US voltage standards and NEC guidelines
In the United States, standard residential power is provided via 120V/240V split-phase 60 Hz service, where lighting and general outlets use 120V and heavy appliances (dryers, EV chargers, HVAC) use 240V.
Commercial and industrial facilities typically utilize 120/208V wye or 277/480V wye three-phase systems.
Under the National Electrical Code (NEC), branch circuits serving continuous loads (operating for 3 hours or more) must be sized so the load does not exceed 80% of the breaker rating (e.g. maximum 16A continuous load on a 20A circuit).
Worked example: Level 2 EV charger
A residential Level 2 EV charger connected to 240V drawing 32 Amperes continuous load:
Real power P = 240 V × 32 A = 7,680 Watts (7.68 kW).
Under the NEC 80% continuous load rule, a 32A charger requires a dedicated 40A circuit breaker (32A ÷ 0.8 = 40A).
Inputs for the US power calculator
Voltage
120V for standard US outlets, 240V for residential appliances, or 208V/480V for three-phase commercial equipment.
CALCULATOR QUESTIONS
US 120V / 240V / 480V electric power calculator questions
What is the NEC 80% rule?
The NEC requires that continuous loads (running 3 hours or longer) do not exceed 80% of the overcurrent protection device rating (e.g. 12A max on a 15A breaker, 16A max on a 20A breaker).
How do you calculate amps from watts in the US?
Amps = Watts ÷ Volts. On a 120V circuit, a 1,500W space heater draws 1,500 ÷ 120 = 12.5 Amps.