Loading Electric Circuits for Safety

When adding only a few receptacles, lights, or a small appliance, tap into an existing circuit. Tap power from a nearby receptacle, junction box, switch box, or ceiling fixture box and run cable to the new electrical box. First, make sure that you don't overload the circuit.

Indexing circuits
An index telling which electrical loads -- lights, receptacles, and appliances -- are on each circuit should be posted on the inside of a service panel door. However, this index may not be accurate. Whenever evaluating a circuit, test to make sure which electrical loads are on what circuit.

If there is no index, make one. Shut off one circuit and test to see which lights, receptacles, and appliances have turned off. Write down the results. Repeat for all the circuits until you have covered all electrical loads. Draw the index and tape it to the service panel.

Indexing circuits is easier when two people work together: One stands at the service panel while the other tests plugs, lights, and appliances and notes the findings. Walkie-talkies or cell phones ease communication.

Figuring the total load
Once you've identified all the loads on a circuit, total the wattage. If the wattage is not printed on an item's label, there should be an amperage rating (amps). Multiply the amps times voltage to get the wattage. For example a 4-amp, 120-volt tool consumes 480 watts (4X120=480).

Add the wattage of all the bulbs in each fixture. For each receptacle, include the appliances or tools usually plugged into it. Include hardwired appliances such as a dishwasher. Once you've totaled all the electrical loads on a circuit, determine if adding new services overloads the circuit. If no convenient circuit has enough available capacity, install a new circuit.

Major appliances, such as electric ranges and dryers, are on dedicated 240-volt circuits. Only one user connects to each circuit.

To find the capacity of a circuit, multiply amps times volts (usually 120). Most local codes demand that all the users on a circuit must not add up to more than 80 percent of total capacity. This 80-percent figure is the circuit's "safe capacity."

15-amp circuit
Total capacity: 1,800 watts
Safe capacity: 1,440 watts (12 amps)

20-amp circuit
Total capacity: 2,400 watts
Safe capacity: 1,920 watts (16 amps)

30-amp circuit
Total capacity: 3,600 watts
Safe capacity: 2,880 watts (24 amps)

To find how many watts a 120-volt appliance or tool consumes, look at its nameplate. If it shows only the amperage, multiply that times 120 to find watts. For example an 11-amp table saw uses 1,320 watts (11X120=1,320). Here are some typical wattages; yours may vary considerably. (All 240-volt appliances should be on dedicated circuits, so you never need to figure whether they can fit in a circuit.)

Dishwasher: 1,000-1,500 watts
Garbage disposer: 400-850 watts
Hair dryer: 500-1,500 watts
Microwave: 600-1,500 watts
Refrigerator: 200-700 watts
Table saw: 800-1,500 watts
Television: 100-350 watts
Window air-conditioner: 500-1,500 watts