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In building wiring, multiway switching is interconnection of two or more light switches to control lighting from more than one location. This allows lighting in a hallway or stairwell to be controlled from either end.

Contents 1 Three-way and four-way 2 Multiway switching 2.1 Two locations 2.2 More than two locations 3 Carter system 4 California 3-way 5 External links 6 References

Three-way and four-way

Three-way and four-way switches make it possible to control a light from multiple locations, such as the top and bottom of a stairway, or either end of a long hallway. These switches are externally similar to normal, single-pole switches, but have extra connections which allow two circuits to be controlled. Toggling the switch disconnects one circuit and connects the other.

Electrically, a three-way switch is a single-pole, double-throw (SPDT) switch:

By connecting two of these switches together back-to-back, it can be arranged that toggling either switch changes the state of the light from off to on, or on to off:

A four-way switch has two pairs of terminals which it connects either straight through, or crossed over:

By connecting one or more four-way switches in-line with three-way switches at either end, the light can be controlled from three or more locations. Toggling any switch changes the state of the light from off to on, or on to off:

Multiway switching

Multiway switching is a method of connecting switches in groups so that any switch can be used to connect or disconnect the load. This is most commonly done with lighting.

Two locations

Switching a load on or off from two locations (for instance, turning a light on or off from either end of a flight of stairs) requires two SPDT switches. There are two basic methods of wiring to achieve this, and another not recommended.

In the first method, mains is fed into the common terminal of one of the switches; the switches are then connected through the L1 and L2 terminals (swapping the L1 and L2 terminals will just make the switches work the other way round), and finally a feed to the light is taken from the common of the second switch. A connects to B or C, D connects to B or C; the light is on if A connects to D, i.e. if A and D both connect to B or both connect to C. This method requires two wires between the switches.

The second method is to join the three terminals of one switch to the corresponding terminals on the other switch and take the incoming supply and the wire out to the light to the L1 and L2 terminals. Through one switch A connects to B or C, through the other also to B or C; the light is on if B connects to C, i.e. if A connects to B with one switch and to C with the other. This method requires three wires between the switches.

Using the first method, there are four possible combinations of switch positions: two with the light on and two with the light off.

Off	On

More than two locations

For more than two locations, the two cores connecting the L1 and L2 of the switches must be passed through an intermediate switch (as explained above) wired to swap them over. Any number of intermediate switches can be inserted, allowing for any number of locations. This requires two wires along the seqeunce of switches.

Using the first method, there are eight possible combinations of switch positions: four with the light on and four with the light off. N.B. This diagram also uses the American electrical wiring name from the table above.

Off	On

As mentioned above, the above circuit can be extended by using multiple 4-way switches between the 3-way switches to extend switching ability to any number of locations.

Carter system

The Carter system was a method of wiring 3-way switches in the era of knob and tube wiring. Two of the four switch combinations are dangerous, and this wiring method has been prohibited by the National Electrical Code since 1923.

In the Carter system, the incoming live (energized) and neutral wires are connected to the traveler screws of both 3-way switches, and the load is connected between the common screws of the two switches. If both switches are flipped to hot or both are flipped to neutral, there is no light; but if they are switched to opposite positions, there is light. The advantage of this method is that it uses just one wire to the light from each switch, having a hot and neutral in both switches.

The major problem with this method is that two switch combinations apply the live (hot) wire to the outer shell of the light socket, presenting a dangerous electrical shock hazard. This method is prohibited in building wiring where the neutral and live conductors must be distinguished. The light socket shell (for an Edison screw lampholder may still be energized even with the light off, which poses a risk when changing a bulb. Switching a neutral conductor can create a shock hazard.

California 3-way

It is requested that a diagram or diagrams illustrating a California 3-way be included in this article to improve its quality. For more information, refer to discussion on this page and/or the listing at Wikipedia:Requested images. (February 2009)

External images
	THIS EXAMPLE, is an a hand sketched jpg that is an example of the California 3-Way.

The California 3-way, also known as a coast 3-way, is a method of wiring using four conductors and two three-way switches. It allows for a hot receptacle at both ends of the circuit as well as switched light outlets on both ends. Its main benefit is that it saves on conductors passed back and forth between two locations, such as a house's entrance and a detached garage. Its main drawback is that it's wired up to the 3-way switches in a non-standard manner that can cause confusion and mis-wiring for anyone who follows the initial installer. When wired correctly, it does not pose an electrical code violation as the neutral is never switched.

The California 3-way system should not be confused with the Carter system. The California 3-way is legal, but confusing.

External links

• "California 3-way" — 2003 message thread with an accurate drawing of a California 3-way
  • — correct drawing referenced in above forum
• http://i125.photobucket.com/albums/p66/JohnJ0906/carter.gif, an unattributed animated GIF.
  • Above image came from the post by 480sparky (dated 03-08-2008, 08:10 PM) in the message thread at http://www.electriciantalk.com/f2/three-way-switching-standard-coast-carter-2229/ — this same thread appears to gives a reasonable explanation of the Carter system v. California 3-way.

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References