I just installed a nest thermostat and need to hookup the c wire inside the furnace. I read where you need to connect the c wire to the secondary wire on the 24v side of the transformer. There are 2 yellow wires leaving the 24v side of the transformer. 1 wire goes up the panel and connects to a white wire (top of first pic) that is going out to the ac unit outside and the other yellow wire connects to a blue wire which is connected to the red thermostat cable (very bottom wire nut). From the looks of my wiring diagram, I believe I just need to connect the unused c wire to the wire nut with the yellow and white cable but I’m not 100% sure. Any help would be awesome. enter image description hereenter image description here [enter image description here

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    Can you get us another view of where the thermostat cables are wired to the furnace please? Jul 26, 2021 at 2:02
  • Can you snip open the white cable tie, spread out the cables without disconnecting anything so it can be clearly seen what is connected to what, and try to highlight the wires that form part of the circuit that has been colored blue in one of the answers? You can connect "C" to anywhere along that circuit, either a convenient existing point, probably one of the wire nuts, or worse case cut one of the wires and connect there.
    – jay613
    Dec 23, 2021 at 15:15

2 Answers 2


From the furnace electrical diagram that you posted in the question I have shown the "C" or "common" side of the the 24V transformer circuitry as traced by the blue line. The opposite side of the transformer is the "R" side and I traced that with a red line.

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Your furnace may very well not have a distinct connection point for the "C" wire to route off to the thermostat which is often the case in older furnace designs.

With the AC power to the furnace turned off you will have to investigate the locations of the components showing on that diagram that connect to the blue line I traced. There may be a way to connect in your "C" wire at the terminal of a heat relay or blower relay. Some clever work is sometimes required but if those relay connections use push on spade type connections you can replace with a new crimp on spade connector with a double wire crimped into it.


In most but not all furnace setups the R and C are the 24V AC supply. Your thermostat already has the R as it is used to power all the relays etc when the thermostat requests that. The other side of these relays is already connected to C in the furnace. There for R is the 'hot wire' C is the 'common wire', normally not needed by a thermostat unless it requires power from the transformer to operate. You new thermostat needs this for power. You need to check the energy requirement of the Nest and be sure your transformer can support the additional load. Here is a link that will explain it much better than I can. https://thesmartcave.com/thermostat-c-wire-common/

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