Typical Residential AC Wiring

240- and 120-Volt Power

• Power companies transmit AC current at a high voltage along the power lines and feed it through a transformer to step the voltage down to 240 volts for residential use.

  Two cables carry the power from the transformer to the main panel. Each cable is "hot," or energized, and connects to a bus bar designed to accept a series of snap-on circuit breakers. A third cable, which is neutral, connects to a third bus bar and leads back to the transformer. When a circuit is connected to either of the hot buses and the neutral one, the voltage in that circuit is 120 volts.

Hot and Neutral Wires

• Each residential circuit supplying lights and small appliances consists of a hot and neutral wire. The hot wire is black and connects to one of two hot bus bars in the panel via a circuit breaker. The neutral wire is white and connects directly to the neutral bus bar.

  In order for the breaker to protect the circuit, it should be rated for the maximum load the circuit can support: usually 15 or 20 amps. Circuits rated at 240 volts power large appliances, and connect to both hot bus bars. This requires the use of two hot wires connected to two circuit breakers.

Color Coding

• Residential wires are color coded to prevent miswiring, as are the terminals on standard electrical devices. The cables from the power company are black and red. These colors are reserved for wires that will connect to them and are therefore always hot. The bus bars to which they connect are brass, making this the color of the hot terminals in all household electrical devices.

  The neutral wire leading from the panel back to the transformer is white, and the bar to which it connects is silver. Electrical devices have an analogous silver terminal to which the white wire in the electrical cable connects.

Grounding

• The electrical code requires grounding for all residential circuitry as a safety precaution. A path to earth, usually supplied by a metal stake pounded into the ground and connected to a ground bus bar in the panel, prevents charge build-up on electrical devices and potential electrocution hazards.

  A ground wire in all electrical cables in the house, usually left bare, must connect directly to the ground bus. All electrical devices, including switches and receptacles, include a green grounding terminal, and all the ground wires in all cables leading to and from each device must connect to this ground terminal.