Calculating Circuit Load

by: Dale Cox

By code, the number of conductors allowed in a box are limited depending on box size and wire gauge. Calculate total conductors allowed in a box before adding new wiring, etc. Check local regulations for restrictions and permit requirements before beginning electrical work. The user of this information is responsible for following all applicable regulations and best practices when performing electrical work. If the user is unable to perform electrical work themselves, a qualified electrician should be consulted. How to Read These Diagrams

photo 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 = Watts)

For a 120 volt circuit with a 15 amp breaker this would be 1800 watts, the total load from lighting and appliances should not exceed 80% of this total, or 1440 watts. Likewise the total amps drawn on the circuit at one time should not exceed 80% of the breaker capacity. For 15 amp circuits this would be 12 amps.

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.