Thermostat wire color guide

Colors and their usages vary by system and installation – there is no standard, and furnaces/thermostats don’t enforce the usage of one color over another. As far as a thermostat or furnace is concerned, all the copper wires in the bundle are exactly the same. It’s up to the human installer to hook terminals up to colors in a way that is generally agreed upon to be the standard. What you find in your system might vary from the charts shown below.

Common Thermostat Connection Points and Colors

Wire colorTerminal labelPurpose
RedRPower, 24V ac transformer
RedRcPower from cooling, 24V ac
RedRhPower from heat, 24V ac transformer
Black/BlueCDeliver continuous 24V ac power from furnace to thermostat
WhiteWPrimary heat relay
BrownEEmergency heat relay
YellowYPrimary air conditioner control
YellowY2Secondary air conditioner control
GreenGFan control
Orange/BlueO/BOrange or blue - reverse valve control
VariesAHeat and cool active to any relay
VariesLEmergency heat active lamp
VariesXSystem malfunction lamp
VariesPDefrost control lamp

About thermostat wire

Thermostat wire is a bundle of 18 or 20 gaugeĀ copper conductors. Each individual conductor is sheathed in a protective, flexible coating of a different color. The exact number of colored wires varies, as does the gauge of the wires. 18 gauge wires are slightly thicker than 20 gauge wires.


When shopping for thermostat wire, it’s common to see a number like “18/5” on the packaging. The first number refers to the gauge, the second number to the quantity of individual copper conductors inside the bundle.

At your thermostat, thermostat wires emerge from the wall and are stripped at the ends to reveal the copper inside. The copper ends are hooked to terminals on the thermostat’s base.thermostat_wires_wall

Over in your furnace and other accessories, the other ends of these wires are hooked to similarly labeled terminals.