You could measure your nominal voltage. At home, I have 116 volts and at work measure 128 volts. That may affect breaker operation slightly.
Basically, the rated amperage is what it takes to start a device. A 1/3 HP motor may require 1400 watts to start-up then requires about 800 watts during operation. A compressor motor even require more at start-up. However, a space heater uses it rated wattage throughout the operation cycle.