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Repair a Two Way and Three Way Switch using a Wiring Diagram

 
 
Dear Mr. Electrician: I have a set of Three-Way Switches in my house that have worked for many years without problems.  Now I find that one switch has to stay in the up position at all times just so the other switch will turn on the light.  How do I diagnose and fix this?

Answer
: In some countries these are called two way switches.  I am guessing that one of your three-way switches broke down.  Due to multiple wires being hot in this particular type of switch wiring, it can be a little tricky for an amateur to diagnose which switch has died.  I suggest that you change one switch.  If that doesn't fix it, then change the other one.  It's a good idea to replace both switches at the same time anyway as the other one could fail soon after.  Be very careful to identify the LINE and LOAD wires BEFORE you disconnect the switches.

The LINE wire is usually the easiest to identify because it is hot at all times.  It should be terminated on a copper or black screw on the 3-way switch.  A Voltage Tester is good for testing the live or dead wires.

All three-way (Two-way) switch wiring has the same basic components:  Wires consisting of a line, a load, a neutral, a pair of travelers, and two 3-way switches.  If you are trying to troubleshoot a 3-way switch operation, then you will need to identify the function of each wire.  Try to do this BEFORE you disconnect any wires from the switches.

Between each three-way switch or 4-way switch is a pair of "Travelers" which are connected to the common terminals.  On one of the 3-way switches the LINE or hot wire gets connected to the copper or black screw terminal.  On the other three-way switch the LOAD (The LOAD is the wire that feeds power to the light fixture) wire gets connected to the copper or black screw terminal.

Below is a simple diagram that can be applied to all three-way switch and two-way switch connections.

NOTE: 2-7-2018 - An updated answer with additional photos and words can be found on my blog here.
Three Way Switch and Two Way Switch Wiring Diagram
Ugly's Electrical References

There are several three-way switch wiring methods that can be used and it is usually the installer that determines what is the best way for his or her purposes.  The "National Electrical Code" requires that a neutral conductor (White wire) be installed at light switch locations (Article 404-2(C).

I think that the most common method used to wire a 3-way switch is to bring the two wire LINE cable into one switch box and bring the two wire LOAD cable into the other switch box.  A 3 conductor cable with a ground is then installed between the two 3-way switch boxes.  With this 3-way switch wiring method a neutral wire is at both switch locations.

At the LINE switch box, the black wire of the two wire LINE cable gets connected to the copper or black screw terminal on the three-way switch.  The red and black wires (Travelers) of the 3 wire cable get connected to the common terminals on the three-way switch.  It doesn't matter which traveler goes on which common terminal.

The white LINE neutral wire gets spliced to the white wire of the three conductor cable.  At the other end, the white LOAD neutral wire gets spliced to the white wire of the 3 wire cable.  If 4-way switches were added in-between the two 3-way's, the travelers would be attached to the 4-way switch according to the manufacturer's diagram, and the white wire just gets spliced through.

At the LOAD switch box, the black wire of the two wire LOAD cable gets connected to the copper or black terminal on the three-way switch.  The white LOAD wire gets connected to the white wire of the three wire cable.  The red and black travelers in the 3 wire cable get connected to the common terminals on the three-way switch.

The above method is good because it ensures that the white line neutral conductor is available at each switch box as required by the "National Electrical Code".  In addition this method requires only an additional 3 wire cable (With ground) between the switches.

Please note that not every existing 3-way switch installation has the same wire colors.  It is important to identify the LINE and LOAD wires before replacing 3-way switches.  Note which wires are connected to the copper or black screw terminals.

Another variation of three-way switch wiring is to bring all of the cables into one box and branch off from there to each switch and light fixture.  For example, the line and load are brought into one 3-way wall switch electrical box.  From there a four conductor cable (2 travelers, a ground, and the LINE or the LOAD) or a conduit would need to be installed over to the second 3-way switch.  At the second 3-way switch, the black wire would be connected on the copper or black screw terminal on the switch.  The bare or green wire is bonded to the metal switch box and onto the green screw on the 3-way switch.  The white wire is capped off for a future device that may require a neutral conductor.  The remaining two wires will be used as the travelers and get connected to the common screw terminals.  When a white wire is being used as a traveler, it needs to have its color changed.  I sometimes use blue electrical tape to differentiate it from other wires.

At the first switch box, the black wire that was run to the second 3-way switch in the 3 conductor cable can be connected to either the black LOAD wire or the black LINE wire.  The black LINE wire or the LOAD wire that is not connected to the second 3-way switch, gets connected onto the copper or black terminal of the first 3-way switch.  The LINE and LOAD white neutral wires should be spliced together.  The traveler wires from the second switch get connected to the two common terminals on the first switch.  The ground wires should all be spliced together and one or two pigtails connected from that bunch should be connected to the metal box with a 10/32 screw and to the green screw terminal on the 3-way switch.

Still another example of an alternative 3-way wiring method is to bring all of the cables into the ceiling light fixture electrical box.  From there a 4 conductor cable or conduit would need to be installed to one of the 3-way switch locations.  A five conductor cable or a conduit would need to be installed at the other switch location as required by code to have a neutral conductor at the switch.  Please note that a larger ceiling electrical box would be needed to accommodate all of the wires in this particular installation.  A minimum of 24 cubic inches is required if #14 wire is used.  See Tables 314.16(A) & (B).

At each switch the black wire gets connected to the copper or black screw.  In the ceiling light fixture electrical box, one black from a 3-way switch gets connected to the black wire on the light fixture.  The other black gets connected to the line black wire. The LINE white wire gets connected to the light fixture.  In the ceiling light fixture box, the travelers get spliced through color to color.  The grounds are connected together, to the metal box, and also to the light fixture.

 
Four way switch wiring diagram
A four-way switch is required in between the three-way switches if more than two switch locations are wanted.  You would just continue from one three-way switch box with the 4 conductor cable (Two travelers, a neutral, and a ground wire) in and out of each four-way switch box.  The white wires get spliced through.  The red and black wires are connected to the four way switches (Follow the switch manufacturer's diagram).   Ground wires are spliced together and bonded to the metal switch box and to the green screw on the switch.  At the three-way switch box, the travelers are connected onto the common color screws.  The two white neutral wires are spliced together.  The LINE or LOAD wire is connected to the colored screw terminal (Depending on which switch you are working on).

 

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Updated June 17, 2018

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