Household electrical circuits are rated according to the number of amperes they can carry at once; the total current draw of the circuit is spread out across all appliances connected to it. One of the most common circuit ratings is 15 amps. This may not seem like much, but some devices only require 0.5 amperes to function. Learning the capacity of a circuit and what it means can prevent you from overloading the circuit.
Electrical Power and Current
Electrical devices have a power output rating in watts that is derived from multiplying the input voltage by the current draw. For example, if you plug a 60-watt lamp into a 120-volt power socket, you would divide the wattage by the voltage to determine the current. In this case, the light draws 0.5 amps to produce 60 watts of power.
A single circuit may only produce 3 watts of power per square foot for lighting and power receptacles, according to the National Electrical Code. Therefore, circuits must be designed and loaded according to how much space they cover. For a 1,200-square-foot building, the total maximum power output would be 3,600 watts. At 120 volts on average, not including more power-intensive appliances, the current draw would be 30 amps. Therefore, two 15-amp circuits would be needed.
Maximum Load vs. Safe Load
As a rule, no one device or series of devices should draw a continuous load of more than 80 percent of the circuit's available current. For example, a 15-amp circuit at 120 volts produces a maximum of 1,800 watts. No more than 1,440 of that should be used continuously. This allows for power surges and prevents the circuit breaker from tripping.
Overloading a circuit will cause the circuit breaker to trip and prevent a fire, assuming the breaker is working properly. Electrical current, when sent through a conductor, produces heat via electrical resistance. Too much current produces more heat than the conducting material can handle and therefore can cause a fire or damage the components of the circuit.