How to Design Electrical Circuits

Draw an accurate scale floor plan of your home, including any doors and windows. If possible, mark any plumbing or duct work that might cross the route of the new circuit.

Estimate the capacity you expect the circuit to bear. Different rooms have different requirements. A kitchen, for example, usually requires 20-amp circuits because of its power-hungry appliances. If you will be adding a couple of outlets or fixtures in a bedroom, you can probably get by with a 15-amp circuit. Do not exceed more than 1,440 watts on a 15-amp circuit, or 1,920 watts on a 20-amp circuit. Determine the total wattage by adding up the wattage of the fixtures you will be attaching to the circuit.

Make sure your existing service panel can handle the additional load. On the breaker box, check the amperage rating. Most newer homes have 100 or 200 amp service, which means the house can probably handle at least one more circuit. You will also need space in the box for additional breakers.

Map the estimated path of the circuit. Try to run the cable as straight as possible for ease of construction and to reduce cost--electrical cable can be expensive. Carefully measure the distance from the service panel to the end of the circuit.

Buy cable appropriate for the type of circuit designed. Depending on how you will be running the cable you will have to choose either BX metallic-sheath cable or non-metallic sheathed cable. A 20-amp circuit requires 12-gauge cable, while a 15-amp circuit requires cable no smaller than 14-gauge. A knowledgeable salesperson at a hardware store can help you choose the right cable. A good rule of thumb is to add 20 percent to your measurement for unexpected turns and for waste.