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NEST install re: C Wire

Community Member

I must be missing something.   I have a situation where I am going to use the G wire (fan) as my C wire.  What I don't understand is does the C Wire have +24 volts on it or is it the return path for +24 volts from one of the R wires?

My AC unit where the G wire goes does not have a terminal labeled "C" so am not sure if I should be connecting it to +24 volt source or should it go to one of the terminals that the "W" wire goes to so as to provide a return path.

Thanks in advance


Silver Product Expert
Silver Product Expert

The power in low voltage HVAC systems is 24VAC - an alternating current across two wires.   Use can use a voltmeter in AC mode to find both sides (it will not read the correct voltage until you have both sides of the alternating current).   Alternatively, you can attempt to trace the 24VAC wires from the transformer and determine which one is R and then by default, the other is Common.

Community Member

Good grief did not occur to me that it was AC vs DC.

Thank you this should set me on the right path.