How to Install a Dimmer Switch

Install a toggle, electronic or slide dimmer.

By Jeff Gorton

From The Family Handyman

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About This Project

Overview and Preparation

Step-by-Step Instructions and Pictures of How to Install a Dimmer Switch

See More Home Electrical Projects

It doesn’t take long to replace an ordinary light switch with a full-feature dimmer. But while you’re at it, to make your home safer, you should upgrade the wiring to meet the latest requirements of the National Electrical Code. Our step-by-step instructions will show you how to install the dimmer, concentrating on details that will guarantee a safe installation.

The tools you’ll need are inexpensive and will come in handy for all your electrical projects. You’ll need a screwdriver, wire stripper, inexpensive voltage tester and needle-nose pliers to install a dimmer.

Double-Check for Hot Wires in the Box
Turn on the light and have a helper watch as you switch off the circuit breakers, or unscrew the fuses one at a time until the light goes out. Leave this circuit turned off while you work.

In Photo 1, we’re using a non-contact voltage detector to double-check for voltage before removing the switch. These detectors are available at hardware stores and home centers for about $12. I prefer this type of tester because it’ll detect voltage without direct contact with the metal conductor. That’s huge— it means you can check potentially hot wires before you handle them. After you unscrew the switch and pull it away from the box, probe around inside the box with the detector to make sure there are no other hot wires from another circuit.

Make Sure the Box Is Large Enough
Too many wires and devices stuffed into a box can cause dangerous overheating, short-circuiting and fires. The National Electrical Code specifies minimum box sizes to reduce this risk.

To figure the minimum box size required by the electrical code, add: 1 for each hot and neutral wire entering the box, 1 for all the ground wires combined, 1 for all the clamps combined, and 2 for each device (switch or receptacle) installed in the box. Multiply this figure by 2 for 14-gauge wire and 2.25 for 12-gauge wire to get the minimum box volume in cubic inches.

To help determine the gauge of the wire in your switch box, look at the amperage of the circuit breaker or fuse in the main electrical panel. Fifteen-amp circuits are usually wired with 14-gauge wire and 20-amp circuits require 12-gauge or heavier wire.

Compare the figure you get with the volume of your existing box. Plastic boxes have the volume stamped inside, usually on the back. Steel box capacities are listed in the electrical code. We’ve listed the volume of the most common steel boxes. If you have a steel box, measure it (Photo 2) and consult the chart to see if it’s large enough. If your box is too small, replace it with a larger one. It’s possible to replace a box without cutting away the wall, but it’s a tricky job. I’d recommend just removing about a 16-in. square of drywall or plaster and patching it after the new large box is installed.

Common Metal Box Sizes