Understanding US Home Power Outlets and Electrical Load
In the United States, standard home power outlets typically deliver 120 volts of alternating current (AC) electricity. The most common outlets are rated for 15 amps, which means they can supply up to 1800 watts of power. Calculated as voltage multiplied by current, 120V x 15A 1800W. Some outlets, particularly those designed for heavy appliances such as dryers or ovens, may provide 240 volts and can handle higher amperage for greater power delivery.
No Power Is Coming Out, but What Voltage Is Provided?
Even if no power is visually emanating from the outlet, the outlet still provides 110–120 volts at 15 or 20 amps, depending on the breaker size. Knowing this, you can quickly multiply 120V by 15A to determine that the outlet supplies 1800 volt-amperes or 1.8 kilowatts (kW).
The Basic Equation: Watts Volts x Amps
Most common 15A, 120 volt receptacles can deliver 15 x 120 1800 watts without any other loads on the circuit. For 20 amp receptacles, 20 amps x 120 volts equals 2400 watts. According to code rules, circuits should be used at 80% of their nominal capacity. Thus, 1440 watts for a 15A circuit and 1920 watts for a 20A circuit. Actual power starts flowing only after the appliance is plugged in and switched on.
Residential Receptacle Circuits in the US
Residential receptacle circuits in the US are typically supplied at 120 volts via 15-amp or 20-amp breakers. These breakers are 80% rated, meaning they can continuously carry up to 80% of their rating and even more for short periods, depending on the breaker's time-current characteristic. You can therefore connect a continuous load of 20 x 120 x 0.8 1920 VA or 2400 VA to a 120-volt receptacle for short periods.
For 240-volt receptacle circuits that supply heavy appliances such as electric stoves, clothes dryers, electric hot water heaters, refrigerated air conditioning units, etc., a 30-amp, 2-pole breaker typically used to protect these circuits supplies 240 x 30 x 0.8 5760 VA continuously or 7200 VA intermittently.
Note that circuits with multiple receptacles connected to them are limited to the maximum load the breaker can supply, regardless of how many devices are contributing to that load. If one device is pulling the maximum current for which the breaker supplying that circuit is rated, attempting to add additional load to other receptacles on the same circuit can risk tripping the associated circuit breaker.
Understanding these principles is crucial for ensuring safe and efficient energy use in your home electrical system.