Question

What is a 3 way switch? Why do they come in pairs? How does it work?

Answer 1

Multiple switches may be located at several approaches to a room in order to turn the room's lights on or off from any one of those locations. They do this by continuing hotness toward the light on two alternate paths -- wires called "travelers". Hotness (voltage) enters into the system at one "three-way" type switch, which we will call the "hot end." According to the position of its handle, hotness "travels" on to the next switch on one of the two traveler wires. If this next switch is the only other switch, it is also of the 3-way type and can be called the "leg end." It will pass the hotness on to the light(s), but only if its handle happens to be in the position that puts the light's wire in contact with the particular traveler that is bringing the hotness. Video. If there were any more switches between these two (electrically), they would simply pass hotness along but are able to change which traveler wire the power continues on; to do this, these (third, fourth, etc.) switches must be "four-way" type switches, a different animal than the first and last switches in the system. Four-way switches have four terminals to connect two traveler pairs to (the incoming pair and the outgoing pair). Three-way switches have three terminals, with the one that is not for the two travelers being called the "common". At the hot end, the incoming hot wire is connected to the common terminal. At the leg end the wire attached at the common is the one that goes to the light(s). This will all be made clear in the diagrams that follow.