High voltage systems typically have a thermostat that is labeled 120 V or 240 V, and has two thick wires with wire nuts.

Can you run a Nest Thermostat without a common wire?

Can You Install Nest Thermostat Without A C Wire? You can install a Nest Thermostat without a C-wire by using a C-wire adapter. A C-wire adapter mimics the functionality of a traditional C Wire without any of the inconveniences of having to wire a C wire all the way from your HVAC system.