I have 2 bedrooms and I want to replace the light switch for a dimmer. I though it would an easy job for a beginner like me but after I opened the first switch, I realized there was a 3 wires (I was expecting 2 + the ground wire).

After reading on the switch, I determined the red wire is a common wire (on the old switch, it was plugged in a "quick connect" hole with the letter "C"). The second room also have the same setup.

I'm a bit confuse. I though the common wire was for a three-way switch only. Considering that each bedrooms switch are independent and that there's only one switch in each room, can someone help me understand if I have a three-way switch or not.

* EDIT * Please note that their was no Common wire in any of my switches. See accepted answer for details.

For the record, here's a schema of my final circuit (I have a ceiling light but the schema is still valid) enter image description here

  • The switches are "interdependent"? Do you mean that if you toggle one switch, the lights in both rooms change state?
    – Tester101
    Aug 20, 2014 at 12:35
  • A photo or diagram might be helpful.
    – Tester101
    Aug 20, 2014 at 12:35
  • @Tester101, no... I meant to say "dependent" (i.e. the switch has no effect on the light in the neighbor room). After TDHofstetter anwser, I realized the guy placed 2 wires under the same screw because he didn't want to do a splice. Aug 20, 2014 at 18:53
  • Note that a three-way switch is just a two-way switch with one extra terminal. If you ignore the '2' terminal, the '1' and 'C' act as a normal switch. Mar 29, 2016 at 22:21

1 Answer 1


One red wire on one screw, right? And two black wires under the other screw (or poked into those dang quick-connect holes I hate so much)? If so, that's an ordinary switch, not a three-way. If it actually has three screws on the body of the switch, though, it's a three-way... and given your description of things, there's no really good reason for that.

If indeed you have a red wire "going to one location" and two black wires "going to another location", then ONE of those black wires should be the power feed and the other should be power continuing on to the next switch ON THAT FLOOR, unswitched, so as to supply power to the next lighting area. The red wire should be the one headed to the light fixture.

Might not be, though. Sparky guys wire all sorts of things in different (often incorrect) ways, so if you run into an ambiguity it's Best To Test.

To verify my suspicions, could you post a quick photo of the wire connections to that first switch?

  • The red cable is in it's own quick-connect hole. The black cables are side by side, one in a quick-connect and the other one in the screw. There's only 2 screws on the switch so this confirm that this is an ordinary switch. I'll start an other question about the wiring of the 2 dimmers to avoid confusion. Thanks for your help. Aug 17, 2014 at 23:19

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