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.
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.