I'm trying to install a smart thermostat in my home. It has only an external air conditioner (model P3RD-042K, here's the manual/wiring diagrams), no heat. The current thermostat has an unused blue wire running to it, but on the air conditioner side it's not to connected to anything. I looked around, couldn't find a c terminal to connect it to. There aren't any terminals at all, just a mess of wires from different parts of the unit. I've included a picture of the wiring. Is there somewhere I can hook up the blue wire? Thanks!


And here's a closeup of the transformer label: Transformer Label Edit: After looking at the image, I suspect that 'COM' is for common. Is that correct?

  • Can you get a close-up pic of the label on the terminal block on the side of the transformer on the left side of the box? It's the thing with the push-on "spade terminals" and has some red, some black & one white wire connected to it.
    – brhans
    Commented Jun 27, 2023 at 14:46
  • @brhans, I added it in. Now that I look at it, there is a terminal suspiciously labeled "COM", but it already has something attached to it...
    – Rafael
    Commented Jun 27, 2023 at 16:10
  • You're missing pieces. How does the cold air get inside your house and move around? Commented Jun 27, 2023 at 18:03
  • @Harper-ReinstateMonica there's a fan somewhere around. It's not attached to the air conditioner, but the green wire goes there.
    – Rafael
    Commented Jun 27, 2023 at 18:05
  • 1
    It's normal & expected that there's already something else attached to the COM (aka 'C') terminal. The other 24Vac stuff in your AC unit needs that too (it's not just for the thermostat).
    – brhans
    Commented Jun 27, 2023 at 19:43

1 Answer 1


That's a 24V transformer all right, but "COM" is energized at 120V.

A transformer has a high-voltage side and a low-voltage side. The voltage proportion is decided by the ratio of number of turns in the winding. Suppose the secondary has 24 turns; the primary would need 208 turns if supplied from 208V or 240 turns if supplied from 240V.

So, to save costs and reduce inventory, they wind the primary of the transformer from the bottom up to 208 turns, insert a "tap" there, and then continue winding to 240 turns and have a terminal there. 208 vs 240V connect to their own terminal but they must also connect to the bottom terminal which is COMmon to both of them.

Because of the way North America does 240V and 208V, each pole is 120V to ground.

The other end of the transformer, the secondary, should have two terminals. One is R and the other is C. If you attached C to R by accident, nothing would happen.

When I give this advice I am relying on your claim that there's nothing in the air handler but a fan with a G wire going to it. If there's a transformer there too, all bets are off.

  • You are a 100% right about the 'COM'. I was so sure that it would be a C wire that I connected the blue wire to it to attach to my smart thermostat, but before I put in the thermostat I (wisely/less stupidly) decided to check the voltage on it. I was shocked (no pun intended) to find that it was 115v to ground. If would've attached my thermostat I would've fried it for sure.
    – Rafael
    Commented Jun 28, 2023 at 2:13
  • Thanks for your answer, but I am still unsure where the C terminal is. Is it one of the two terminals on the other side that are still visible on the label, or further inside the AC? Also when you wrote "If you attached C to R by accident, nothing would happen", did you mean that if I shorted R to C (sounds bad), or if I attached the blue wire to R (smart thermostat probably wouldn't work)?
    – Rafael
    Commented Jun 28, 2023 at 2:19
  • 2
    If you connect the blue wire to R, the thermostat just won't power up. But it will after you correct it. Just make sure you're on R and C (it should measure ~24V between them). Commented Jun 28, 2023 at 2:25
  • Correct answer! I tried testing for continuity between R and one terminal, but it was shorted to both for some reason (AC voltage, IDK). So I just traced the terminal back to R, and hooked up C to the other. It works now. Thanks!
    – Rafael
    Commented Jun 28, 2023 at 17:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.