Replacing switch as in pic. Old Switch has no black terminal screw. How do I tell which is the common wire? Red is typically traveler wire but It has the white wire connected to terminal as well which I don’t think is common? enter image description here

  • 1
    Search the switch for markings, then. That picture is too underexposed to correct in Photoshop, I tried. Jul 13 at 3:24
  • No not really at all
    – Rdu27613
    Jul 13 at 11:04
  • This answer to the linked question above should tell you how to determine which is the common.
    – FreeMan
    Jul 13 at 13:11

2 Answers 2


Use a non-contact voltage tester like this Klein from Amazon:

Klein NCVT1

See which wires are live in various switch positions. Typically you will have either:

  • One wire always hot = incoming hot
  • The other wires alternate depending on switch position = travellers


  • One wire hot only when the light is on = switched hot
  • The other wires alternate depending on the other switch's position = travellers

If everything lights up an NCVT all the time (except when the breaker is off) then you'll need to test with a multimeter. Some NCVTs are more sensitive than others to phantom/induced voltage.

  • they'll all show as hot. phantom voltage.
    – Jasen
    Jul 13 at 7:15

The easiest way to tell which wire is what on a switch is to kill the power to the switch (turn off the breaker for the switch) and use a continuity tester on the wires contacts. The two travelers will be disconnected from each other no matter which position the switch is in.

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