How to Install Dimmer Switches

Old electrical box too small? Here's how to solve that problem and install a new dimmer switch.

Step 1: Gather the materials

We’ll demonstrate this procedure on a light controlled by a single switch in a single-switch box. If the light is controlled by two switches, be sure to buy a three-way dimmer switch. (See “Buying a dimmer switch” below for more details.) If you have a double box (for two switches), calculate the required box size and install a larger double box if needed.

You won’t need special skills to take on this project, but you should know how to make safe, solid wire connections (twist-on connectors, screw terminals, etc.).

You’ll need three special tools for this job: a voltage tester (we like the non-contact type, Photo 1), a mini hacksaw (Photo 4) and a keyhole saw. All are available at home centers and full-service hardware stores.

Begin by calling your local building inspections office to see if you need an electrical permit. The permit means that your work will be inspected to ensure that you didn’t make any mistakes. If all goes well, you can complete this project, including box replacement, in about two hours.

CAUTION

If you discover aluminum wiring, call in a licensed electrician who is certified to work with it. This wiring is dull gray, not the dull orange that is characteristic of copper wire.

Back to Top

Step 2: Open up the switch box

First shut off the power at the main panel. To find the correct circuit, turn on the switch you intend to replace and then turn off the circuit breakers one at a time until the light goes off. If you have fuses instead of breakers, remove the fuse. Note: Before turning off any circuits, shut down any computers to avoid data loss. Also, you may lose your settings on clocks, security systems, lighting systems and other devices if power to them goes off.

Next, unscrew the switch plate cover. Ideally it’ll pop right off, but if the plate edges have been painted to the wall or if wallpaper runs up to it, gently score around the cover’s edge with a utility knife before removing it.

As a standard precaution, we recommend that you check the wires in the box with a voltage tester to confirm that all are dead (Photo 1). The non-contact type we show doesn’t have to actually touch a bare wire or metal terminal to tell if voltage is present. Getting it to within 1/4 in. of the wire or touching the wire insulation will do. Occasionally you’ll find wires from more than one circuit in a box. If you find hot wires, continue to turn off breakers until you shut off all circuits to the box. Once you are sure the power is off, pull out the switch and disconnect it (Photo 2).

Next, make sure you have a ground wire in the box. The National Electrical Code now requires all switches to be connected to a ground wire. If you don’t see one, you’ll have to run a bare copper or green insulated wire (14 gauge) to a known electrical ground. Metal boxes are sometimes grounded through metal conduit. If you’re not familiar with ways to test a metal box for grounding, call a licensed electrician for guidance.

Buying a Dimmer Switch

Dimmer switches are available in many styles and configurations, including slides, knobs and touch-sensitive dimming mechanisms. However, check these key things:

  • Capacity (how many lights it can control). The capacity will be measured in watts. Add up the wattage of the bulbs in all the fixtures the switch controls to make sure it falls within the switch rating listed on the package or instructions.
  • Single-pole or three-way. Buy a “single-pole” switch if one switch controls the lights or a “three-way” if you have two switches controlling the same lights.
  • Light type. Standard and halogen bulbs require standard incandescent dimmers. A few fluorescent lights can be dimmed with special dimmer switches, but most can’t. Low-voltage lights may also require special dimmers.

Dimmer switch
Back to Top

Step 3: Calculate the minimum box size

With the switch removed, count the wires in order to correctly size the box (Photo 3). The basic rules:

  • Count all hots (black and any other color except green and white) and neutrals (white) as one each.
  • Count all ground wires combined as one.
  • Count all clamps combined as one.
  • Count devices like switches, receptacles and timers that mount in the box as two. (Most light fixtures that fit over boxes do not have to be counted.) The total count for our box in Photo 3 is 8. Multiply by 2.00 cu. in. per wire for 14-gauge wire (use 2.25 for 12-gauge) to get the 16 cu. in. of space required. The wire gauge is stamped on the plastic sheathing. But since you usually can’t see it, simply compare the wires in the box with any 14- and 12-gauge wires you have on hand.

Plastic boxes generally have the cubic inch volume stamped in the back of the box. Use a flashlight to find it. Metal boxes won’t be labeled, so you’ll have to measure the interior dimensions: height, width and depth (Photo 3). Then multiply to find the volume. By measuring and multiplying, we found that our old box has a volume of about 13.5 cu. in. Since we need a box with at least 16 cu. in., we’ll have to install a new box.

The National Electrical Code lists official volumes of metal boxes.  If you’re not sure about the capacity, call your local electrical inspector with the box dimensions and the inspector can tell you the exact volume.

Back to Top

Step 4: Remove the old box

To replace a box, begin by removing all the wire connectors and separating the wires. If necessary, label them with masking tape so you can properly reconnect them in the new box.

The trick to replacing a box is to do it without marring or breaking into the wall to avoid a time-consuming wall repair and repainting. Most old boxes were nailed to the sides of studs. The easiest way to neatly remove them is with a mini hacksaw (Photo 4). Removal will be tougher if you have to cut through metal straps instead of nails. Use short strokes to protect the drywall at the back of the cavity. If necessary, use a flat screwdriver to pry the box slightly away from the stud to make room for the blade. Work carefully so you don’t damage the insulation on the wires or the wall around the box.

If you have to replace a plastic box that’s nailed into a stud, simply break it out with a hammer and large screwdriver. Break it open to release the wires and pull the nails or cut them off with the mini hacksaw. Then pull the remaining pieces of the box from the opening.

Back to Top

Step 5: Install the new box

The best new box to use in this situation is called a remodeling or old work box. These boxes are designed to clamp directly to the drywall and are available in several sizes. We recommend a plastic box that’s available in a 20-cu.-in. size (available at home centers and hardware stores), because it’s large enough to work in almost every situation. (The largest metal remodeling single box you’ll find is 18 cu. in.) It’s far easier to enlarge the wall opening (Photos 5 and 6) for a larger box than to make it smaller. However, if the wiring runs through metal conduit or has metal sheathing, you’ll have to use a metal replacement box with matching clamps so that you can reconnect the conduit or sheathing to the new box.

It’s important to cut the hole so the box fits snugly. If you overcut it, the box clamps might not hold. So keep the pencil tight to the box when you outline it (Photo 5). Undercut the hole slightly (Photo 6), then test-fit the box and enlarge the hole as necessary.

Installing the new box takes a little finesse. Start by taping the cable sleeve to the insulated wires with electrical tape (Photo 7) to help them slide through the box clamps. Then work the wires into the box. Don’t overstress or break off the plastic clamps; the code requires that they effectively clamp the cable. With the wires in the box and the flanges snug against the drywall, tighten the fastening mechanism (Photo 8). But don’t overtighten. You could break the mechanism or crack the drywall.

Follow the manufacturer’s instructions for connecting the new switches. Our switch had stranded wire leads. Use wire connectors to fasten them to the solid wire (Figure A and Photo 8). If the existing wires are too short to work with, simply attach 6 in. of additional wire to them with wire connectors.

Figure A: Switch Connections

Follow the manufacturer's instructions when installing the dimmer switch.

Dimmer switch connections
Back to Top