What to Know About Light Switch Wiring Before Trying DIY Electrical Work

Before you attempt to fix a faulty light switch, make sure you understand the basics of switch wiring with our guide to power-through and end-line switches.

Being a smart homeowner means knowing what is going on in your home and how it works, including the electrical elements. Behind your walls, there are wires running up and down that make it possible for a light switch on the first floor to turn on a tall chandelier hanging from the ceiling. Often located near the doorway, switches allow us to control ceiling-mounted fixtures, wall sconces, and even some electrical outlets with a simple flick or tap. While it might seem like magic, it's just a matter of understanding how your light switch wiring works.

stone wall with switch
Michael Partenio

Especially if you're planning to complete any DIY electrical work, such as repairing or replacing a switch or adding a new light fixture, it's important to know the basics of how these switches control your electricity. By taking the time to learn how your switch wiring system works, you can reduce your risk of electric shock or fire when working with switches. Check out our comprehensive guide below to learn the ins and outs of light switch wiring, including information on power-through and end-line switches.

End-Line Switch Wiring
Dave Toht

How Does Light Switch Wiring Work?

The way a light switch is wired depends on whether the power comes into the light box or the switch box first. These two varieties of electrical switches are called power-through switches and end-line switches, and the diagrams below explain how each type works.

illustration of light wiring
Illustration by Dave Brandon

Power-Through Switches

With power-through wiring, power enters the switch box directly. 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 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.

illustration of light wiring
Illustration by Dave Brandon

End-Line Switches

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

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.

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