Figure 1 (Switches) Commentary. New window.
All the diagrams of this tutorial are to illustrate most of what you might run into in a home -- or might need to reconstruct if you undid some connections. They will not be so helpful if your wiring is a conduit or knob-and-tube system. FIG.1 may be the granddaddy of all house wiring diagrams of connections. The letters on the cables in it are my code for the main ways that 2-wire (flat, black-white) and 3-wire (round, black-white-red) cables and their wires may be functioning in any electrical box in a home. The Cable-and-Wire Functions Chart above explains the meaning of these code letters.
The big blue wirenut loaded with black wires distributes electrical "hotness" from the black of one of the p-cables ("p" is for "power"). This p-cable is bringing the circuit's hot and neutral to this 9-gang (!) switch box. Which p-cable is doing this doesn't matter for our purposes at the moment. That black wire's hotness gets extended to a lot of other black wires by virtue of their all being in contact in that big wirenut. Likewise "neutralness" is spread to several other white wires by that p-cable's white. That cable is coming from somewhere earlier in the circuit and so, ultimately from a circuit breaker in the electrical panel.
The two other p-cables are passing the circuit (constant hotness and neutralness) on to other places/boxes downstream -- further along. One might be going to another switch box and the other to an outlet or light box. Actually if you look carefully there is one other cable that takes hotness from the big wirenut without passing through any switch... Yes, cable-f.
3- Cable-f's red wire is getting switched by switch-3. Whatever it is that is getting switched -- a light or outlet -- is also getting a constantly hot black wire, for whatever reason. Maybe a light's box is just going to pass that hotness on to an outlet beyond it. The red is fed with the hot and neutral (=power) wires.
1 & 2- OK, that leaves "L" and "h" and "m," on the left side of this box. That's enough to deal with at the moment. Cable-"L" means it goes to switch a light or plugged-in lamp. Its black is being switched on or off by switch-1. Switch-2 is pretty simple too and was the basic way of switching house lighting through the 1960s. But since then, the switch-1 arrangement -- where the neutral wire goes with the switched hot up to the light -- has become more common. Then what is the white wire going out in cable-h from switch-2? It is a constant-hot coming to our box from the light or switched-outlet box. Why not just use the hot available out of our big box? Who knows? People wire in various ways. And why do I call this wire usage "h"? For the white wire of a 2-wire cable being hot. Some call this cable function a switch loop.
4 & 5- Cable-m, having the extra red wire, is able to go to switch two things, each having its own switch. M is for "multiple." An example would be a fan/light, whether it be a ceiling "paddle" fan with a light on its bottom, or a bathroom exhaust unit that includes a light. Can such things share a single neutral wire? Yes, and if you think about it, the various things on a given circuit are usually all sharing its one neutral that lands in the main panel.
As we seem to be going from left to right, the rest of the switches are different. They are not "single-pole" switches, inside which a single contact point makes or breaks the connection between the two wires. They are three 3-way switches and one 4-way switch. When they break connection from a second wire, they make connection to a third, generally. See 3-way switches. Of these next switches, only switch-7 seems to be using the circuit's hot. Are the others running from other circuits? Well, they could be, but let's say they are not. Since they are all working in conjunction with partner switches from the other side of the room or down the hall, those locations may have our circuit's hot available at them, and we only want it hooked up at one end of a pair of 3-ways switches.
7- So what about switch-7? It is being the "hot end" switch, and its partner (wherever that is) will be the "leg end" -- finally giving or withholding hotness from the light they control. The white wire of cable-7 is not a neutral; it is a sometimes hot "traveler" -- hot or not, according to the switch-handle's position. Like switch-2, 7's light must be getting the neutral it needs from elsewhere.
6- Backtrack to switch-6, perhaps the most common wiring look of 3-way switches. Its travelers are part of a 3-wire cable ("t" -- for threeway), which is more commonly the case for 3-way systems (in contrast to cable-x). But rather than being the hot-end of its 3-way system, switch-6 is the leg end, feeding the system's decision (hot or not hot) up to the light on nearby cable-L, along with the light's neutral, which has been brought over in cable-t from the hot-end switch (on the other side of the room).
8- If switch-2 and cable-h looked like they only had eyes for each other, so do switch-8 and cable-n. Each cable attaches only to its switch... Since cable-n's red and white are attached to same-color screws, they are travelers. Then is its black a constant hot (making this 3-way switch the hot end of the 3-way system)? It might be but I'm not saying that it is. It might be a hot or it might be the switched "leg" to the light, making this switch the system's leg end... I needed a category and code-letter for all the 3-way switch wiring methods that use 3-wire cable but do not carry a neutral in them. That's why I call switch-8's cable "n".
9- The best for last. On the far right is a 4-way switch -- switch-9. This one shows how the traveler pairs from two three-wire cables connect to the two pairs of traveler terminals on the switch. The reds would not have to be on the right side or even on the same side. The main thing is that each cable's travelers hook to a pair of same-color terminals. Are these white wires neutrals? In this case, yes, and that is why the cables are labeled "t", meaning the three-way cables that have a neutral and two traveler wires. To review how a 3-way switch system works see 3- and 4-way switches.