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Originally Posted by chachee52

https://www.amazon.com/Transformer-T...7MT5VWQN&psc=1

If you don't have the c-wire you can get this as I mentioned before.
My house doesn't have one and this is what I did. It was easier than snaking wire all around to pull it up from the basement.

Quote from Nest.

“Warning: Some 3rd party C wire adapters can cause problems or damage your Nest thermostat, even if they claim to be Nest compatible. We recommend using a Nest Power Connector to ensure compatibility and best performance with your thermostat.”

Unless the Thermostat has an isolated 24V PAIR of inputs you are asking for trouble. If it depends on a C wire for power you are mixing two 24V systems.

Some thermostats do allow for a simple external power with separate terminals or a jumper. Only use that if the manual clearly states it accepts a separate 24V source. Nest does not.