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How to Install a Dimmer Switch

There's more to changing a switch than connecting a few wires. You have to check grounding and box size for a safe, first-class job. It's easy to install a dimmer switch, but make sure to upgrade the wiring if necessary to make it safer and meet Electrical Code requirements. We'll show you how to assure a safe installation.

By the DIY experts of The Family Handyman Magazine

Step 1: Gather tools and turn off the power to the switch

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.

To begin, turn off the power and 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. 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.

Caution: Call an electrician if the original switch is connected to two white wires. This may indicate a dangerous switched neutral.

Caution!

If you have aluminum wiring, don't mess with it! Call in a licensed pro who is certified to work with it. This wiring is dull gray, not the dull orange that's characteristic of copper.

Step 2: 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 in Figure A.

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.

Tip: If the circuit breaker is labeled “15 amp, ” the wires are probably 14-gauge, or 12-gauge for 20-amp circuit breakers.

Figure A: Common Metal Box Sizes

Height/width/depth (inches) Volume (cubic inches)
3 x 2 x 2-1/4 10.5
3 x 2 x 2-1/2 12.5
3 x 2 x 2-3/4 14.0

Step 3: Test your ground before you connect it

New dimmers have either a green grounding wire or a green ground screw that you'll have to connect to a grounding source if one is available. Houses wired with plastic-sheathed cable almost always have bare copper ground wires that you'll connect to the dimmer. But test first using the procedure shown in Photo 3 to verify that the wire is connected to a ground.

Some wiring systems, like ours, rely on metal conduit for the ground. If you have one of these systems, Photo 3 shows how to test the metal box to verify that it's grounded. If it is, attach a short ground wire to the metal box with either a metal grounding clip as shown in Photos 4 and 5 or a green grounding screw screwed into the threaded hole in the back of the box. Then connect it to the dimmer.

If testing reveals your box isn't grounded, you can still install the dimmer, but you must use a plastic cover plate and make sure no bare metal parts are exposed.

Step 4: Install the new dimmer

Some dimmers, like the one we're installing, have stranded wires attached. Photos 7 and 8 show how to install this type of dimmer. Others have screw terminals instead. For these, strip 3/4 in. of the insulated covering from the wires in the box and bend a loop in each with needle-nose pliers. Place the loop clockwise around the screw terminals and close the loop around the screws with the needle nose pliers. Then tighten the screws.

It doesn't matter if you reverse the two switch wires to a single-pole dimmer. But if you're replacing a three-way switch with a three-way dimmer, label the “common” wire (it'll be labeled on the old switch) when you remove the old switch so you can connect it to the “common” terminal on the dimmer.

In most cases, the two switch wires will be some color other than green or white, usually black. But one of the wires may be white if your house is wired with plastic-sheathed cable. Put a wrap of black tape around the white conductor to label it as a hot wire.

Electronic dimmer switch

Electronic dimmer switch

Toggle dimmer switch

Toggle dimmer switch

Buying Dimmers

If the switch you're replacing is the only switch controlling the light, buy a standard single-pole dimmer. If the light can be switched on and off from two or more switches, buy a three-way dimmer switch. But you won't be able to dim the lights from every switch location unless you buy a set of special dimmers with advanced electronics and install one at each switch location.

Most dimmers are designed to handle 600 watts. Add up the wattage of all the light bulbs you'll be dimming. Then read the dimmer package to make sure it can handle the load. Heavy-duty 1,000- and 1,500-watt dimmers are also readily available. Read the package if you'll be installing dimmers side by side in the same electrical box because the wattage rating is reduced to compensate for extra heat buildup.

Finally, you have to use a special device, not a dimmer, to control the speed of ceiling fans and motors. Most fluorescent lights can't be dimmed without altering the fixture.

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How to Install a Dimmer Switch

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