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I bought a Wifi Thermostat but my old thermostat did not have a Common Wire. I can't use any of the swapping the G for the C solutions because I have an electric heater. I was told by Honeywell that I needed to run a new wire from the "C" terminal on the HVAC to the thermostat, but there is nowhere on the HVAC that is labeled "C."

I've searched everywhere for an explanation but I haven't been able to find anything analogous to my situation. All the other diagrams or solutions I've seen indicate a "C" Terminal somewhere on the control unit, but there is nothing like that on mine.

American Standard Electric Heater Wiring Diagram

I am assuming that I can piggy back off one of the connectors on the circuit board (below) to create the common wire, I'm just not sure exactly where I should do it.

Circuit Board

Below is the wiring to/from the thermostat, the twisted blue/brown/black wires are extras that were run through the wall but don't connect on either end. The thicker wires are the ones coming from the control panel which are then connected to the thinner wires running to the thermostat via the wire connectors.

Wiring to Thermostat

Can anyone tell me where would be best to connect the new Common Wire or if it is even possible?

P.S. I can take more pictures of the actual wiring if it helps. Just let me know what section would be helpful to see.

  • This worked on my office's American Standard/Tran USA 4TEE3F31B1000BA A/C system with a new Ecobee3 smart thermostat. The "B" circut on the A/C's control board worked perfectly with the "C-Wire" on the new thermostat. Thank you! – Lockdown Oct 17 '18 at 20:02
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On the schematic, the BL wire in the section labeled "Low Voltage Field Connection" near the bottom of the diagram. That's the C wire.

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It appears as though the "C" wire is bonded to the chassis, so you should be able to connect the C wire from the thermostat to the chassis as well.

In the last photo, if you follow the yellow wire from the thermostat cable. It connects to a yellow wire from another cable, which I would assume leads to your A/C system. The blue wire from the A/C cable is attached to another blue wire, that blue wire ties in with all the other "C" wires. So you could connect the C wire from the thermostat, in with those blue wires as well.

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  • Thanks! I'm gonna give that a shot as soon as I have some time this weekend. Will report back, hopefully with confirmation that it worked! – Kenny Mar 17 '15 at 3:35
  • Worked perfectly, thanks again. I connected the extra blue wire that was run through the wall to the wire connector you labelled "C," plugged it into the thermostat and away she went. – Kenny Mar 24 '15 at 2:31

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