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When wiring a common ground (C wire) for a dual transformer system, does it matter which transformer you connect the ground too? I'd assume that the ground on either transformer would serve as a ground for the power from either transformer, rather than having the two linked.

Edit:

This isn't for a specific installation, just more of a general rule. I realize that the wiring would make a difference, but is there a situation that would be true 95% of the time?

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  • It depends on the installation, and the equipment involved. Do you have a schematic or wiring diagram? – Tester101 May 13 '15 at 16:13
  • This isn't for a specific installation, just more of a general rule. I realize that the wiring would make a difference, but is there a situation that would be true 95% of the time? – Iron May 13 '15 at 16:18
  • You'll have to check the documentation for the thermostat you're using, or contact the manufacturer. Some thermostats have an R and Rc terminal, and would likely be powered by R and C (heating transformer). Others have Rh and Rc, and could be powered by Rh and C (heating transformer) or Rc and C (cooling transformer). It depends on the circuitry in the thermostat. – Tester101 May 13 '15 at 16:45
  • Assuming it's powered by Rc and C, would it matter if the ground is hooked up to the heating transformer? I thought a ground would be a ground. – Iron May 13 '15 at 17:14
  • Being powered by Rc and C would mean that the R wire coming from the cooling transformer would be connected to the R terminal, and the C wire coming from the cooling transformer would be connected to the C terminal. It's better to think of the C wire as "neutral", not ground. – Tester101 May 13 '15 at 18:16
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In a "common ground" dual transformer system the commons (or "ground") for the heating and cooling transformers must be connected to each other and supplied to the thermostat through a single connection at the "C" terminal. In this arrangement, it is very important to keep the Rc and Rh (power from the cooling and heating transformers) separated and the thermostat must be designed and installed to handle separate heating and cooling power supplies. If the commons for the two transformers are not connected, then a single thermostat will not be able to control both the heating and the cooling units unless you install separate control relays that are all actuated by either the heating transformer or cooling transformer along with the thermostat (typically the heating transformer is used as the power supply in this case). If the two transformer commons are isolated in this arrangement then, yes, each common for the cooling and heating are only for the cooling and heating units, respectively. This could not be called a "common ground" system, however.

As a general rule, provided the heating transformer has a sufficiently large VA rating, I would recommend disconnecting the separate cooling transformer and instead power all the controls from the heating transformer only to reduce the possibility that someone will accidentally use an incompatible thermostat or cross separate Rc and Rh supplies in the future.

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