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About This Project
At first glance, exchanging a standard switch for a dimmer seems like it would be easy.
Simply disconnect the wires from the old switch and connect them to the new one. But chances are, your situation will be more complicated than that. The National Electrical Code now requires larger box sizes to accommodate a wide range of bulky devices like dimmer switches and ground fault circuit interrupters (GFCIs). If your home is more than about 10 years old, the box used for many of your switches may be too small to meet current codes.
In this article, we’ll walk you through the critical steps for installing a new dimmer switch, including the toughest part: knocking out an old electrical box and installing a larger one without damaging the wall. We’ll also tell you how to calculate the size box you’ll need.
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” for more details.) If you have
a double box (for two switches), do all the calculations 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, $15, a mini hacksaw ($6) and a keyhole saw ($5). 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.
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.
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.
As a standard precaution, we recommend that you check the wires in the box with a voltage tester to confirm that all are dead. 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.
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.
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