Calculating Circuit Load

  • email this page
  • print this page

–Check local regulations for restrictions and permit requirements before beginning electrical work–

lit incandescent lightbulb

An overloaded circuit can heat up to extreme temperatures creating a serious fire hazard and a threat to life and property. To protect against an overload always know the capacity of the circuit and how much power is drawn by the appliances before adding a new outlet. To calculate the capacity multiply the amp rating of the breaker by the voltage of the circuit or:

(Breaker Amps X Branch Voltage = Wattage Capacity)

For a 120 volt circuit with a 15 amp breaker this would be 1800 watts, the total load from appliances should not exceed 80% of this total, or 1440 watts.

To determine the total watts drawn, calculate the load of all the devices plugged into the circuit. First determine which electrical appliances are plugged in by turning off the breaker and counting all the devices that stop working. Next, add the watts drawn by each device, for example, a circuit with a ceiling fan, TV and two lamps with 60 watt bulbs adds up to 1,010 watts if all these devices are running at the highest speed, all at the same time. This is 56% of the available capacity for a 15 amp/120 volt circuit, making it a good candidate for adding a new electrical outlet.

Finding the Wattage for Appliances

The wattage rating information for a particular device may be on a label or imprinted on the back somewhere. If no wattage rating is displayed, the load can be calculated by multiplying the amps drawn (appliance amps) by the voltage of the circuit. If neither the amps nor the wattage ratings for a device is known the chart below lists common household appliances and the average watts drawn for each to aid in calculating the load of a circuit.

appliance wattage chart