How complicated can 3 way switch troubleshooting be? Consider this. Since a 2-switch system has 3 wires to be connected to 3 terminals at each box, it works out that only 1 out of 9 possibilities will work. Each 4 way switch in-between would worsen the odds by a factor of 3. And any defective switch or connection further complicates things. What is going to be easier -- trial and error or understanding the 3 way switch system?
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. 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.
The British perhaps use more common sense in naming these "2-way switches", but in North America we are stuck with more technical terms. I think I've noticed some Canadians and others talking about "tree-way switches". Anyway, I refer to these switches, wires, and the lights they control as a 3-way "system" not a 3-way "circuit", because that can be confused with the branch-circuit and breaker that such a system is part of.
pivoting the contacts 
There are two abnormal and rare 3-way systems that may be encountered. They are called by various names (California, Hollywood, coast, farmer’s, French, Chicago, Carter, lazy Susan, lazy neutral). The names are confused between the two kinds. One is illegal and presents possible shock danger. The other (British, I think) seems allowed by code and is so different in its concept that few electricians here would even recognize it. What I am saying about 3-way systems is not meant to apply to either of these. Basically, the illegal one attaches a hot permanently to one traveler and a neutral to the other; the commons each extend from their terminals to the light fixture. The legal one likewise attaches a hot to one traveler but the leg (to the light) to the other; another wire runs between the common terminals. Here is a diagram of these Rare 3-ways.
Since there is a wide variety of ways that multi-switch systems are wired in practice, I give, below, a number of 3-way and 4-way switch diagrams, so you can perhaps recognize your own version. To see some 3-ways in the context of a whole circuit, see the Typical circuit.
I will try to describe here what color the insulation on the wires of multiple-switch systems will commonly be, if they are not miswired. Lately, Code is wanting any "factory" whites serving as travelers or as the hot-end hot to be re-marked black or red. Here I am only telling what you will commonly find in most homes. The hot wire at the common terminal of the hot-end switch will be black (or rarely: red or white). The light-leg wire at the common terminal of the leg-end switch will be black or (rarely) red. Each traveler pair is contained in one cable, and will be either black and white, black and red, or red and white. Most other white wires present in these switch boxes are neutrals that are connected to each other and not to any of the switch terminals. Any bare or green wires are grounding wires connected to each other. If the switch has an additional green screw, to meet Code a bare piece should be run to it from all the grounds.
Normally encountered 3-way systems (3-way "circuits," as some call them) all share one scheme-theme:
--- S === S --- O
where the "S"s are the switches, the "O" is the light, and the lines shown are wires carrying constant (the line on the left) or switchable (the rest) hotness. The neutrals, not being involved in the switching operation, are not shown yet, since they can come into the picture in a variety of ways. Nor are boxes or cables containing the wires shown yet; nor any additional lights switched with the one shown; nor any 4-way switches, which would interrupt the path of the two traveler wires at any point. Here then are two ways you might see this scheme in the flesh, complete with neutrals, boxes, and cables:
A. Here the hot arrives at one switch box. It can then be connected through to the other switch to be its hot-all-the-time common, or instead it can attach as common of the switch where it arrived to begin with. Either way, the travelers between the switches end up giving hotness or unhotness to the light "leg." This leg either comes directly off the common of the switch nearest (electrically) to the light, or is tied through to the light from the switch furthest from it. This arrangement is a common one:
B. Here the hot for the system arrives at the light box, but its functional connection is at whichever switch it is connected through to. The other switch ends up sending the decision of the travelers back by way of the same cable that brought hotness down from the light box:
C. Here the travelers from one switch to the other are simply routed through the light box, where connectors pass them on through:
If you have begun to grasp the idea of 4-ways, you may be getting the impression that you could invent your own way to wire such a system. It is true. Whatever works, is not against Code, and is safe is possible. This site is not to advise you about design, Code, or installation.
Searchers who need to read the material on this page may be using terms found in this statement: When you replace a two way switch with a tree way switch, the 3 way electrical switch turns the 3 way circuit off.
Switch article by Wikipedia© 2007 Larry Dimock
