preface The simple way to control a light or other electrical device from two locations is to use a pair of three-way switches. Of course you know how this works: If both switches are up or both are down, the light is off. If one is up and the other down, the light is on. (Unless one of them is upside-down.) The wiring is not difficult, but it is not obvious to one who has not encountered it. Three-way switches are so-called because they have three terminals. Its action is "single pole, double throw" (SPDT), although that terminology is not often used in reference to house wiring. Four-way switches have four terminals. Four-way switches are specialized double-pole devices, but cannot accurately be called either single-throw or double-throw. (A double-pole, double-throw (DPDT) switch has six terminals, and could be wired to act as a four-way switch, but this isn't done in house wiring.) In this discussion, I use the term "neutral" conductor, which is the white wire. In a two-wire 120-volt circuit, both conductors carry current -- there is no neutral. However, common usage is to call the white wire the "neutral" (the official term is "grounded conductor"). I also use the term "grounding" conductor, which is the green or bare wire often called the "ground wire". As with all the pages on this web site, this one is informational, and it isn't DIY (Do-It-Yourself) advice. If one is looking on web pages or news groups for wiring advice, one might not be ready just yet. If one insists upon doing it, this should help. diagrams  Figure 1 -- The Plan This is a schematic diagram of a light fixture controlled by two three-way switches. The "hot" wire extends to the left, going off to the circuit breaker. The "neutral" conductor is shown going off to the right, back to the neutral bus at the panel. The current path is closed when both switches connect to the same conductor. Change one switch, and the path is opened. Change it again, or change the other switch, and the path is closed. No matter how the wiring runs, the electrical scheme is the same as this.  Figure 2 -- Plan A
Of course, real life is often more complicated than the plan. Here is a typical installation made with type NM electrical cable. In this case, power from the circuit breaker panel runs first to the light fixture wiring box. The white conductor goes to the white terminal of the fixture, and the black "hot" wire goes to the switch, as one would expect. But notice that the black "hot" conductor is spliced to a white conductor in the fixture box! (Splices are shown as black squares.) Until 1990, there was an explicit exception in the National Electrical Code allowing for this situation, because two-conductor cable always has one white and one black wire and a bare grounding wire (shown here in green). In new work, this white conductor must be identified with tape or paint at each termination or splice. If this were being done with conduit, the electrician would pull two colored wires, and not a white wire. Getting from the middle box to the left-hand box requires three conductors, plus ground. In this cable, the three conductors are white, black, and red. As for the bare "grounding" wire, it makes no difference what kind of circuit is being made up. The ground conductor simply connects each device box to the ground at the circuit breaker panel. (The green dots are connections to the metal box, or the switch or fixture in the case of a non-metal box.) It may sometimes seem more convenient to allow the conductors, shown here bundled into cables, to take separate routes, but that is not allowed. It is important that current flowing in one direction be balanced by current flowing in the opposite direction in a pair of closely-spaced parallel conductors. This has to do with mutual inductance reducing the impedance of the wiring, which reduces power loss and voltage drop, as well as stray magnetic fields.  Figure 3 -- Plan B
Plan B still creates the circuit of Figure 1, and is about the same complexity as Plan A. The difference is that the power runs first to the left-hand switch. The choice is one of convenience. Notice that the white conductor is the expected "neutral", and not a "hot" wire in this configuration. The bare "grounding" conductor works the same as in Plan A.  Figure 4 -- Plan C
Plan C is not much harder to figure out, but it is harder to recognize in place. Notice the seven current carrying wires and three ground conductors entering the middle box. Be certain to mark the wires and take notes before disconnecting the wiring! Here again, we have the black "hot" wire connected to a white wire in the middle box to form part of a switch loop.  Figure 5 -- Plan D
In this case, power runs to the fixture box, and switch wiring runs in two directions from it. Going to the left, is a switch loop with the black "hot" conductor connected to the white conductor. To the right, is the same type of cable, but it is not a switch loop, so the white conductor must be marked with paint or colored tape at both ends to show that it is not a "neutral" conductor. There are eight current-carrying conductors and three grounding conductors entering the light fixture wiring box.  Figure 6 -- Four-Way Switches
Where control from more than two places is needed, one or more four-way switches are added between the two three-way switches. A four-way switch has four terminals, arranged as two pairs. With the switch handle in one position, the first pair is connected to the second pair as shown at "C". With the switch handle in the other position the connections are "crossed" as shown at "B". As many four-way switches may be inserted as are needed. Changing the position of any one of the three-way or four-way switches in this circuit will change the state of the light from off to on, or from on to off. notes on replacing a three-way switch before you take it apart While it is always possible to identify the conductors sticking out of the box, it is far easier to do so before disconnecting the switch. Write down which conductors connect to the ground, common, and switched terminals. Make a drawing, especially if the wiring looks complicated. Where multiple wires are the same color, make small "flags" of different colored electrical tape, or hand-written labels with masking tape. (Masking tape should be removed when you are finished because it is flammable.) If there are other devices in the box, disconnect only one device at a time. three-way switch terminals  This photograph shows a typical three-way switch. At the left of the illustration is the schematic symbol, the same symbol used in the diagrams above.
Connections to the two switched terminals should be evident. This switch has a screw terminal and a push-in connector for each switched contact. The screw and push-in are connected together, so either may be used. The common terminal is distinguished from the other two by the dark color of the screw, and by the word "common" stamped on the body of the switch near the two common push-in connectors. The two push-in connectors and the common screw are all connected together. Any of them can be used for the common connection. On some models, a green grounding screw is included across from the common screw, which sometimes causes confusion. The grounding screw is never associated with a push-in connector. Both push-in connectors are for the common connection. Some models have the grounding screw attached to the mounting strap, instead of on the body. Other models do not have a grounding screw (these are for mounting on a metal box so that the strap is grounded to the box.).
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