I'm installing a new thermostat and the previous one did not have a C wire connected. Tracing in back to the HVAC, I can't see a C output connection? Based on the wiring diagram (and reading other posts), i'm guessing i need to connect it directly to the transformer and the brown wire? Not sure if that's best, but looking for outside help. Bottom line - where should i connect my C wire and how should i connect it? Thank you! enter image description here [![enter image description here][2]][2]enter image description here

1 Answer 1


You are correct

The brown wire from the transformer to the relays in your unit is indeed the C wire you're looking for -- you can cut it and splice it with a bog-standard wirenut, or use a piggyback tab terminal crimped on the end of what you're using for a C wire to make the connection at a device. There may even be a spare terminal connected to it but nothing else -- the wiring diagram on your unit has provisions for a C wire from the unit to an external compressor contactor.

  • thank you for the response! i'll continue to think through how i'll make the connection. i'm not electrical savvy, but will connecting the new "C" wire to the pump relay where the brown wire connects the best way? (ie i don't know how to splice it)
    – chris
    Commented Sep 2, 2017 at 23:55
  • Can i connect the C wire to the ground? (second picture, left side, it's connected to nothing) I don't think it'll work, but youtube videos mention it. Or can i connect it at the cooling unit relay? It looks like the brown wire is an input there and the cooling unit has a "Y" output.
    – chris
    Commented Sep 3, 2017 at 2:08
  • You can connect to the brown wire going out to the cooling unit relay, yes Commented Sep 3, 2017 at 2:24

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