Dear Mr. Electrician: I have two switches that control one light in my kitchen. I am not sure if they are 3-way switches or 2-way light switches. 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: Two-way electrical light switches and 3-way light switches are the same thing, they just have different names according to country. I am guessing that one of the 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 3-way switch failed. 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.
You cannot rely on a color coded schematic wiring diagram to figure out what each wire’s function is in an existing installation. The original installers used whatever materials and wiring methods were acceptable at the time of installation. There are old existing two-way and 3-way electrical switch installations that do not follow acceptable industry practice or electrical code requirements. That is why it is important to identify the function of the wires for two-way and 3-way electrical light switches.
All three-way switch and 2-way switch wiring installations have 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. Do this before you disconnect any wires from 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 one of the 3-way switches. The other three-way switch will have the LOAD wire connected to the copper or black screw. A voltage tester is good for testing the live wires with neutral and/or earth ground.
It is possible to simply replace the switches without having to identify the function of each conductor. In this case you would just remove one wire at a time and put it on the same terminal on the new switch. Take note of the color of the screw terminals on the OLD and the NEW switches. With older wires where the color is not very distinguishable, I use colored electrical tape to identify the conductors. It is very helpful and makes replacement much easier the next time the switches need to be changed. I always have black, white, red, green, and blue electrical tape on my truck.
Between each three-way switch is a pair of “Travelers” that 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 (The LOAD is the wire that feeds power to the light fixture) gets connected to the copper or black screw terminal.
Below is a simple schematic diagram that can be applied to all three-way electrical light switch connections. This is only a wiring schematic and not the actual wiring method used for 3-way switch installations.
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. In some countries the three-way switch is called a two-way switch. The National Electrical Code requires that a neutral conductor (White wire) be installed at most light switch locations however article 404-2(C) only requires the neutral at one 3-way switch location.
I think that the most common method used to wire a set of 3-way and 2-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 three conductor cable with a ground is then installed between the two 3-way switch boxes. This is depicted in the diagram at the top of this post.
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 connected to the white wire of the 14/3 cable. At the other end the white LOAD neutral is connected to the white wire of the 14/3 cable. 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. In addition this method requires only a 3 conductor cable (With ground) between the switches.
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 and two-way light switches. Note which wires are connected to the copper or black screw terminals.
A variation of the above three-way and 2-Way switch wiring is to bring all of the cables into one wall switch box as depicted in the diagram below, and branch off from there to each switch and light fixture. The LINE and LOAD are brought into one 3-way switch electrical box. From there a 3-wire cable (2 travelers, a ground, and the re-identified white wire) 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.
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 neutral wires should be spliced together. The traveler wires from the second switch get connected to the same two common terminals on the first switch. The ground wires should all be joined 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.
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 three conductor cable or a conduit would need to be installed to one of the 3-way switch locations. A four conductor cable or conduit would need to be installed to the other 3-way switch location as required by code to have a white neutral conductor in one of the switch boxes. A larger ceiling electrical box would be needed to accommodate all of the wires in this particular 3-way switch 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 LOAD wire on the light fixture. The black wire from the other 3-way gets connected to the LINE black wire. The LINE white neutral wire gets connected to the light fixture. In the ceiling light junction box, the travelers get spliced through color to color. The grounds are connected together, to the metal box, and also to the light fixture.
The ground wires are not shown on these diagrams to save space and make for easy viewing. However during the installation, all ground wires would be joined together and a pigtail for each switch would be added. All metal electrical boxes need grounding.
You might find my post depicting 4-Way switch wiring diagrams useful.