How to Wire a Dimmer Switch: 2-Wire and 3-Wire Installs
To wire a dimmer, turn off the breaker, confirm the power is dead, and connect the dimmer's two leads to the two black wires in a single-pole (2-wire) box. In a 3-way (3-wire) circuit, connect the dimmer to the common wire and the two travelers, and keep a standard 3-way switch at the other location. Always match the connections to the instructions that come with your dimmer.
ELEGRP DM19-WHBest overall
Kasa Smart HS220Smart app and voice control
Lutron DVCL-153P-WH-6Trim lever for LED tuningStart by figuring out which setup you have
People use "2-wire" and "3-wire" to describe two different things, so it helps to sort that out first. In most cases, a 2-wire install is a single-pole circuit: one switch controls one light. You will see two insulated wires (usually both black) plus a bare copper or green ground wire.
A 3-wire install usually means a 3-way circuit: two switches control one light. The box holds three insulated wires (a common and two travelers) plus ground. Sometimes "3-wire" also refers to a box with a white neutral wire. Smart dimmers like the Kasa HS220 need that neutral, according to its listing, while most basic dimmers do not.
Before you buy, check the load, neutral and wiring-type points on our best dimmer switch comparison. It lists which models support single-pole only and which support both single-pole and 3-way.
Safety first
Household wiring can shock or start a fire if it is handled wrongly. If you are unsure at any step, stop and call a licensed electrician. Local codes may also require a permit or a pro for some work.
- Switch off the breaker for the circuit, not just the wall switch.
- Remove the cover plate and the old switch, then test every wire with a non-contact voltage tester before touching it.
- Test the tester on a known live circuit first so you know it works.
- Never work on a box with exposed conductors while standing in water or on a wet floor.
- Use a stable step stool if the box is high, and wear safety glasses when you remove screws and strip wires.
Gather tools and check your bulbs
You need a screwdriver, a non-contact voltage tester, wire strippers, wire connectors (wire nuts) and a pencil or masking tape for labeling. Many dimmers come with wire connectors and a wallplate, so look in the box first.
Next, check the bulbs. Use only dimmable bulbs, and add up the wattage of everything on the circuit. For example, the ELEGRP DM19-WH lists 300W for dimmable LED and CFL and 600W for incandescent and halogen. The BESTTEN LUXDMWK600 lists 150W for LED and CFL, with a 3W minimum LED load. Stay under your dimmer's limit.
Dimmers are also made for lights, not for every load. The BESTTEN listing notes that it cannot control fans, so do not use a dimmer on a fan or an outlet unless the product says it is made for that.
How to install a dimmer with 2 wires (single-pole)
This is the simplest case. Once the power is verified off, pull the old switch out gently and note how it was connected before you disconnect anything. A quick phone photo helps.
- Label the two black wires if they are not already marked. In a single-pole box, one brings power in (line) and one goes to the light (load). On a standard single-pole dimmer, they are usually interchangeable, but check your instructions.
- Disconnect the old switch and straighten the wire ends. Trim any damaged copper and strip about the length shown on the dimmer's strip gauge.
- Connect the ground wire (bare copper or green) to the dimmer's green ground lead or screw.
- Connect one dimmer lead to each black wire using wire connectors. Tug each connection gently to confirm it is firm.
- Fold the wires into the box carefully. Dimmers are bulkier than switches, so avoid pinching any wire.
- Screw the dimmer to the box, attach the wallplate, restore power and test the full range.
How to install a dimmer with 3 wires (3-way)
In a 3-way circuit, only one of the two switches should be a dimmer. The other stays a standard 3-way switch. The ELEGRP and BESTTEN listings both say to use only one dimmer in a 3-way or 4-way circuit. If you put dimmers at both locations, you can get flicker or failure.
The tricky part is finding the common wire. On the old 3-way switch, the common screw is usually a different color from the other two (often darker) and is sometimes labeled COM. The other two terminals are the travelers. Mark the common wire with tape before you remove the old switch.
Then connect the dimmer's common lead to the common wire, and connect the two other dimmer leads to the two traveler wires. On most 3-way dimmers, the travelers are interchangeable. Connect the ground, tuck the wires in, and test from both switch locations. The remaining standard switch will turn the light on at the level you set on the dimmer.
If your dimmer is a single-pole only model, such as the Kasa HS220, it cannot be used in this setup. Check the wiring type before you buy.
Quick reference: what goes where
Use this table as a memory aid, and let the instructions that come with your dimmer have the final word.
| Circuit | What you see in the box | Dimmer connections |
|---|---|---|
| Single-pole (2-wire) | Two black wires plus ground | One lead to each black wire, ground to ground |
| 3-way (3-wire) | Common wire, two travelers, plus ground | Common lead to common wire, other two leads to travelers |
| Smart dimmer with neutral | Above wires plus a white neutral | Neutral to the neutral lead, as the product instructions describe |
Troubleshooting and finishing touches
If the lights flicker or buzz at low settings, the first thing to check is the bulb. Not every LED is truly dimmable. Some dimmers also have a low-end trim, such as the Lutron Diva LED+ trim lever or the side adjustment wheel on the BESTTEN LUXDMWK600. Adjusting it can fix flicker or dropouts at the bottom of the range.
If the light does not work at all, turn the power off again and check each connection. A loose wire connector or a swapped common wire in a 3-way box is the usual cause. If the breaker trips when you restore power, shut it off and call an electrician.
Once everything works, mount the wallplate and label the breaker if it was not already marked. If you are still choosing a model, compare the load, neutral and trim details on our best dimmer switch page.
Frequently asked questions
Can I install a dimmer with no neutral wire?
Yes, if the dimmer does not require one. Most basic dimmers in our comparison list no neutral requirement. Smart models vary, and the Kasa HS220 needs one.
Does it matter which black wire goes where on a single-pole dimmer?
On many standard single-pole dimmers it does not, but follow the diagram that comes with yours. Some models mark a line or load lead.
Can I replace both 3-way switches with dimmers?
No. Use one dimmer and one standard 3-way switch. Two dimmers in the same circuit can cause flicker or failure.
Why is my dimmer warm after installation?
Dimmers can feel slightly warm in use. If it feels hot, turn it off, check that the load is under the rating, and ask an electrician to look at it.