Find the Common Wire on a Three-Way Light Switch

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Old 01-24-13, 11:51 AM
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Find the Common Wire on a Three-Way Light Switch

I have a home built in the late 1950s. I want to replace a three-way light switch. My problem is that all the screws on the old three-way light switch appear to be the same color (brass). There are three wires connecting to the switch: one black, one red, and one white. The black wire connects to the only screw on the right of the switch. The red wire connects to the screw on the top left and the white wire connects to the screw on the bottom left. It appears that the black wire exits the box through one conduit while the red and white wires exit together through another conduit. Common sense (which I lack sometimes) tells me the black wire is the common wire and the red and white wires are the travelers, but this is all new to me and is based on an hour's worth of reading on the Internet. I don't have a voltmeter. If someone could help, I'd appreciate it. Also, if I wire it up wrong, is it dangerous?
 
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Old 01-24-13, 12:15 PM
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An analog multimeter can be bought for less then $15 and give you many years of use. You can't work without essential tools.

One can assume that the common on both switches will be in the same position so if you find it for one you will know it for both. My guess it is the lone screw on the one side. There are a couple of ways to determine if that is common. In all cases make a diagram of current connections and remove a switch. Test each wire to a known good ground. On one of the switches you will get 120 volts. That wire will have come from one of the commons.

Another way is to test the switch itself using the resistance checker of your multimeter. Set to lowest ohm scale. Touch the probes together so you will know what continuity is. Test for continuity between the probable traveler screws, the two on the same side of the switch. If no continuity between them regardless of switch position then they are the travelers and by elimination the lone screw on the other side is common.

No harm if you misidentify which is common. If it were me I would jjust assume the lone screw on the one side was common and connect that way.
 
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Old 01-24-13, 01:25 PM
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It would not be dangerous if you got the wires in the wrong spots. It would affect the proper operation of the switches.
 
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Old 01-25-13, 03:13 AM
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Thanks Ray and PCBoss. I will try that today.
 
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