||The examples and perspective in this article may not represent a worldwide view of the subject. (May 2015)|
In building wiring, multiway switching is the interconnection of two or more electrical switches to control an electrical load (often, but not always, lighting) from more than one location. For example, this allows lighting in a hallway, stairwell, or large room to be controlled from multiple locations. While a "normal" light switch needs to be only a single pole, single throw (SPST) switch, multiway switching requires the use of switches that have one or more additional contacts and two or more wires must be run between the switches. When the load is controlled from only two points, single pole, double throw (SPDT) switches are used. Double pole, double throw switches allow control from three or more locations.
In alternative designs, low-voltage relay or electronic controls can be used to switch electrical loads, sometimes without the extra power wires.
Three-way and four-way
This article follows American usage. Readers in the most other countries should read "two-way" for the American "three-way" and "intermediate" or "crossover" switch for the American "four-way".'
The controlled load is often a lamp, but multiway switching is used to control other electrical loads, such as an electrical outlet, fans, pumps, heaters, or other appliances. The electrical load may be permanently hard-wired, or plugged into a switched receptacle.
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, either end of a long hallway, or multiple doorways into a large room. These switches appear externally similar to Single Pole, Single Throw (SPST) switches, but have extra connections which allow a circuit to be controlled from multiple locations. Toggling the switch disconnects one "traveler" terminal and connects the other.
Electrically, a typical "3-way" switch is a Single-Pole, Double-Throw (SPDT) switch. By correctly connecting two of these switches together, toggling either switch changes the state of the load from off to on, or on to off. The switches may be arranged so that they are in the same orientation for off, and contrasting orientations for on.
A "4-way" (intermediate) switch is a purpose built Double Pole, Double Throw (DPDT) switch, internally wired in manufacture to reverse the connections between the input and output and having only four external terminals. This switch has two pairs of "traveler" terminals that it connects either straight through, or crossed over (transposed, or swapped). An intermediate switch can, however, be implemented by adding appropriate external wiring to an ordinary (six terminal) DPDT switch.
By connecting one or more 4-way (intermediate) switches in-line, with 3-way switches at either end, the load can be controlled from three or more locations. Toggling any switch changes the state of the load from off to on, or from on to off.
Number of wires
In this article, when comparing different wiring methods or systems the focus is on the number of wires connecting the switches to each other. Sometimes an additional wire is present in each cable to carry an unswitched neutral or hot wire which passes through the switch mounting box, but is not normally connected to the switch. If the switch has a pilot light or a receptacle, additional hot or neutral connections to the device may be required. Grounding (earthing) wires are not counted.
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 several arrangements of wiring to achieve this.
In the traveler system, also called the "common" system, the power line (hot, shown in red) is fed into the common terminal of one of the switches; the switches are then connected to each other by a pair of wires called "travelers" (or "strappers" in the UK), and the lamp is connected to the common line of the second switch, as shown.
Using the traveler system, there are four p