7 Things to Know About 3-Way Switch Wiring

A 3-way switch makes it easy to turn on a light fixture from two separate locations in the home. Wiring one is slightly different from wiring a single-pole switch.

3 way switch wiring

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In a standard, single-pole switch, one light switch controls one light fixture—on/off. Things get a bit trickier with 3-way switch wiring, in which there are two switches and one light. However, when you understand how electricity travels in this type of circuit, it all starts to make sense.

While installing new electrical wiring should almost always be done by a licensed electrician, DIYers can often successfully replace old switches with new switches—if local codes allow. Replacing a 3-way switch is a straightforward task, but it’s more complex than replacing a single-pole switch. Anyone preparing to replace a 3-way switch should have a working knowledge of switch wiring. If you’re not comfortable replacing a switch, call an electrician.

Safety is always the top consideration when doing any wiring. The electricity must be turned off at the breaker box, and a voltage tester should be used to test the wires in a switch box to make sure they’re not hot before going any further.

1. 3-way switch wiring enables you to turn on/off a light from two different locations.

It’s not pleasant to come home late at night and stumble to the other side of a dark room to find the light switch. That’s where 3-way switches are handy—they allow users to turn on a centrally located light from different sides of a room or from the upper and lower ends of a stairway. A 3-way switch setup will even work with a dimmer switch, as long as the dimmer switch is designed for 3-way wiring.

Individual 3-way switches resemble single-pole switches. However, they are not labeled “OFF” or “ON,” because they either allow or stop the electrical current based on the other switch position in the setup. In essence, a 3-way switch is a toggle switch.

2. Two different types of wire cable are used in a 3-way setup.

Two different types of wire cables are used in wiring a standard 3-way switch, most often 14/2 cable and 14/3 cable. The 14 stands for the gauge of wire (rated for 15-amp circuits) and the following number, 2 or 3, represents the number of conductor wires in the cable. The number of conductor wires in the different cables are important, because an extra wire is required in one section of the 3-way switch setup. Without a 14/3 wire, it wouldn’t be possible to have both switches control the light.

how to wire a 3 way switch wire detail

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Some homes may have 12-gauge wire rather than 14-gauge wire, which just means the wire is rated to carry more amps. The 12-gauge can carry 20 amps. Houses built since the mid-1960s likely contain non-metallic sheathed cable (NM), commonly called Romex, after a popular brand of wire.

3. Each wire has its own purpose.

14/2 NM Cable contains two conductor wires: one black and one white. It also contains a third, bare copper wire. The cable runs from the power source to the first switch box in the typical 3-way setup described here, but other wiring configurations also are possible (see below). The following wire colors are standard, but different wire brands can use different colored wires.

  • Black wire: This is a hot wire that carries electricity from the power source to the first switch in a typical 3-way setup. It’s also called the “common wire” or the “line wire.” Unless the breaker is off, this black wire is always hot.
  • White wire: This is the neutral wire, and its purpose is to complete the electrical circuit. In all electrical circuits, power must return to the energy source, and this is the job of a neutral wire.
  • Ground wire: The ground wire is a bare copper wire or a green wire, and its purpose is to provide a measure of safety. When the circuit is operating correctly, the ground wire doesn’t carry any electricity. If a problem occurs, such as a short circuit, the ground wire transfers excess electricity to the ground (the earth).

14/3 NM Cable contains a bare copper wire and three conductor wires: one black, one white, one red. In a typical 3-way setup, the 14/3 cable runs from the first switch box to the second switch box.

  • Black wire: A black wire is a hot wire, but also a traveler wire. In a 3-way setup, the black wire (along with the red wire) is a traveler wire. This is because power travels from one switch box to the other through both wires, but only through one wire at a time and is determined by the configuration of the toggle switches.
  • Red wire: The second hot/traveler wire is the red wire that serves the same purpose as the black wire between the two switch boxes. Depending on toggle switch configuration, either the red wire or the black wire will be hot if the light is on, but not both.
  • White wire: Still considered the neutral wire, the white wire from the 14/3 cable serves to carry electricity back to the power source to complete the circuit.
  • Ground wire: This wire also serves the same purpose as it did in the 14/2 cable, to carry excess electricity to the ground in the case of a short or a fault.

4. Each screw terminal has its own purpose.

A standard 3-way switch features four terminals, each represented by a colored screw. The screw location is often similar from switch to switch. Some manufacturers locate the screw terminals in different spots, so be sure to study the diagram that comes with the switch.

3 way switch wiring

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  • Black screw terminal: The black (or darkest in color) screw attaches to the black common wire from the 14/2 cable. The terminal may be labeled COM.
  • Green screw terminal: The green screw is the ground terminal. Two ground wires will be found in both switch boxes, one from the 14/2 cable and one from the 14/3 cable. Both of those ground wires should be connected to one another, and then connected to the green screw in each box.
  • Two traveler terminals: In addition to the black and green terminals, there are two other screw terminals that are often brass. These are the traveler terminals. It doesn’t matter which traveler wire (red or black) connects to which traveler terminal as long as it’s the same in both switch boxes. For example, if the red traveler wire is on the top traveler terminal in the first box, it should also be on the top traveler terminal in the second box.

5. Neutral wires connect to one another, not to the switches.

In a typical 3-way switch, the white neutral wires do not connect to the actual switches. Instead, they connect to one another. This creates an uninterrupted return circuit to the power source, which is generally a bus bar terminal on the breaker panel.

Neutral wires can be connected by twisting both wires together in each switch box, but today’s lever nut connectors make it so much simpler. Wire connectors, such as Aigreat’s Lever Nut Connectors, work by lifting a lever, inserting the end of the wire, and then pushing the lever back down to lock the wire in place.

6. In wiring a 3-way switch, there are always two traveler wires that connect one switch to the other.

For a 3-way switch setup to work, electricity must be routed through either one traveler wire or the other, and the route depends on whether the toggle switch is in the up or down position.

When the first switch turns the light on, the electrical current runs through one of the traveler wires. However, if the second switch is used to turn the light off, then the current will run through the other traveler wire when the first switch turns the light on. Think of how a conductor switches a moving train from one track to another—that’s how a 3-way switch routes the electricity along either the red or the black traveler wires so both switches can control the light.

3 way switch wiring

Photo: istockphoto.com

7. The 3-way switch may have a different wiring configuration.

While the wiring configuration described above is typical, it isn’t the only way to wire a 3-way switch. The configuration is determined by where the power enters the circuit (at a switch or at the light fixture). Alternate wiring configurations should only be done by an electrician.

If the DIYer opens a switch box to find a white wire with black tape on it—the white wire is hot. This doesn’t mean replacing the switch requires a pro, because replacing a 3-way switch does not require running new wire–it’s simply a matter of disconnecting one switch and connecting the new one.

No matter the wiring configuration, the simplest way to successfully replace an old 3-way switch with a new 3-way switch is to label each wire with the terminal that it’s connected to before removing the wires from the old switch. Then, it’s a simple process to connect the correct wires to the correct terminals of the new switch.

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