How to Install an Electronic Dimmer Switch

1. Disconnect and Remove the Existing Switch

   Once the power is off, you can safely remove the old switch.

   Loosen the screw terminals on the switch and remove the wires. If the switch has been connected with push-in fittings, there will be a slot to insert a small screwdriver or nail that will release the wire. 

   Straighten the wires and check on their condition. Badly nicked wires will need to be clipped off and re-stripped to provide bare wire to connect to the dimmer switch.

   There will be two or three wires to disconnect from the switch. One will be an insulated black wire and is a "hot" wire. Another insulated wire may be white, but with black electrical tape wrapped around it to indicate it, too, is carrying hot current. The third wire, if present, will be a bare copper ground wire.

   Tip

   In an older home, you may encounter a switch box that has no ground wire. In that case, an exception in article 404.9(B) of the current National Electric Code (NEC) allows a dimmer without a connecting ground wire to still be installed if it is a replacement. 

   Simply remove the dimmer wire, or cut it short and cap it with a wire nut. Also note that if the dimmer is installed without a ground wire as under this NEC exception, it needs to have a plastic, noncombustible wall plate.

2. Connect the Ground Wire to the Dimmer

   Assuming you have a ground wire in your switch box, it's time to connect it.

   If there is insulation on the dimmer lead for the ground wire, use the combination tool to strip away about 5/8 inch of wire. Hold the dimmer ground wire alongside the circuit ground wire and fasten them together with a wire nut twisted over the ends in a clockwise direction. 

   Tip

   After twisting wires together with a wire nut, securing the connector with a couple of wraps of electrical tape is a good idea, but not required.

3. Connect the Load Wire to the Dimmer

   Electronic dimmers can get a little particular about which wire goes where. You'll likely notice two different colored wires on the back of your dimmer. One wire will be black and the other will be a color, probably red or blue. Let's assume blue for this tutorial.

   If there is pre-cut insulation on the dimmer lead for the blue wire, remove it so you expose about 5/8 inch of wire. Hold the exposed end of the dimmer blue wire alongside the circuit LOAD wire and fasten them together with a wire nut twisted clockwise over the ends. Turn the pair of wires until there is a slight twist in them both to ensure a strong connection.

   Tip

   How do you tell which wire is which? As you recall, one of the insulated wires you disconnected from the original switch was black, and the other wire was likely white and tagged with black tape. This white wire is a hot wire. The black tape means the previous installer marked it to indicate it is not a neutral wire but is also carrying hot current.

4. Connect the Hot Wire to the Dimmer

   Connect the remaining black wire, which is the hot wire. The hot wire provides power to the circuit and runs from the switch to the source of electrical power in your panel. 

   If there is pre-cut insulation on the wire, remove it so you expose about 5/8 inch of wire.

   Hold the exposed end of the dimmer wire alongside the circuit hot wire and fasten them together with a wire nut twisted clockwise over the ends. Turn the pair of wires until there is a slight twist in them both—then the connection is tight enough.

5. Test the Dimmer for Proper Operation

   Now that the wires are connected, test the operation of the dimmer. Start by turning the power back on at the electrical service panel.

   Some dimmers such as the Leviton Decora have an air gap switch that locally controls power to the switch. When pulled out it cuts power, and when pushed in it allows power to the switch. Make sure it is pushed in.

   Use the touch plate, lever, or slide bar to turn the dimmer on. If the lights turn on, you are all set to go to the next step. If it doesn't work, skip down to the troubleshooting section.

6. Install the Dimmer Coverplate

   Now that you have the dimmer all wired up and working, carefully fold the wires back into the switch box, then press the dimmer assembly into the switch box.

   Install the two dimmer plate mounting screws. Place the plastic cover plate over the dimmer and fasten it into place with the two finish screws.

Troubleshooting the Dimmer

If the dimmer isn't working, there are a few things to look for that might help.

• The black and blue dimmer wires may be switched. Try reversing the wiring.
• The air gap switch may be pulled out, which means no power is getting through. Make sure the air gap switch is firmly pushed in.
• The bulb may be burned out, or it might be a CFL or LED bulb that isn't dimmable. In either case, you'll need to get a new bulb.
• The breaker might be tripped or the fuse might be burned out. Reset the breaker or replace the fuse and try again.

If the light comes on but it flickers, you might have a bulb of fewer than 40 watts, a loose wire connection, or a bad fixture bulb connection. Start by replacing the bulb with one of higher power, and make sure it's tight in the socket.

If that doesn't help, turn off the power and check the wire connections in the switch box to ensure they are firmly wired together. Turn the power back on and test it out.