How to Design a Safe Circuit

Determine the total wattage that your circuit will bear. If you are adding a circuit where there are powerful appliances, such as in a kitchen, plan on a 1,900-watt, 20-amp circuit. The total wattage is calculated by adding up the wattage of the fixtures that will run off the circuit. Most other rooms operate on 15-amp circuits capable of carrying no more than 1,440 watts. Most household circuits are 120 volts, regardless of amperage. However, appliances such as stoves and dryers run on 240-volt circuits.

Check your service panel to ensure it can handle another circuit. Check that the amperage rating is at least 100 amps and that there is space in the box for additional circuit breakers. The amperage rating of a circuit breaker panel is listed on a placard on the front of the panel, and most modern homes are certified for either 100 or 200 amps. If your service is less than 100 amps, do not add any more circuits; contact an electrician to discuss increasing the service to 100 or 200 amps.

Create a scale drawing of your home, marking the location of doors, windows and any plumbing or air ducts. Use this drawing as a map for the circuit and to calculate the amount of cable you will need. Plan to run the cable as straight as possible. Cable is expensive and the more twists and turns the cable must make, the more difficult it is to thread it. A good rule of thumb is to add 20 percent to your measurement for unexpected turns and for waste.

Get appropriate cable. Choose BX metallic-sheath cable for running in exposed areas like attics or basements. Non-metallic sheathed cable can be used for circuits that run behind walls or ceilings. A 20-amp circuit requires 12-gauge cable, while a 15-amp circuit requires cable no smaller than 14-gauge.