Debugging a Three Way Switch

We had a number of three way switches in our house that didn't work when we moved in. In fact, not one worked right. Whomever wired the house back in 1986 either didn't care or didn't know how to hook them up correctly.

If you're not an electrician, or are not familiar with their standardized technical terms, the troubleshooting guides available might be confusing or useless. The information that comes with the switches seems to be the same. After pondering this issue for a few minutes the answer presented itself.

1. The "common" wire is either the load (to the light) or the hot wire (from the breaker box).

2. A "traveler" wire is just a wire that only connects to the switch. It doesn't lead to the load or the breaker box.

To identify the "common" wires, the simplest method is to disconnect the switches and use a voltage detector pen (such as a Fluke VoltAlert Non-Contact Voltage Tester) to find the hot wire. Once the hot wire is located, the other two wires in that box will be traveler wires. Mark the hot wire with a piece of tape, then temporarily connect it to one of the traveler wires. This is the "hot" switch.

At the other switch box, the "load" switch, use the voltage detector pen to locate the hot wire from the other switch. This will be a traveler wire. Touch it (carefully!) to the other wires in succession. When the lights come on, you've found the second common wire, the load. Mark that with tape.

The rest of hooking up the three way switch is painlessly easy. Locate the common terminals on the switches. Usually, they are a different color than the other terminals. Connect the common wires to the common terminals and the traveler wires to the other terminals. Test the circuit by switching on the light from one switch, then going to the other and trying to switch it off. Then go back to the first switch and turn it on again and finally, back to the other switch to turn it off. That's it!

Testing the switch
It's pretty easy to test the switches to see if they have gone bad.

On the "hot" switch, the voltage detector pen should see voltage on the common terminal and one of the traveler wires. Flipping the switch should change the voltage to the other traveler wire.

On the load switch, locate the hot traveler wire and when the switch is moved, the load (common wire) should alternate on and off. Repeat the test to check the other traveler wire by flipping the "hot" switch and test again.

Here are some really great circuit diagrams from the Electrical101 website.

The switch on the left is the "hot" switch and the one on the right is the "load" switch. Both switches down, light off. Note the common wires at the one running from the bulb to the switch on the right and from outside the image to the switch on the left. The other wires are travelers.

Here, the left switch has been switched up, which energizes the circuit through one of the traveler wires to the common wire for the load.

Now the right switch has been flipped up which cuts off the current and turns out the light.

Finally, the right switch is flipped down, which connects the hot traveler wire to the load and turns the light on.


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