How to extend an electrical circuit wiring

How power is distributed in a house; extending a circuit

Caution: Before working on an electric circuit, shut off power to it at the fuse box or circuit breaker panel.

Electricity flows along a continuous wire circuit from a power plant to a power-drawing device such as a lamp or appliance and back again. The amount of current flowing past a given point in a wire is measured in amperes or amps. The pressure that forces current through a wire is measured in colts. The use of electricity is measured in watts, which are equal to volts multiplied by amperes.

Power enters most houses by means of three wires that pass through a meter to a distribution panel. Two hot wires carry 120 volts each; the third wire is neutral. One hot wire and the neutral wire form a 120-volt circuit needed for light fixtures and small appliances. The two hot wires supply the 240 volts used by heavy appliances.

In a typical circuit, a hot wire, wrapped in black (sometimes red or blue) insulation, delivers current at120 volts to a series of receptacles, light fixtures, and switches. A white-or gray-insulated neutral wire carries current at zero volts back to the distribution panel. Because the neutral wire must provide an uninterrupted path, it is connected to receptacles, but not to switches, fuses, or circuit breakers, which can cut the flow of current. In some circuits, a white wire may be hot; in that case, its ends should be painted or taped black.

Each circuit must have a grounding system to provide a safe path to earth for abnormal current flow. In a well-wired house, a continuous green-insulated or bare copper wire connects every receptacle, switch, and junction box in a circuit to the neutral bus bar in the distribution panel. (In some older systems. the circuit wires’ metal sheathing acts as ground.) A main ground wire connects the bus bar to an in-ground water pipe or to a copper rod driven into the soil. White wires are also grounded through the bus bar to keep them neutral (at zero volts’).

Wiring gauges and cable types

Wiring is coded by number; the higher the gauge, the thinner the wire and the smaller the amperage it can carry. A wire too thin for the amperage it must carry is a fire hazard. Don’t use a fuse or circuit breaker of a higher amperage than the wire’s capacity. Consult local codes and the National Electric Code before starting a project.

Circuit wires are protected by nonmetallic (usually plastic) sheathing or by steel or aluminum armor. Armored cable is found in older homes, and in a few areas is required by the local electric code. Nonmetallic (Romex) cable is used in most modern houses.

Extending a circuit

To extend a circuit to a new box, turn off power to the circuit. Open an existing box near the opening for the new one; see how the box is wired. The easiest point from which to extend a circuit is an end-of-the-run receptacle not under switch control. Run cable from the existing box to the new one. Secure the cable to both boxes. Install the new box; connect its cable to a fixture, receptacle, or switch. To connect new cable to an existing end-of-therun receptacle, attach the black wire to the unused brass terminal, the white wire to the unused silver terminal ;join ground wires with a wire connector.