Use this tag for any time lights, fans, or devices are controlled from more than one wall switch location.
Multiway switching uses extra wiring and slightly special switch devices to allow a load or loads to be switched by multiple switches, with each switch able to control the on-off state of the load.
At a minimum, two three-way (North America) or two-way (Commonwealth) switches are used to allow control of the load from two locations, with two travellers run between the switches.
In a conventionally wired, or traveller system configuration, four-way (North America) or crossover/intermediate (Commonwealth) switches can be used at intermediate points to allow control of the load from three or more locations. These switches either pass the travellers through unchanged or swap them around, depending on their position.
The California system, in most applications, sacrifices four-way/intermediate switching for having switched and unswitched hots at both endpoints on the loop. This is commonly used with partially switched receptacles, but requires different switch wiring, and is somewhat obsolete due to its lack of expandability.
The obsolete and DANGEROUS Carter system can be found in old buildings, often alongside knob-and-tube wiring. It switches the neutral wire as well as the hot, and poses an unreasonable shock hazard as a result. The NEC prohibits it in 404.2(A), which requires all multiway switching to be done exclusively in the hot wire.
Switches can also be connected in series or parallel if other logical functions are needed, such as an override or shutoff switch. While not true multiway switching, such uses may be considered alongside more conventional multiway setups.
"Smart" devices such as dimmers, timers, and remote controllable switches have varying levels of multiway switching support:
- Some devices (often smart switches or timers, but many cheaper/older dimmers as well) don't have any support for multiway switching. Timers and smart switches that return power via the neutral, though, can be adapted to multi-way applications using a suitably rated relay.
- Other low-end smart-switches, timers, and dimmers support switching-only control from a single remote location, often using one of the travellers as the switched hot and the other as an auxiliary wire, connected to the hot or switched via a standard switch.
- Higher end devices (usually dimmers) will have remotes available that communicate with the master dimmer in the set over the traveler wires interconnecting them. These can provide full multi-location switching and dimming control.
- Finally, multi-way control can be emulated using building automation protocols (even a humble X10 setup can do this). This may be an option if the existing building wiring is insufficient or poorly designed.