I had the extra wire in the wires run from my thermostat to my A/C, but the old thermostat did not need the C wire, so it was cut and stuff on both ends. While installing the new thermostat I realized that a C wire is needed, so I pulled out the stuff wire in the wall and attached it to the C on the thermostat. Went down stairs, opened the A/C unit and found that there isn't a C connection on the board. There is a W, R and G and it appears the Y goes out to the A/C compressor outside. My question is, am I able to cut one of the wires from the transformer on the 24v side that I think is the common side, it's grounded, and simply use a wire nut to connect everything back together while splicing in the wire that was unused, now making it the C wire? here is a picture. I was going to cut 1 of the 3 blue wires. It's hard to see in the pic, but there are 3 square terminals on the blue side that 3 wires attach to. Please help and thank you =) http://i.imgur.com/Hk0US7J.jpg

side note, only terminals on the a/c board are W, R and G. The yellow wire doesn't even attach to the board, but connects to another wire that looks like it runs out to my a/c compressor outside.


The wire that is connected to the Y wire, and runs out to the condensing unit, should be in a cable with another wire. That other wire should attach to the "common" side of the transformer. Find that wire, and attach your new C wire to it.

  • Thank you, and I will try this as soon as I get home. Both the yellow and the other wire were attached with wire nuts, so hopefully this won't take too long. I'll make sure the other wire is running to the common on the transformer. – Nate Jun 2 '15 at 20:00
  • Got home, the other wire goes straight to ground, doesn't connect to the transformer. Now what to do? – Nate Jun 2 '15 at 21:47
  • In the image, you can see where the blue and green are grounded together. The blue wire of those 2 is what connects to the return wire from the ac outside. Should I just ground the C wire? Even the terminal on the common side of the transformer is grounded. – Nate Jun 2 '15 at 21:54
  • @Nate You probably can just connect it to ground, though it's not exactly "proper". Use a ring connector, and put it under the same screw that the blue wire is under. – Tester101 Jun 2 '15 at 22:36
  • connected it to the ground, plugged everything in, pressed cool and everything started, the heater, the ac and the fan, couldn't get it to turn off, unplugged the thermostat and things shut off. Took the thermostat back and got a new one, hook it up and everything works perfectly now. So, moral of the story, if it's an open box electronic, get a different one or gain more grays on top! Thanks for all your help because I really do appreciate it. – Nate Jun 3 '15 at 1:16

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