Thermostat Wiring and Terminal Designations

Here is a list of each common terminal and what their commonly used for. Also listed is what colors are commonly used. NEVER ASSUME these are what you have. Verify all your wires by following each to each piece of equipment. This will help you to be able to match your equipment with the terminal.

- R, Rc, Rh Red wire, hot side of transformer
- C various color wires Usually the common side of transformer
- Y Yellow wire, Energizes the compressor, cooling or heating and cooling for heat pumps
- W White wire, heating for most units
- G Green wire, energizes the fan or blower relay.
- O Orange wire, energizes cooling on most heat pump units
- B Blue and sometimes Orange wire, energizes heat on most heat pump units
- B or X Blue, Black, or Brown wire, the common side of the transformer when the t-stat needs a common. Some brands use B as the common (York, GE, and Trane)
- E Pink, Gray or Tan wires, Emergency heat relay on heat pumps with auxiliary heat
- T Tan or Gray wires, For units with outdoor anticipator reset control (Trane, American Standard, GE, and Carrier)
- W2 Pink, or Brown wires energizes second stage heating on 2 stage units
- Y2 Blue or Pink wires, energizes second stage cooling on 2 stage units
- L Can be most any color, energizes service indicator lamp on units that are equipped with that.

The wires that always cause the most confusion are the B and X wires.
Check out the B terminal first. The NEMA standard for the B terminal to be the heating changeover valve but with some units it is different.

Trane, GE, York and some other manufactures which includes older Honeywell thermostats often use it for the common side of the transformer.
“X” Usually is the standard for common, but just like “B” some manufactures do use “X” or “X2” as the terminal for the emergency heat relay.