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: I am guessing that one of the three-way switches broke down. It can be a little tricky for an amateur to diagnose which switch died due to multiple wires being hot in this particular type of wiring. 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 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 with a built-in continuity tester is good for testing the live or dead wires.
All three-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. Note the colors of the screw terminals on the existing 3-way switches. The dark or red color screw is the most important to get right.
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 a line or hot wire gets connected to the copper or black screw terminal. On the other three-way switch the load wire gets connected to the copper or black screw terminal.
Below is a simple diagram that can be applied to all three-way switch connections.
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 now requires that a neutral conductor (White wire) be installed at most switch locations.
I think that the most common method used to wire a set of 3-way switches 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.
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 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 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 five conductor cable (2 travelers, a neutral, 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 requires a neutral.
The remaining colors will be used as the travelers and get connected to the common screw terminals. At the first switch box, the black wire that was run to the second 3-way switch can be connected to either the black load wire or the black line wire. The black line wire or 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 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 light fixture electrical box. From there a 5 conductor cable or conduit would need to be installed to each of the 3-way switch locations. At each switch the black wire gets connected to the copper or black screw. In the 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 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.
A four-way switch is required in between the 3-way switches if more than two switch locations are needed. 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. Ground wires are spliced together and bonded to the metal switch box and to the green screw on the switch. At the other end you connect the other 3-way switch with either the line or load.