How To Install a 3 Way Dimmer Switch

This tutorial is on how to install a 3-way dimmer switch. Now I’ve learned that many people call these two-way switches, but this is incorrect. They are three-way switches. A 3-way switch is used when you want to switch an electrical device from 2 or more locations. 

Three-way switches can commonly be found at either end of a hallway, near the doors of a huge room. Now, this circuit is already using 3-way switches, but the homeowner wants to be able to dim their lighting. So I will install a Lutron Ariadni AY-603P, 600 watts, preset, incandescent dimmer switch.

The first thing you need to do is determine what size dimmer switch you need. To do this, add up the maximum fixture wattage(s) this switch will control. So let’s say you have 6 recessed cans, all using 65-watt lamps (bulbs) that this switch will control.

However, the maximum wattage lamp you may use in most recessed cans is 75 watts. You need to size your dimmer switch according to the fixture’s maximum wattage rating and not the existing lamp inside of the fixture. So, you need a dimmer switch with a minimum wattage rating of 450 watts (6 x 75 = 450 watts).

A label on the fixture states the maximum wattage lamp allowed for that fixture. If you have a 100-watt lamp in a fixture rated for 60 watts maximum, then now is the perfect time to correct this.

The extra 40 watts puts off just enough extra heat to make this a fire hazard. This is one of the most common mistakes that I see.

If you want to be able to dim the circuit from the either light switch, then you need a master and slave combination similar to a Lutron Maestro.

However, the homeowner would prefer the dimmer switch installed in one location in this application. Depending upon your location, this is approximately $150.00 (labor + materials) cheaper than buying and installing the master and slave combination.

How to Wire a Three-Way Dimmer Switch Step-by-Step

Things You’ll Need:

  1. Three-way dimmer switch
  2. Screwdrivers
  3. Wire strippers
  4. Voltage tester
  5. Electrical tape

Step # 1 – Locate your breaker box and turn off the power to this circuit.

Step # 2 – Remove the switch cover plate.

Step # 3 – Remove the 3-way switch

Step # 4 – Disconnect the wires from the old switch. Note the wire on the common terminal (odd colored screw), as this will need to connect to the common terminal on the new switch. In this installation, there are 2 black wires. So, I need to identify the common wire. I usually disconnect this wire last, hold it, grab the new switch and connect it to the common terminal on the new switch first. The placement of the common wire is essential. If it is not placed on the common terminal, then the 3-way switching will not work correctly.

Step # 5 – Connect the wires to the 3-way dimmer switch, ensuring the common wire is connected to the common terminal. Grounding is also critical when installing dimmer switches. Ensure you connect the ground wire to your dimmer switch.

Step # 6 – Install the 3-way dimmer switch

Step # 7 – Install the switch cover plate

Step # 8 – Turn on the power and test. When testing 3-way switching, I like to turn on the light from one switch. Go to the other switch and turn off the light. Then go back to the first switch and turn on the light again. If everything works, then you installed a 3-way dimmer switch correctly. If it does not work, then you probably did not place the standard wire on the common terminal.

Installing the 3 way dimmer switch is a relatively simple DIY task. One rule is that switches and dimmers are permanently wired “hot.” This means that only Black or Red wires are ever connected to the switch (or dimmer) itself. However, in your particular installation, a White wire is used as a “hot” wire on both the dimmer and the switch. In this case, the White wire must be colored Black or otherwise identified as a “hot” wire by wrapping it with a piece of black electrical tape.

This is how to wire your circuit:

Take a note of the Black and Red wires that were coming out of the same slot on the back of the dimmer. Using your original image – not the one posted by ‘Harper’ – the Black wire on the dimmer is connected to the Black wire going to the light fixture(s). The Red wire on the dimmer is connected to the Red wire going out to the switch. The other Red wire on the dimmer (the one in the same slot as the Green ground wire) is connected to the WHITE wire going to the switch. Mark the White wire as black because it is now “hot.” The Green ground wires are all connected in the box.

Wiring the switch is equally easy. The Black wire connects to the brass screw on the switch. The Red wire connects to the silver screw on the same side of the switch as the Black wire. The White wire, which must also be identified as Black with a piece of electrical tape, is connected to the other silver screw on the other side of the switch. Finally, the bare ground wire is connected to the green screw on the switch. Wiring the light fixture(s) should be self-explanatory. And that’s it.

Worked in IT for 10 years, specialising in computer measurement, resource and performance management and complex problem solving. Changed careers to HVACr in 2015.

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