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To your right is a house wiring diagram of a typical U.S. or Canadian circuit, showing examples of connections in electrical boxes and at the devices mounted in them. You may prefer to print out the text plus the image.
This page takes you on a tour of the circuit. The boxes are shown as light areas. The dark background represents the area between boxes -- usually inaccessible -- where the cables containing the wires shown in the diagram run in the ceiling, wall, and floor framing of the home. As you can see, between any two boxes, either two or three wires run, corresponding to two or three-conductor cable. After you have become familiar with the circuit shown here, you can compare it directly with a housewiring diagram that follows the cables between these boxes as they might be routed in the rooms of a home; for this see Floorplan. For a GFI wiring diagram go to GFIs.
Ground wires (bare or green wires) are not shown. Connections are shown either as wires making contact with the side screw terminals of devices or as wires bundled into blue wire connectors. The diagram is not meant as a guide for doing wiring; for example, the white wires connected to some switches here should nowadays be taped differently (e.g., red or black). The diagram is more to familiarize you with what you may encounter in existing homes.
NOTE: Where two wires are shown as contacting a single side-screw on a receptacle or switch, this is not to encourage anyone to ever put more than one wire under a screw (they are only meant to take one). Instead, it is just to indicate that the two wires do electrically connect with each other and with the terminal. But this should be done using any combination of screws, back-holes, clamps, and pigtailing with wirenuts -- all within the capacity of each option.
This circuit starts with A4 receiving hot (black) and neutral (white) wires from the main electrical panel -- imagine the panel to the left. A4 passes hots and neutrals to nearby receptacles A3 and A5 by means of their wires' contact with the terminals on A4. A3 and A5 are the beginnings of the two main branches of this circuit, and we can identify several sub-branches that are developed beyond them.
Looking ahead, A3 will feed a string of boxes one direction (A2, B2, A1, B1, C2) and another string through B3, C3, D3, D2, D1, C1, and B4. Meanwhile the branch at A5 provides connection out to B5, C5, D5 ,D4, and C4. It also sends power through A6 to B6, C6, and D6. A6 itself also splits power out to A7, B7, C7, and D7.
Let's follow the sub-branch that goes up from A3. A3 needs to get its hot and neutral connections passed on so that two lights (A1 and B1) can burn -- not all the time, but according to what the switches at A2, B2, and C2 say. A2 is a normal single-pole switch, as seen by its two side screws. According to the position of its handle, it will either let its light (A1) get the hot connection or not; the neutral connection at A2 (a wire connector) is not affected by the switch.
Meanwhile, however, A2's box needs to help the switches at B2 and C2 receive power so they can run their light (B1). These two switches (B2 and C2) are three-way switches, as seen by their 3 side screws. If either of their handles moves, it changes the hot's connectedness to the light. This particular set of 3-way switches is set up in this way: B2's switch sends hotness to C2's along either the red or the white (called "travelers") according to which gold traveler terminal B2's switch internally passes its hotness on to from the black terminal. Similarly, the black at C2 will be made hot or not, depending on which gold traveler terminal the switch internally connects it to. This black wire (the "light leg") is the one whose hotness or unhotness will let the light burn or not, so back at B2's box it must be bundled with the black that goes to B1, where the light is. If your house is newer you may find that the white from B2 to C2 has been colored to show that it is not a neutral, and you may also find a white neutral as a fourth wire going from the other whites in B2 to C2; this is in case a fancy switch that needs a neutral is installed at C2.
Let's go back now to the other sub-branch that goes out from A3. It will run a light (B3) and three receptacles (D1, D2, and D3). The light will be controlled by switch B4, D2 and D3 by switch C3, and D1 by switch C1 (switched receptacles!). Here's how. A3 feeds power to the B3 box, where the light gets its neutral connection immediately; but the light's hotness will depend on switch B4, once B4 has received constant hotness from B3 by a wire connector at B3, which also sends hotness on to the rest of this sub circuit on the black going to C3. Don't rush on. And, yes, a white wire is being used as a hot down to B4; that's the way a cable of two wires comes -- black and white. For some time now this white should be colored differently when installed. In fact wiring done under the 2011 NEC code should use 3- not 2-conductor cable to provide a neutral for possible special switches. In that case the switched wire (black here) would be red, the hot wire (white here) would be black, and the neutral (none here) would be white -- connecting with the other whites at B3 but to nothing at B4 unless the type of switch called for a neutral.
Now, C3 has received its hot, and the neutral bundle at B3 has also given C3 its neutral. C3's switch is mainly going to switch the left half of the receptacles at D2 and D3. So the neutral simply ties through in C3's box to reach D3, D2, and D1. But the hot at C3 must also tie through (besides giving hotness to one end of the C3 switch) in order for the right half of these receptacles to be able to run things plugged into them all the time, regardless of what any switches are doing to the left side. The two halves of these receptacles have to be isolated from each other (on the hot side only!) by breaking off the metal tabs that normally join the halves electrically. Compare the look of the gold terminals in D3, D2, and D1 with the other receptacles on the page.
The red wire from C3 to D3 is hot or not, according to the switch C3, and this switchedness is passed on to D2's left side, as is D3's constant hot passed on to the right side of D2.
D1's left side is controlled by a different switch -- at C1. D1 has received neutral and constant hot from D2, but for C1 to switch D2's left side, the white wire from D1 to C1 must take hotness to C1, so the C1 switch can send hotness or deadness back to D1's left side. The hot-side terminal tab of D1 must be broken away to prevent the left side from being hot all the time (unswitchable).
We're over half-way through! Next see how A5 sends power out to run lights B5 and D5. A5's neutral goes to connect directly to B5's light, but to reach light D5 it must go the same path as the other wires in its cable, which want to relate light B5 to switch C5 and want to pass a constant hot on to switches C4 and D4, so those can end up switching D5. So the neutral from B5 ties through box C5 to give D5 its neutral connection. No switched light (like B5 or D5) personally needs a constant hot in its box, but since it was convenient for the cable guy to run A5's power toward D5 by way of box B5, the hots tie through at B5 and at a terminal on C5's switch and at D5 itself, so as to run through the 3-way switches, C4 and D4, which finally determine the hotness of the black wire from D4 to light D5.
Back at B5, hotness for the light comes back (or not) from switch C5 on the red. At box D5 hotness is sent to C4, first via a white wire to D4 and then from there to C4 via a black. Switch C4 passes hotness to one of the travelers (red or white) that go back to switch D4, which in turn passes or does not pass such hotness on to light D5 on the black wire.
If your house is newer you may find that the white from D5 to D4 and the one from D4 to C4 have been colored to show that they are not neutrals. And since the 2011 NEC code you might not even find this kind of "dead end switch" wiring coming down from the light because a true white neutral is now required to be brought to both switches in case special dimmers are installed at them.
Let's continue on another sub-branch. A6 receives good hot and neutral paths from A5. A6 will power our last two sub-branches of the whole circuit. First is the line from A6 through B6 and C6 to D6. This scenario is similar to the one just described for A5 sending power through light and switch boxes B5 and C5. In this case, however, power from A6 reaches the switchbox first, rather than the light box. Comparing the diagrams of the two switchboxes and the two light boxes, they are identical. The difference, we could say, is that now the red wire switching light C6 is "flowing" in the same direction as the other wires that extend power further out, whereas the red for light B5 "came back from" switch C5. This (row 6) sub-branch ends with receptacle D6 receiving power, with no one to send it on to, except of course to whatever is plugged into it.
The last section of the circuit involves A6 sending power down to a 3-way switching scheme that has yet another look. A7 and B7 tie the neutral through their boxes for lights C7 and D7 to use. The hot at A7 connects to the non-traveler terminal (called the "common"), and its hotness is passed by the switch onto either the black or red. Inside switch B7 one of these travelers is in contact with the common, so that the black from B7 to the lights will be hot or not. Finally, light C7, besides attaching to the incoming black and white itself is involved by its wire connectors in passing this same switched power on to its friend D7.
To see how the entire circuit might have served part of a house see:
1. Beginner. The circuit in the diagram was working fine, until one day nothing in rows 1, 2, or 3 worked anymore. A tester registered no hotness in these three rows at any of the terminals, wire connectors, or bare-copper ends of the wires themselves. But everything in rows 4 through 7 still worked, other than B4 of course. Testing was not done on the working rows. I will tell you that there is a poor connection or contact somewhere. Exactly where is it?
2. Intermediate. The beginner solved the problem above, but a year later something else went wrong. Maybe a beginner made all the connections in this circuit. This time light D5 stopped working. Everything else still worked. No, the bulb was fine. And testing revealed that hotness was getting everywhere it should and that all the switches were doing their jobs. Which connection is bad, or if there is more than one possible place, how many and where?
3. Advanced. The intermediate solver traced down the problem and fixed it, but another year later (most homes don't suffer this badly), a final problem showed up. If you solve this one, your home will not bother having any more problems, because it will know it has no chance of fooling you. One day light B1 stopped working, no matter what positions switches B2 and C2 were in. You found the bulb was not the problem, but your courage was low about looking into the three-way switches, so you hired a handyman to fix things. He told you -- correctly -- that the neutrals were all good, hotness was getting through switch B2 properly to its traveler terminals, and that the wire connectors for the blacks in B1 and B2 were connecting the blacks well. Having checked these things, and still finding that no combination of switching by the two switches turned the light on, he assumed -- incorrectly -- that switch C2 was defective inside and replaced it. All three wires he connected to the switch terminals were solidly connected, and the light was shining when he left. Then you found that when B2 is in a particular position, C2 can't make the light go on. I will tell you that only one thing was wrong to begin with, and it is something else that is the problem now-- something he did. What was the problem, and what sort of thing is the problem now?ANSWERS
1. The bad connection has to be at where the black wire from A4 to A3 is connecting at A4. That is what would account for no hotness registering anywhere in rows 1-3. If the bad spot were at the A3 end of the same wire, that end of the wire would itself show hotness, even though the screw it was supposed to connect to would not.
2. If hotness was getting everywhere, the bad connection must be a bad neutral (white). Since everything worked before D5 on the circuit, the bad neutral could only be at three possible points: a.) At D5's own wire connector for whites; b.) At the white-wire connector in C5, which wouldn't bother that switch's operation; or c.) At the white-wire connector at B5 (the light at B5 could itself have a good white connection, but the white that goes from B5 to C5 might not).
3. Based on his tests, the handyman was right to figure something was wrong at C2 but wrong to assume that meant the switch was bad internally. That could have been true, but since I said he was wrong, you therefore know that the only other possibility would be the poor quality of C2's black-wire connection to the switch's "common" terminal. Since he only worked on changing C2's switch, he must have hooked the new one up wrong -- with the black on one of the traveler terminals (we don't know which one).