Wire My ThermostatThermostat Wire Colors Chart (And Why You Shouldn’t Trust It)
Updated 2026-09-12
Thermostat cable (18/5, 18/8) comes with standard insulation colors, and installers usually follow a pattern. Usually. Here’s the common convention:
| Color | Usual terminal | Job |
|---|---|---|
| Red | R / Rh / Rc | 24 V power |
| White | W / W1 | Heat (aux heat on heat pumps) |
| Yellow | Y / Y1 | Cooling / compressor |
| Green | G | Fan |
| Blue or black | C (sometimes B) | Common (constant power) |
| Orange | O / O/B | Heat pump reversing valve |
| Brown | W2 / AUX / E | 2nd-stage or emergency heat |
Why letters beat colors
Installers run out of colors, reuse cable, or wire a heat pump with a furnace-style cable. White on Y and blue on O/B are both common in real homes. Google, ecobee and Honeywell all tell you to label wires by the terminal letter they came off, not the color. Take a photo before you disconnect anything, then use the wiring tool with the letters.