The switch-leg portion of electrical circuits controls the flow of electricity to lights or receptacles. The type of circuit wired for a switch depends on where electricity is delivered to the circuit: at a switch or at the light or receptacle. Switches that control lights or receptacles from more than one location require yet another switch-leg circuit.
An outlet is any device where the homeowner has access to the electricity. This includes light fixtures and standard receptacles, permanently wired large appliances, and large appliances that connect with a plug and receptacle, such as a dryer or a range.
Any outlet controlled by a switch uses some form of switch-leg wiring. Outlets controlled by more than one switch have more complex switch-leg wiring than simple, one-way switch circuits.
Point of Use
When power is delivered to the point of use, it is not connected directly to the outlet. The hot wire that carries current to the outlet connects to a wire that routes the electricity to a switch. Another wire travels from the switch back to the outlet. The switch turns the electricity on and off. The switch and the two wires that travel between the outlet and the switch make up the switch-leg portion of the circuit.
Electrical current is sometimes delivered to the switching point. This is common when multiple switches in one wiring box control different outlets on the same branch circuit. The switch-leg is still made up of two wires, but the hot wire connects directly to the switch and the two wires connect directly to the outlet. The second wire completes the circuit by connecting to the house neutral wire in the switch wiring box, instead of in the outlet wiring box.
Outlets controlled by more than one switch use three-way and four-way switches to control the flow of electricity. Although the hot wire might deliver the current at the point of use or at the switching point, the switch-leg wiring is the same. A hot wire delivers current to the first switch. Two wires called travelers carry current between switches. A wire connected to the last switch in the leg carries the current to the outlet.
The first and last switch in the leg are always three-way switches. Additional switches are four-way switches. This allows any switch to turn the current on or off.