Replacing a 3-way Switch with a Dimmer

Published: 28th July 2006
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Do you have a room that has two entrances and would like to have a switch at each entrance to control a light? Or more commonly, did your electrician set one up but now you want to replace a switch with a dimmer? Fear not, some simple diagrams should help sort it out.

First of all, there are 2-way, 3-way, and 4-way switches; each with a different purpose. Two-way switches are most common and only have two terminals in addition to the ground screw. These are very simple in nature and simply either break or complete the circuit to turn a light on or off.

If you have two switches that control a light, you must use 3-way switches. Usually we see people get in trouble when they want to replace one switch with a dimmer. The dimmer in this situation must be a three way and the three wires must go to the correct terminals. (at the end of this article we explain how to look at a switch and most of the time get the hook-up correct. When all else fails, use a continuity tester.) Make sure the package says the dimmer is a 3-way switch.

Three-way switches will have 3 terminals in addition to the ground screw. One hot(usually black) wire either comes from the power panel into the switch or one hot wire exits the switch and goes to the light. In between the switches are two wires called travelers. These are considered switched hot wires and can be typically black, red, or sometimes a white wire has black tape wrapped around it at each end to designate it as a hot and not a neutral wire.

Also note that the dimmer function and the switch are separate. In other words you set the dimmer to a certain level and then either switch can turn the light off or on to its dimmed state. If you turn the light down to nothing with the dimmer, flipping either switch won't turn the light on. That is why we never recommend putting a dimmer on both switches. It just gets way to confusing when the light doesn't turn on as to whether it is the dimmer turned all the way down or that a switch doesn't work.

Let's take a look a some diagrams to understand how the circuit and switches work.

First here is an example of a 3-way switch setup. Light is off as there is no path for the hot.

Switch 1 Switch 2

---- ---- OFF

Hot | / 2|-------+2 | -------

----|1 | | 1|----| Light |

| 3|-------+3 / | -------

---- ---- Neutral|


Switch 2 is moved, Light is ON as there is now a path for the hot.

Switch 1 Switch 2

---- ---- ON

Hot | / 2|-------+2 | -------

----|1 | | 1|-----| Light |

| 3|-------+3 | -------

---- ---- Neutral |


Either moving Switch 1 or Switch 2 will break the hot. And from the Off state, either Switch will make the connection.

The wires in-between the the 3-way switches are called travelers. So use the red and the black for your travelers between switches. On the 3-way switches, you cannot just connect the hot to one of the terminals and the travelers to the remaining two. Look at a diagram on the switch or most times there is a single terminal on the top or bottom for the in/out hot and then two terminals (one on each side) at the other end are for the travelers. Connecting the hot to the side that has one terminal and the travelers to the side that has two terminals is usually NOT the way to do it.

If you didn't get the wires on the correct terminals, then you will find that it sometimes takes flipping both of the switches to get the light to turn on or off. With the proper wiring, any single switch that is flipped should cause the light to go on or off.

If you have more than two switches controlling a light, then you will also have 4-way switches in the circuit. We'll cover that in another article.

And please remember to turn the power off first. BZZZT sounds or arc welding your switch is not a good thing.

About The Author:

Alison Leaderman is in charge of online marketing for Ready Electric Wholesale Lighting and Electrical Supplies business. Ready Electric is a well established electric supply company for contractors and has established an e-commerce website to serve both contractors and the public with wholesale and deep discount pricing.

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