What's a Circuit
An electrical circuit is a loop of wire that carries power from the breaker box at the service entrance to the appliance power outlets and back to the breaker box. This wire loop can safely handle an electrical load that is less than the circuit’s rated capacity.
But if the circuit is overloaded for a significant period of time, the wiring can get hot enough to start fires. To prevent fires, a safety device called a circuit breaker disconnects the power if a circuit is overloaded.
Household electrical circuit capacity is typically rated in amps of current or watts of demand. If the electrical demand of the appliances connected to a circuit exceeds the rated capacity, your circuit breaker will trip and cut off the power.
Modern household electric circuits can safely handle 20 amps or 2,400 watts at 120 volts, while heavy-duty circuits are rated for 30 amps or 3,600 watts. The difference between a 20-amp and a 30-amp circuit is the size of the wiring that carries the electrical current.
A bigger wire can safely carry a bigger current.
You need to know the electrical demand of appliances you want to connect to a circuit. This information in amps or watts is on a nameplate on your appliances.
To add up demand, you want all your appliances on the same measurement. If your appliance is rated in watts, you can convert watts to amps by dividing the watts figure by 120 volts.
If your appliance is rated in amps, you can convert amps to watts by multiplying amps times 120 volts. You add up the amps (or watts) for each appliance to determine their total load on your circuit.
Appliances with an electric motor may require a momentary starting surge that can be at least twice the power needed to run. You have to allow for this.
If you want to connect a dishwasher, microwave and refrigerator to the same 20-amp kitchen circuit, you must consider that a typical dishwasher or refrigerator draws about 6 amps to run but needs 12 amps for a second or so to get started. An average microwave draws about 7 amps to run.
Each of these appliances will run fine on the same 20-amp circuit if they are run one at a time. But if you tried to run two at a time or all three at once, you could overload the circuit capacity and trip off the circuit breaker.
In this instance, you might be better off putting the dishwasher or refrigerator on a separate circuit to avoid the inconvenience of having to reset your circuit breaker all the time.