want to operate above the 80% point so the number would be 120/16 = 7.5 Ohms.