About Single-pole Switches

A single-pole switch is the most common switch for residential wiring. A single-pole switch controls a single device and can function as anything from a light switch to a furnace cutoff switch. It has two terminals and a ground. The switch is always wired so that it interrupts a hot wire rather than a neutral wire, ensuring that when the switch is in the off position, the power flow is stopped. A switch can be physically located either somewhere between the breaker box and device it controls or somewhere after the device it controls. You can tell by looking at the box where in the circuit the switch is located.

If a switch box has only one cable coming into it, it is at the end of the circuit, or an end-of-run switch. The cable supplying power from the breaker box runs directly to the box holding the device the switch controls -- typically a light. Instead of going to the light, however, the power connects to a black wire in a second cable. The black wire carries the power to the switch, and the white wire in the cable carries the power back to one side of the light. The other side of the light connects to the white wire that travels back to the breaker box, completing the circuit. Because the white wire carries power whenever the switch is on, it's tagged with a piece of black tape in each box to show that it's potentially hot.

A middle-of-run switch is in a box with two cables. One of the cables comes in from the breaker box to supply power. The other cable travels to the device the switch controls. The hot black wire from the breaker box connects to one of the terminals on the switch -- it doesn't matter which one. The black wire in the cable leading to the controlled device connects to the other terminal. When the switch is on, power flows from the breaker box through the switch and to the load through the black wires. A white wire connected to the load carries the power back to the switch box, where it is spliced to the white that travels back to the breaker box.