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 color | Terminal label | Purpose |
|---|---|---|
| Red | R | Power, 24V ac transformer |
| Red | Rc | Power from cooling, 24V ac |
| Red | Rh | Power from heat, 24V ac transformer |
| Black/Blue | C | Deliver continuous 24V ac power from furnace to thermostat |
| White | W | Primary heat relay |
| Brown | E | Emergency heat relay |
| Yellow | Y | Primary air conditioner control |
| Yellow | Y2 | Secondary air conditioner control |
| Green | G | Fan control |
| Orange/Blue | O/B | Orange or blue - reverse valve control |
| Varies | A | Heat and cool active to any relay |
| Varies | L | Emergency heat active lamp |
| Varies | X | System malfunction lamp |
| Varies | P | Defrost control lamp |
About thermostat wire
Thermostat wire is a bundle of 18 or 20 gauge 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.
Over in your furnace and other accessories, the other ends of these wires are hooked to similarly labeled terminals.