Thermostat Wiring Colors Code [HVAC Wire Color Details] Jul 04, 2008Thermostat 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. Guide to Thermostat Wiring Color Code: making install HVAC systems with more functions need more wires to communicate to and from the thermostat. The most common is 18/5 (5 strand, 18 gauge). The 5 strands of wire provide enough functions for the modern thermostat. The most common benefit of having 5 conductors is being equipped with a common wire, known as the c wire. I’ve found the best place to purchase thermostat wire is Amazon.