Switch Wiring: Power-through and End-line

The way a light switch is wired depends on whether the power comes into the light box or the switch box first. If power comes into the switch box first, the neutral white line from the service panel and the white line that leads to the light are spliced together. The hot black wire from the power source travels through the switch and from there to the light. Flipping the switch interrupts the flow of electricity to the light, turning it off and on.

If the line carrying power comes into the light box first, the circuit must still be wired so the switch interrupts the black line. The white wire from the service panel is wired to one side of the light. The black wire is spliced to a black wire in a cable that runs to the switch. That cable's white wire is also connected to the switch and runs back to and is connected to the light. Flipping the switch interrupts the flow of electricity, and the switch does its job.

Because the white wire from the switch to the light is now a hot wire, it must be marked to show it is hot. Wrap a piece of black electrician's tape around it at both ends or color the end black with a marker to show that it carries power.

With through wiring, power enters the switch box. The feed wire (the hot wire coming from the service panel) runs to the switch before it goes to the fixture. Two cables enter the switch box -- one supplying power and one going to the fixture. The neutral wires are spliced, and a black wire connects to each switch terminal. (Ground wires are not shown.)

If power goes to the fixture first and then to the switch, you have "end-line" wiring. Only one cable enters the switch box, coming from the fixture. Here the white wire to the switch is marked black to indicate that it is hot. (Ground wires are not shown.)