How to Install a Dimmer Switch

Go to your breaker panel and flip the breaker that feeds the switch you are replacing. Stick a note on the panel so nobody flips it back on while you are working.

Bring a non-contact voltage tester back to the switch. Touch it to a known-live outlet first to confirm the tester is working. Then test the switch wires through the faceplate and again once you have the switch pulled out of the box. Do not skip this. Breaker labels lie all the time.

Take the cover plate off first. Then unscrew the two mounting screws holding the switch to the box. Gently pull the switch forward until you can see the wires behind it.

Do not yank. The wires are short on purpose and the insulation can crack if you stress them. Once the switch is clear of the box, test the wires again with your voltage tester to be absolutely sure there is no power.

Most single-pole switches have two wires. One is the live coming in (called common), one is the switched live going out to the light (usually labeled L1). On older houses the live might be red and the neutral black instead of brown and blue.

Take a phone photo of the wiring before you touch anything. Then wrap a small strip of colored tape around each wire and write what it is - common, L1 - in pen on the tape. When you get to the new dimmer, you will know exactly which wire goes where.

Every dimmer has a load rating printed on the back. The one shown here is rated 5 to 150 watts for LEDs. Add up the wattage of every bulb on that switch and make sure the total falls inside the dimmer's range.

Eight 5-watt LED spotlights pull 40 watts, which fits comfortably. Eight 60-watt incandescent bulbs would pull 480 watts and need a different dimmer. Also confirm your bulbs say dimmable on the package. Non-dimmable LEDs will flicker, buzz, or burn out the dimmer.

The back of the dimmer has three terminals: L1, L2, and a pair of squiggly arrow marks that indicate common. Your labeled common wire goes into the common terminal (the squiggly one). Your labeled L1 wire goes into the L1 terminal.

Push each wire fully into its terminal and tighten the small screw firmly. Give each wire a gentle tug to confirm it is locked in - some terminals look closed but do not actually grip the conductor.

If your box has a metal back box and an earth (ground) wire, connect it to the green ground screw on the dimmer faceplate. Plastic box plus plastic faceplate does not need an earth connection.

Fold the wires back into the box behind the dimmer. Start the supplied mounting screws by hand first so you do not cross-thread them. Then tighten them down with a screwdriver.

Before you fully tighten, hold a small level across the faceplate edge and adjust so the switch sits straight. A crooked switch is the kind of thing you will notice every single day. Finish the screws once it is level, snap the cover plate on, and you are done with the build.

Head back to the panel and flip the breaker back on. Return to the switch and turn the dimmer all the way up. The light should come on at full brightness.

Now run the knob through its full range. The light should dim smoothly without flickering, humming, or strobing. A little buzz at the lowest end is normal with some LED bulbs. Heavy flicker or audible hum means the bulbs are not properly dimmable or the load is wrong for the dimmer - swap one and try again.