How to Wire a 3 Way Light Switch

Here is a detailed explanation of one of the hardest electrical circuits to figure out. This is one of the easiest ways to do 3-way light switch wiring. Be sure to check the "Tips" section to see several other accepted ways to wire this circuit.

Steps

1. 1
   Select the proper wire size. The size of all the wires in any circuit must be the same size. If originating from an electric panel or fuse box, #12 copper is the minimum size when connecting to a 20 amp circuit breaker or fuse; #14 copper is the minimum size when connecting to a 15 amp circuit breaker or fuse (it has not been permissible to use aluminum wire on circuits of these capacities for many years). If obtaining power from a nearby electrical outlet, etc.; the new wiring must be the same size as those that supply the outlet or other circuit device.
   
   
2. 2
   Select the proper cable type. The power supply or feed cable should be "2 wire" (or conductor) cable plus a ground wire. Common cable types with these features are:
   
   
   • NM (often called "Romex") and UF type cables (both have plastic jacket wrapped around 2 [or more] insulated wires - one white, one black and others - and a single uninsulated wire), NM is used indoors and UF is used outdoors when subjected to sunlight or when buried underground.
   • BX, MC and AC types are armored cables. They are very similar with only slight variations (construction consists of a metal jacket formed by an interlocking, helically wound band of steel or aluminum around two [or more] insulated wires - one white, one black and others possibly including one green). Cables lacking a green insulated utilize the outer metal jacket as the ground conductor. None of these metal jacketed cables types are permitted to be installed outside or underground. If the power source is from an armored type cable that lacks a full size (#12 or #14) ground wire, a metal box must be used to extend the ground from the armored shell to the box and to the circuit's ground wire via special green, hex headed grounding machine screw that is threaded into the pre-tapped hole in the metal box, or special green ground clip.
   • All of these cables have "trade names" which are basically derived from the number of insulated non-ground conductors and construction type; such as: "twelve two Romex" or "fourteen three BX". A 12/2 NM, BX, AC, or Romex cable has two #12 conductors, plus a full sized #12 ground. A 14/3 NM, BX, AC or Romex cable has three #14 conductors , plus a full sized #14 ground. There are special connectors for armored cables and unique precautions and instructions for each. Use of the similar in appearance Romex connector on an armored cable is not permissible. NM or Romex type cable is easier to work, requires no special tools or preparation and costs less to purchase. Because of these reasons, it enjoys widespread use.
3. 3
   Shut power off. This is a very important step. Please do not skip it.
   
   
4. 4
   Install a 2 wire cable between the power source (outlet box, electric panel, etc.) to the first switch box. Leave 8 - 10 inches of wire inside both boxes (source and first switch) before cutting the cable to facilitate easy connection to the switch and power source. Add the ground wire to the circuit ground wire(s) with a wirenut connection. If the power source is an electrical panel or fuse box, the cable should be cut at least long enough to reach the furthest termination point (breaker or fuse, ground and neutral bar) without the need of splicing. The ground wire should be connected to either 1) the neutral terminal bar or, 2) the ground terminal bar (but only if equipped with a separate ground bar). If all the existing ground wires connect to one bar and all the existing white wires connect to a different bar, maintaining separate ground and neutral connections is required. Never introduce ground wire(s) to a terminal bar with only white or gray insulated wires connected or vice-versa. Connect the black wire to the hot supply or the circuit breaker / fuse and the white to the neutral supply or neutral terminal bar in the electrical panel.
   
   
5. 5
   Install a 3 wire cable from the first switch box to the light fixture's box. Leave 8 - 10 inches of wire inside each of the boxes before cutting the cable to facilitate easy splicing and connection to the switch and fixture. A 3 wire cable has an "extra" wire when compared to 2 wire cables, and this wire is nearly always covered in red insulation. This third wire is necessary for 3-way switch installations.
   
   
6. 6
   Install a 3 wire cable from the second switch box to the light fixture's box. Leave 8 - 10 inches of wire inside each box before cutting the cable to facilitate easy splicing and connection to the fixture.
   
   
7. 7
   Connect ground wires. All the ground wires in any electrical box or enclosure must be connected together by wire nut, lug or other approved method. Provide as many shorter lengths (8 inch) of bare or green insulated wire as needed (one for each) to this wire-nutted group to allow connection to green ground terminal screws on each device in the box (switches, outlets, fixtures, etc.). If the switch or junction box is metal, it too must be securely grounded with a green ground screw or approved grounding clip. This must be done at each and every box that a cable enters and at each device that provides a termination point for ground. It is highly recommended that these ground connections be completed first, then gently folded into the rear of the box - out of the way - leaving only the shorter ground leads out for easy connection to devices. No ground connection is made to plastic, fiber or other non-conductive box.
   
   
8. 8
   Connect feed wires in the first switch box. Connect all ground wires as previously described. The 2 wire cable feed from the power source enters the bottom of the switch box and the hot (black) wire connects to the common or shunt terminal on the 3-way switch. There is only one such terminal on a 3-way switch. This terminal is usually identified as the one having a different color terminal screw (often significantly darker) from the other two terminal screws (not counting the green ground screw). In the picture of the circuit above, the shunt or common terminal is on the lower right side both of the switches.
   
   
   • Connect the 3 wire cable's white insulated wire (neutral) directly to the 2 wire 'feed' white insulated (neutral) wire with wire nuts (there is no connection of any white wires to this switch).
9. 9
   Connect 3 wire cable in the first switch box. The 3 wire cable enters through the top of the first switch box. The red insulated wire connects to either one of the 2 unused terminal screws (shown in the image above to be on the top left and right sides of both 3-way switches). It does not matter to which terminal screw this wire is connected.
   
   
   • Connect the black insulated wire to the remaining unused terminal screw on the switch.
10. 10
    Connect wires in the light fixture's box. Connect all ground wires as previously described, if not already completed. In the light fixture's box, there will be two 3 wire cables. One 3 wire cable originates at the first switch box and has its white insulated wire as a neutral; the other 3 wire cable originates at the second switch box and its white will become what is called the "switch leg".
    
    
    • Identify the switch leg. Mark both ends of the white wire in the 3 wire cable that was installed between the second switch and the light fixture box by wrapping it with black electrical tape. This alerts others working on the circuit later that the white wire is no longer neutral. This is a newer electrical code requirement, but has been a common practice whenever a white or gray wire is / or can become hot. Since this happens most often at switches, it got the name "switch leg".
    • Connect the two red insulated wires together with a wire nut.
    • Connect the black wire coming from the first 3-way switch and the white "switch leg" wire coming from the second 3-way switch (has black tape wrapped around it) together with a wire nut.
11. 11
    Connect 3 wire cable in the second switch box to the switch. Connect all ground wires as previously described, if not already completed. Connect the black insulated wire to the shunt or common terminal screw of the switch (once again, the common screw terminal is the different colored screw from the rest of those on the switch).
    
    
    • Connect the red insulated wire to one of the two unused terminal screws (it does not matter which).
    • Connect the "switch leg" (white wire with black tape) to the remaining unused terminal screw on the switch.
12. 12
    Connect the fixture. The light fixture's box should have only a black wire, a white wire and a ground wire to supply power to the fixture.
    
    
13. 13
    Finish up. Check all wire nuts for tightness and exposed neutral and hot conductor surfaces. Fold all wires carefully into the boxes and secure devices and fixture with screws. Install plates and covers. Restore power and test.