air Conditioning Thermostats -- How To Wire A Thermostat Air conditioning thermostats wire are color coded in white, yellow, green, red, and (C) common wire. I had seen common wires are sometime blue, black or brown. Some thermostat wires that coming from the air conditioner unit with four wires and there is no common wire. Thermostat Wiring Colors Code [HVAC Wire Color Details] Thermostat Wiring and Wire Color Chart – Thermostat Wiring Colors Code; Tstat Terminal Designation: Color of Wire and Termination: R – The R terminal is the power. This is a red wire and comes from the transformer usually located in the air handler for split systems but you may find the transformer in the condensing unit.How to Wire a Thermostat · Single Transformer Central Air Compressor Wiring - Ask Me Help Desk Central Air Compressor Wiring. You will have (S)tart, (C)ommon and (R)un on the compressor. S will come from the Start/Run capacitor, R will come from the Start/Run capacitor and C will come from the contactor (Load) side..