Standard wall switches control power to light fixtures on a simple on-off basis. However, dimmers allow you to vary the lighting intensity by controlling the amount of voltage a fixture receives.
Old-style dimmers operated using a rheostat—a large variable resistor. As you increased resistance in the rheostat, the voltage to the lamp was decreased and the energy that would have gone to brighter light instead wound up as wasted heat energy dissipated in the dimmer.
But times have changed, and now we use electronic dimmers. Electronic dimmers use an ingenious little device called a Triac. A Triac is a three-terminal, solid-state electronic switch or relay that quickly turns power to a lamp on and off up to 120 times per second. Since this power sequencing occurs so fast, it is not perceptible to the naked eye. It generates very little heat and saves energy.
We'll walk you through the steps of replacing a single-pole switch with an electronic dimmer using incandescent or halogen lighting. You can create mood lighting and save energy in one easy project.
If you have a three-way switch configuration, in which the light fixture is controlled by two switches, then you can install a dimmer to replace one of the switches, but make sure to buy a switch identified as a three-way dimmer.
First things first: Before any repair is performed on an electrical circuit, you need to make sure the power is off.
Turn off power to the circuit feeding the switch. Do this by going to your electrical service panel and either removing the fuse or turning off the circuit breaker feeding power to the switch.
Remove the cover plate on the switch you are replacing, and carefully pull the switch from the electrical box by pulling gently on its mounting straps.
Using a neon circuit tester, test the switch terminals to see if power has been shut off. Touch one probe of the circuit tester to each of the screw terminals on the switch, while touching the other probe to either the bare copper grounding wire or the grounded metal electrical box. If the tester does not light up, the power is off.
If you have a standard compact fluorescent bulb (CFL) or LED bulb, then dimmer switches will not work. If you want to dim a CFL or LED bulb, you will need a special dimmable bulb, which costs more than standard bulbs.
Equipment / Tools
- Neon circuit tester
- Combination tool
- Wire nuts
- Electrical tape
- New electronic dimmer switch
Disconnect and Remove 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 usually be two or three wires to disconnect from the switch. One will usually 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.
No Grounding Wire?
Sometimes in an older home or apartment, you may have 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 as shown in the photo above. Also note that if the dimmer is installed without a ground wire as under this NEC exception, then it needs to have a plastic, noncombustible wall plate.
Connect Ground Wire to 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.
Using Electrical Tape
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.
Connect Load Wire to 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.
This blue wire is to be connected to the LOAD wire in your switch box. The load wire runs from the switch to the light fixture.
Identifying the LOAD Wire
How do you tell which wire is which? As you recall, one of the insulated wires you disconnected from the original switch was black, with the other wire, white, hopefully, tagged with black tape. This white wire is the LOAD wire. The black tape means the previous installer marked it to indicate it is not a neutral wire but is also carrying hot current.
Connect the blue wire to LOAD wire. 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.
Connect Hot Wire to 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 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 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.
Test 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.
Install Dimmer Coverplate
Now that you have the dimmer all wired up and working, you need to put it back in the wall and cover it up.
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 in place with the two finish screws.
If the dimmer isn't working, the black and blue dimmer wires may be switched. Try reversing the wiring.
Look at the air gap switch. It may be pulled out, which means no power is getting through. Make sure that air gap switch is firmly pushed in.
Look at the bulb. It 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.
What about the circuit breaker or fuse? 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.
Bartelt, Terry. Industrial Control Electronics. Delmar Cengage Learning, 2012