by D. O'Larde

Created on: November 19, 2008   Last Updated: April 08, 2011

A three-way switch is an electrical device which has the unique ability to not only control an electrical load but also provide a second three-way switch the ability to control the same electrical load. Essentially, a three-way switch has a pair of contacts that are alternatively normally closed or normally open depending upon which way the switch is flipped or pushed. Internally, one side of each contact is connected to form a common terminal located externally. The opposite ends of the pair of contacts are located externally to form a second and a third terminal, thus calling it a three-way switch.

One of the most common applications of three-way switches is when you want to control an electrical lighting load over a stairway from the first floor of the house to the basement in either of two locations. Conveniently locate one three-way switch on the wall upstairs and a second three-way switch on the wall downstairs. This scheme will give you the ability to turn on the light over the stairway before going downstairs or upstairs and turn it off when you have safely reached your destination. To accomplish this scheme the three-way switches must be wired properly.

When you buy a new three-way switch it usually comes with a wiring instruction. As you can realize by now you need two three-way switches to control an electrical lighting load from either one of two locations. The two three-way switches are wired right when the following conditions exist.

With the two three-way switches positioned in their respective locations one wire connects their second terminals and another wire connects their third terminals. An interchanged connection in one or the other switch will not alter the functional operation of the scheme.A wire connects the common terminal of one three-way switch to the hot side of the electrical circuit and another wire connects the common terminal of the other three-way switch to the hot terminal side of the electrical load usually a lighting load. To complete the wiring there must be a wire that connects the other side of the electrical load to the neutral side of the electrical circuit.

To really appreciate how the scheme works it might help to know exactly the positions of the contact points in both three-way switches when the light is off and when the light is on. Also, and perhaps more importantly, you might want to know how the wiring makes it possible to control the electrical load from either one of two locations.

When the light is off the closed contact point in one switch is in series with an open contact point in the other switch. When you flip or push the switch in either one of two locations, the open contact point closes and in series with the closed contact point in the other switch. The circuit completes and energizes the light. Note that when the light is on the path of the electric current between the two three-way switches can be either one of the two wires wired in between the two switches depending upon which contact point is closed in each of the switches.

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