How to Install a Dimmer Switch (page 3 of 3)

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How to Install a Dimmer Switch-Installation

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 (B120R made by Carlon and available at Home Depot and other home centers), 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 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.

Switch Connections

Figure A Switch connections

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. Undercut the hole slightly, 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 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. 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. If the existing wires are too short to work with, simply attach 6 in. of additional wire to them with wire connectors.