Forgot to label common wire when replacing 3 way switch

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Old 10-30-15, 02:58 PM
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Question Forgot to label common wire when replacing 3 way switch

How do you ID the common wire?
 
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Old 10-30-15, 03:20 PM
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How do you ID the common wire?
The common wire will be either:
  • The black of the power in at one of the switch boxes.
  • Or power out at the other switch box.
  • Or if power comes in at the light the (usually white) wire of the cable bringing power from the light.

Note a non contact tester can not be used to determine which wire is hot. You need to use a multimeter, preferably analog. Measurements are made between the black (in very rare cases it may be a red) and white of the cable with the cable disconnected.
 
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Old 10-30-15, 05:18 PM
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Three wires and three screws. That's nine different possible connections if my arithmetic is correct. You can't hurt anything by trying all combinations.
 
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Old 10-30-15, 05:53 PM
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I am assuming there are only two 3 way switches involved here. Go to the other 3 way switch and remove the cover . Identify the colors on the wires on the switch's switched terminals. At the box receiving the new switch, identify the same two colored wires for the switch's switched terminals. The remaining free wire in the box is the common wire. Good luck.
 
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Old 10-30-15, 10:33 PM
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Two of the wires that are/were connected to the three way switch will go into a three wire cable. Those two wires are the travelers. The remaining wire is the common.
 
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