I'm trying to install a smart thermostat I the home to help with D&R programs in my area. I was able to determine that I have a unused C wire running from the thermostat to the unit, but I believe that the outside air condenser is connected to where my board says "C" for the common wire.

There are two cables visible in the HVAC unit, one with 4 wires + the unused common wire that runs to my thermostat and one with a red and white wire to the condenser. The red wire is connected below the yellow wire and the white wire is where I think my common wire should go.

Is this as simple as having both the white wire from the compressor and the blue common wire connected at the "C" connection on the board? Is it safe to have both on the same screw head?

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  • 1
    It's normal for the outside unit to be connected to the 'C' terminal. You just need to add the 'C' wired for the thermostat to that same terminal alongside the existing wire.
    – brhans
    Commented Jul 10, 2023 at 12:05

1 Answer 1


Yes, there can be multiple wires at the same terminal (whether it be C, Y, etc). In your case, since someone cut that spare blue wire so short, I would strip it and lengthen it using a wire nut and some spare wire. Then add your "new" wire to the C terminal with the white wire.

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