Thermostat Wiring Color Code

Thermostat Wiring Color CodeA thermostat cable is constructed of numerous solid copper wires. Each wire is shielded by a different color jacket, and then the bundle of wires is covered with an outer jacket. Why so many colors, you ask? To make installation easier, of course. If all wires were wrapped in the same color jacket, it would be virtually impossible to know which wire is which and where they each belong on the thermostat board. And it connected incorrectly, the system cannot operate.

The various colored wires make up the thermostat wiring color code. Every color translates to a specific port on the thermostat board. An important side note – if the wires are swapped on both sides the color will not work according to the wiring diagram.

If all is according to the thermostat wiring color code, follow this table to connect the thermostat cable to the board:

Thermostat Wiring Color Code
Marking Wire Color Description
 A  Heat and cool active to any relay
 B  Blue  Change-over relay to heat relay
 C  Black/Blue  Common of 24 Vac transformer
 E  Brown  Emergency heat relay
 G  Green  Fan relay
 L  Emergency heat active lamp
 O  Orange  Change-over relay to cool relay
 P  Defrost control lamp
 R  Red  Power, 24 Vac transformer
 Rc  Red  Power from cooling, 24 Vac  transformer
 Rh  Red  Power from heat, 24 Vac  transformer
 X  System malfunction lamp
 Y  Yellow  Primary cool call relay
 Y2  Yellow  Secondary cool call relay
 W  White  Primary heat cell relay