How to Install a Dimmer Switch (page 2 of 3)

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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.

Calculate the Minimum Box Size
With the switch removed, count the wires in order to correctly size the box. 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.)

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. 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.

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. 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.