Service Entrance Panel
Also called the circuit-breaker panel, the main serviceentrance panel (SEP) is the distribution center for the electricity you use in your home. Incoming red and black hot wires connect to the main breaker and energize the other circuit breakers that are snapped into place. Hot (black or red) wires connected to the various circuit breakers carry electricity to appliances, fixtures, and receptacles throughout the house. White and bare-copper wires connect to the neutral and grounding bus bars, respectively. (Representative 120-volt and 120/240-volt circuits are shown.)
Aluminum Wire. Some older homes have aluminum wiring, identified by the silvery color and an AL stamp. Aluminum wire expands and contracts at a different rate than copper, and this can loosen the copper-to-aluminum connection. So, use aluminum-compatible connectors, marked CO/ALR or CU/AL.

Circuit Voltage
Power rated at 120 volts travels from a black wire on a singlepole circuit breaker to the device. It returns through a white wire from the device to the neutral bus. In a 120/240 volt circuit, 240-volt power flows from one pole of a doublepole circuit breaker to the appliance and back to the second pole on the breaker. Additionally, 120-volt power that runs the lights, clock, and timer travels through a hot wire and back on a neutral white wire.

Single wires can be insulated to carry electricity or bare for grounding. Most household wiring is contained in cable, inside flexible metal (such as BX, pictured at top), or plastic insulation (such as NM, 2nd from top). Cords (such as lamp cords, 3rd from top) are stranded wires in plastic insulation, not to be used as fixed wiring; lowvoltage wire (bottom) is used to wire doorbells and thermostats.


Wire Sizes
Wires have size numbers based on the American wire gauge (AWG) system, which expresses wire diameter as a whole number. For example, No. 14 wire is 0.0064 inches in diameter; No. 12 is 0.081 inches. Smaller numbers indicate larger diameters that can carry more power. The National Electrical Code requires a minimum of No. 14 wire for most house wiring.


Wire Colors
Wires have color-coded plastic insulation to indicate their function in your house's wiring system. Hot wires carrying current at full voltage are usually black, red, or white with black marks (marker or bands of electrical tape), but can be other colors. Neutral wires carrying zero voltage are white or gray. Ground wires can be bare copper or copper clad in green plastic insulation.


Ground Faults on Appliances
A ground fault can occur in an appliance any time that excess or misdirected current causes the appliance to become energized. A grounding system, including an equipment grounding conductor, is intended to provide a low-resistance path for current back to its source to prevent an electric shock or possible electrocution. In a properly grounded system, the current remains within the wiring system and trips the affected circuit breaker at the panel, rather than being directed to a grounding rod. In an ungrounded or improperly grounded circuit, the appliance itself may become energized, which means the ground-fault current will not be sufficient to trip the affected circuit breaker at the panel.