Thermostat Wire Colors: What Each Conductor Does
The standard colour convention for low-voltage thermostat wiring - R, W, Y, G, C, O/B - what each one switches, and why the colour on the wall is not always the colour in the manual.
Thermostat wiring is low-voltage control wiring - typically 24 V AC - and the colour assignment is an industry convention rather than something the electrical code mandates. Installers follow it closely because everyone reads the same diagrams, but nothing prevents a previous installer from having used a blue wire where the convention says white.
Treat the colour as a strong hint and the terminal letter as the truth. Every thermostat and every control board is labelled with the terminal letters (R, W, Y, G, C, O/B). Match letters, not colours, and the job is right regardless of what the last installer did.
| Wire colour | Terminal | Function | Notes |
|---|---|---|---|
| Red | R (Rh / Rc) | 24 V AC hot from the transformer | Rh = heating transformer, Rc = cooling transformer; jumpered on single-transformer systems |
| White | W (W1) | Heat call - first stage | Energises the heat relay or furnace board |
| White (second) | W2 | Second-stage heat | Auxiliary or backup heat |
| Yellow | Y (Y1) | Cool call - first stage | Energises the compressor contactor |
| Yellow (second) | Y2 | Second-stage cool | Two-stage or multi-stage equipment |
| Green | G | Fan / blower | Independent fan control |
| Blue or black | C | Common | Required by smart thermostats to power the display |
| Orange | O | Reversing valve - cool | Heat pumps; energised in cooling |
| Brown | B | Reversing valve - heat | Heat pumps; energised in heating |
| Black | E | Emergency heat | Heat pump auxiliary/emergency strip heat |
Modern smart thermostats need a continuous 24 V supply to run their display and radio, and that is what the C (common) wire provides. Older mechanical thermostats often had no C wire because they were powered by the small current that flowed through the heating or cooling call itself.
If a smart thermostat is browning out, rebooting, or losing Wi-Fi, the absence of a C wire is the first thing to check. Options are pulling a new conductor, using an add-a-wire adapter, or choosing a thermostat that includes a power-stealing kit - each with trade-offs. A multimeter check between R and C should read roughly 24 V AC on a healthy system.
| Situation | What you may see | Resolution |
|---|---|---|
| Heat pump reversing valve | Orange on O (cool) or brown on B (heat) | Check the equipment manual - manufacturers differ on which is energised |
| No C wire at the thermostat | Only 4 conductors (R, W, Y, G) | Add-a-wire adapter or a new cable run; verify the board has a C terminal |
| Two transformers (Rh/Rc) | Separate Rh and Rc, no jumper | Do not jumper them - remove the jumper if present |
| Previous installer recut colours | Blue used for W, white used for C | Trace to the control board and label both ends before reconnecting |
-
1
Photograph the old wiring
Photograph the terminals before removing anything. Colours and letters both need to be recorded.
-
2
Kill power at the air handler or furnace
Turn off the service switch at the equipment, not just the thermostat.
-
3
Match by terminal letter, not colour
The board and the thermostat base are both labelled. The letter wins.
-
4
Check for a C terminal
If the thermostat needs C and the board has no C terminal, that is a design decision, not a wiring trick.
-
5
Measure R to C before powering the thermostat
Roughly 24 V AC confirms the transformer and the C reference are intact.
What colour is the common wire on a thermostat?
What does the red wire do on a thermostat?
What is the orange wire on a thermostat?
Are thermostat wire colours required by code?
Can I use any wire for a thermostat?
Find thermostat wire and low-voltage controls
Get trade pricing, freight quotes, and portal calculators.
Register Now