How To Wire A 3 Way Switch - Wiring Diagram

How to wire a three way light switch is not a particularly difficult task - there are only 3 connections to be made, after all. Making them at the proper place is a little more difficult, but still within the capabilities of most homeowners - that's where understanding a wiring diagram can be of help.

To replace a switch is not difficult at all; simply put the wires back on the new light switch in the same relative position. The problems arise when a new switch is being added, or the handyman forgets which wire went where. Then it is necessary to understand a little more just how a three way switch works, and perhaps how to read and understand a wiring diagram.

There are many possibilities for methods of getting the proper wire to the proper place but in every case the actual wiring has the same effect and uses the same concept - if that effect and concept is known the task becomes much simpler. This article is designed to explain these things for a better understanding of how to wire a 3 way switch, and wiring diagrams are shown for common wiring methods for these switches.

4 way switches are often used in conjunction with three way switches; whenever more than 2 switches are used to control a single light fixture the additional switches are of the 4 way variety. Additional information is available in the article written on how to wire a 4 way switch, with additional wiring diagrams for these switches.

The photos to the right show the back of a 3 way switch (click on a photo to enlarge it). There are 3 screw terminals on the sides of the switch, along with one on the end, and every switch will have the same three terminals. The small screw terminal on the end is the ground terminal and is usually painted green, although the picture does not show that color well. It can often be recognized as the screw that is part of the metal framework of the switch and is not insulated from other metal parts. The green, or bare of insulation, ground wire always goes to this terminal. Older switches often did not have this ground screw, but are no longer legal to use and all current light switches must have a ground screw to attach the ground wire to.

One of the three other terminals is a different color, usually darker, and is called the common terminal. Mechanically and electrically this common terminal is connected internally to one of the other two brass screws, called the traveler terminals. When the switch is flipped the other way that connection is broken, and the common terminal is then connected to the other traveler terminal. This common terminal is always connected internally to one (but only one) of the traveler terminals; which one is dependent on whether the switch is up or down.

It should perhaps be noted that the traveler terminals are essentially interchangeable. Given that each one is to have a traveler wire attached to it, and there are two traveler wires and terminals, it doesn't matter which traveler wire goes to which traveler terminal.

The terms "traveler" and "common" have already been explained, but there are other terms that will be used in this article that also need some explanation.

• Cable. The term "cable" refers to an assembly of two or more wires, bundled together, usually in a sheath of insulating material. Each wire is insulated separately, with the possible exception of the ground wire. This wire may be insulated with a green color or left bare, without insulation.
• Power in. The power in cable is that cable that eventually ends in the circuit breaker panel, or fuse box. It is the cable that provides the power to the lighting system.
• Neutral. This is the white wire contained in the power in cable. It does not terminate at any switch, although it may be present in a switch box and spliced straight through the box.
• Ground. The grounded wire in each switch or light fixture box. It is either colored green or left bare of insulation
• Hot wire. This is the second, black wire, contained in the "power in" cable. It is "hot" at all times unless the entire circuit is turned off at the circuit breaker panel
• Circuit breaker panel. Commonly called a "fuse box" it may contain either circuit breakers or fuses. This panel is where all the power in the building is derived, and where that power may be shut off.
• Two rope. Two rope is the designation given to a cable that has two individual wires, plus a ground wire. These wires will be colored white and black, with a green or bare ground.
• Three rope. Three rope is a cable with three wires, plus a ground. Normally the colors are white, black and red with an additional green or bare ground.

Each of the wiring diagrams used in this article can be expanded to full size by clicking the mouse on them. Please do so as they are very difficult to follow otherwise.

Each diagram will show the two 3 way switches (but not the wall box they are contained in), the various cables and wires used in the configuration being discussed, the light box and light fixture.

To understand the wiring diagram, it must be realized that the electrical current enters the system on the black wire in the power in cable, passes through the switches, through the light fixture, and returns to the white wire in the power in cable. If the circuit is broken anywhere (a switch turned the wrong way, a broken wire, or a bad light bulb) the current will not flow and the light bulb will not light. For discussion purposes, each 3 way switch will be considered to have the common terminal connected to the right hand traveler terminal when in the "up" position and connected to the left hand terminal when in the "down" position. This is not necessarily true, and the convention is only for discussion purposes.

Read the descriptions carefully, and compare them to the diagrams to understand the diagrams. Each diagram will have a description of how the current travels in order to light the lamp.

Once the correct location of each wire is determined, using the wiring diagrams below, the light switch is connected to proper wires and installed in the light switch box. Make sure the power is off before making any connections!

Many residential light switches have a small hole in the back of the switch that wires can be pushed into, and all switches have the screws on the side as shown above. The picture of the older switch above has both the push in holes and screws; the other is an expensive switch that has holes to insert wire but the screws must be tightened as well. Many switches have only the screws, with no holes. There is a "strip gage" on the back of the switch; it shows how much insulation is to be stripped off if the push in method of connection is to be used. If the screws are to be used, a little more insulation needs removed.

If the screws are to be used for connection, bend the end of the stripped wire into a half circle, using needle nose pliers, and wrap the wire around the screw in the clockwise direction. Tighten each screw firmly. Fold the wires neatly back into the wall box and push the switch into the box. Normally the ground screw goes down, toward the floor, but it can be inserted in the up position with 3 way and 4 way switches.

In this example the power in cable enters the light box. This method of running the wire is commonly found when several light fixtures are on one common breaker, and switches for the one in question are both on the same wall. Cables need to be run into the light box, between the two switches, and from the light box to just one of the switches.

Lets follow the current as it lights the lamp in the light fixture. The current enters the light box on a black wire, as it always does. That wire is spliced to a white wire in a two rope cable that goes to the first switch box (not the switch) where it is spliced to the white wire in a three rope cable and continues on to the second switch, at the common terminal. If the switch is up (remember our assumption above?) it will exit the switch on the right hand traveler terminal and continue on the red wire back to the traveler terminal on the first switch. If that switch is also up it will exit that switch from the common terminal on the black wire in the two rope cable from the light switch. Continuing down that black wire it enters the light box, where it goes to the light fixture. The current will pass through the light, exiting the light on the white, neutral, wire and return to the power in cable.

A note about wire colors. The National Electric Code requires that every neutral wire be colored white, and that ground wires be colored green. Only neutral wires may be white in color, but the code makes an exception for white wires in a cable that are not being used for a neutral. These wires (in our example the white wire from the light box to the switch box and from that box to the second switch box) should be colored black (or some other color), using a magic marker or some other method. Many electricians will do this, but many will not, and it can make trouble shooting in the future difficult and can be a safety hazard to anyone else working on the system. I encourage you to take the few seconds necessary to color these non-neutral wires. In this example the only neutral wires are the white wire in the "power in" cable (which is always a white wire) and one of the two wires attached to the light (also always white). All other white wires should be colored.

In addition, the colors shown in these wiring diagrams are common color usages only. Not all electricians use the same color code (except for neutrals and grounds), so the wires could be different colors.

In this 3 way switch wiring diagram the power in line again enters the light box, but 3 rope cables are then installed between the light box and each switch box.  This method might be used when power is available in the ceiling but switch boxes are on opposite walls - it is often easier to run the cable up into the ceiling to the light box instead of between switches.

If the current is again followed, it comes into the light box on the black wire, and to the common terminal on one switch using a (colored) white wire.  Exiting the switch from a traveler terminal it then returns to the light box, but is merely spliced to another wire that goes to a traveler terminal on the second switch. It goes through the switch, again exiting from the common terminal, and once more enters the light box where it goes to the light itself.  The neutral once more goes from the power in cable directly to the light fixture.

This time the electrician has brought power into the first switch, through the second switch and on to the light fixture.  This is a reasonable method for cases with multiple switches in the same box, as other switches then have power available and can operate other lights without having to have a separate power in line run to them.

The major difference here is that the neutral from the power in line has to be taken to the light fixture via the 3 rope.  The white wire must be used here as code requires that all neutral wires be white.

Following the current, it enters the first switch box on the black wire and is connected to the common terminal.  If the switch is in the "down" position it exits the switch on the red wire, entering the second switch at a traveler terminal.  If that switch is also down it exits that switch on the black, common, wire and continues to the light.  After passing through the light fixture the current returns to the second switch box on the white wire, is spliced to another white wire in the 3 rope used between switch boxes and continues to the first switch box where it is spliced to the white power in wire and back to the fuse box.  The circuit is complete and the lamp will light. 

This example shows the power in cable once more entering the first switch box, along with the cable to the light fixture.  This can result in a lot of wires in this box, but can be helpful when the light is near the first switch box.  A larger box may be necessary to contain all the wires.

Following the current for a last time, it enters the switch box on the black wire at the common terminal.  If the switch is up it will exit the box on the red traveler wire and continue to the traveler terminal at the second switch.  If that switch is also up it will exit the switch at the common terminal on the white (colored) wire and return to the first switch box where it is spliced to the black wire in the 2 rope going to the light.  Passing through the lamp, it returns on the white (neutral) wire to the first switch box, is spliced to the white (neutral) wire returning to the fuse box.  Once more the circuit is complete and the lamp lights up.

• Common to all of these wiring diagrams is that the neutral, white, wire from the lamp connects directly to the white, neutral, wire from the power in cable without ever terminating on a switch.  It may or may not be spliced to another white wire in a box, but never terminates on a switch - only on the light fixture.
  

• The power in, black, wire always goes to the common on a switch, often "changing colors" through the necessity of splicing to different cables. No matter what color, one switch will have a common terminal connected directly to the power in black wire.

• The other common terminal on the other switch always goes directly (although perhaps again spliced) to the light fixture. It does not terminate on the other switch.

• There are two traveler wires; they always go directly from one switch to the other. Neither traveler wire ever terminates at the light fixture, the power in cable, or on anything but a traveler terminal, although it may splice to a different cable somewhere.
• Neutral wires are always white, and white wires not connected to the white power in wire should be colored some other color.
• Ground wires are always green or bare of insulation. Each switch, as well as the light fixture, must have a ground wire terminated to it. The only exception is older homes that do not have ground wires in the boxes; if there is a ground wire in the box it must be terminated on the switch and light.
  

Switches in general are not difficult to replace or install, and most homeowners are quite capable of replacing a light switch. Those people adding a new light fixture, with associated 3 way switches, have hopefully found this article useful and informative.

Any of the different methods of wiring a 3 way switch shown here can be used interchangeably; they merely indicate different ways to run the necessary cables. Actually wiring the switches is always the same, the different methods simply can result in easier or cheaper ways to install the cabling necessary.

Regardless of whether you are replacing a switch or installing new switches in a major room remodel, probably the most useful tool you might own is a non contact AC voltage detector. Make sure that whenever doing any kind of electrical work that a good voltage detector is available - it can save a lot of grief.