Three-way and multi-location switching

A three-way switch is not three switches. The name comes from the switch having three terminals instead of two, and the arrangement it makes possible is the useful part: two switches, either of which controls the same light, whatever position the other one is in.

It is the single most useful wiring arrangement in a house, and it exists in exactly the places where walking in the dark is worst: staircases, long hallways, rooms with two doors, and the path between a back door and a kitchen.

An ordinary single-pole switch has an on position and an off position. A three-way switch has no on or off - it just routes the circuit down one of two paths, and the light is on when both switches happen to have selected the same path. That is why flipping either one always changes the state, and it is also why the terminals are not interchangeable during installation.

That last row is the one nobody builds and everybody wants, and it is the clearest case for the wireless approach below - the same problem shows up in stair and landing lighting, where being able to switch from both ends is a safety point rather than a convenience.

Adding a dimmer to a three-way circuit is where most upgrades come unstuck, and the reason is worth understanding before you buy anything.

An ordinary single-pole dimmer expects to be the thing that decides how much power reaches the light. On a three-way circuit, two switches share that job, and a standard dimmer has no way to coordinate with the other one. Fit it anyway and you get a circuit that works from one position and behaves unpredictably from the other.

Manufacturers solve it in two ways.

• A three-way rated dimmer plus a companion switch. The dimmer goes in one position and a matching companion or remote goes in the other. Lutron's Maestro range is built this way - the dimmer handles the level and the companion just tells it to change.
• A smart dimmer plus wireless remotes. A Caseta wall dimmer becomes the single point of control, and any number of battery remotes tell it what to do. This scales past two positions, which a wired three-way cannot.

Both are electrical work on the building. Replacing a switch is not a reversible accessory swap - it needs the owner's permission and generally a qualified person, for the reasons set out in can you change a light fixture in an apartment. Check the neutral-wire requirement too, since older switch boxes frequently have none - covered in best dimmer switches for LEDs.

For a renter, or for anyone who would rather not open a switch box, the whole problem can be solved without touching the circuit.

A battery wireless remote mounts on the wall exactly where the missing second switch should have been. It drives up to ten paired devices, needs no wiring, and Lutron publishes a ten-year typical battery life - so it is not a maintenance item. Paired with a plug-in lamp dimmer on a corded fitting, you get multi-location control of a light that was never wired for it, with nothing installed in the building at all.

This has two advantages over a real three-way circuit, beyond needing no electrician. It is not limited to two positions - a third remote at a third location costs the price of a remote. And the remotes are movable, so if you discover the position is wrong, you peel it off and try somewhere else.

The trade is that it only controls what it is paired to. A hardwired ceiling fixture still needs the wall-side route unless you put a smart bulb in it. The full format comparison is in wireless wall remotes, and the bulb-versus-switch decision is in smart bulbs vs smart switches.