In replacing or installing your home's thermostat, it will be helpful for you to know something about the wiring, including the RC wire. Each wire in the thermostat is color coded, and is assigned a specific terminal to which it should be connected. Connecting a wire to the wrong terminal can create thermostat malfunctions, or can open a circuit breaker.
The RC Wire
The RC wire provides power to operate your home's HVAC cooling system. The wire for this function, if the thermostat was wired correctly, is typically red. This wire connects the cooling system transformer to a contact in the thermostat.