I am looking to install a new thermostat that will require a C wire. I have a single stage A/C unit and my current thermostat has Rc, G, and Y wires. There is also an unused blue wire which I traced to the A/C unit electrical panel - it is not connected to anything. My problem is - the guys that installed my A/C were kind of "half ass" and there is no easy way for me to just connect the unused blue wire in the A/C electrical panel. The unused blue wire is wrapped around the brown wire.

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  • Unused low voltage wires in HVAC and telco, are often wrapped back around the cable. It's a very common practice, which allows the wire to be used as needed in the future. Instead of criticizing the installer for doing this, you should thank them. Unwrap the wire, and use it as your C wire.
    – Tester101
    Commented May 10, 2013 at 12:04

1 Answer 1


You should be able to see onto the board where the rest of the wires connect and find the "C" terminal. It likely already has something connected going to one of the many wire nuts (and yeah, that is pretty sloppy).

You can test for sure using a multimeter. Set it to measure AC voltage, and then between the Rc (or Rh) and C wire you will see around 24VAC.

Once you've traced C and verified it, connect it up to your blue wire. Do yourself (and anyone who works on this in the future) a huge favour and label each wire at the thermostat before you put it back in the wall.

  • Thanks gregmac - my one question is there is no standard board like you'd see in other HVAC units. Also, nothing is labeled R or Rc or anything like that. I can see where the power comes in from my shutoff switch, but it already has other wires connected to it so I'm not sure if I can just add the common wire to that with another wire nut.
    – Patrick
    Commented May 11, 2013 at 19:07

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