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Thermostat Wire Color Codes

The standard thermostat terminal letters and the wire colors commonly used for each. Wire colors are a convention, not a code requirement, so the terminal letter and the equipment wiring diagram govern.

Ref HV-0042NEC 725NEC 722

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Residential and light commercial HVAC controls usually run on a 24 VAC Class 2 circuit supplied by a transformer in the furnace or air handler. The thermostat closes circuits between R (24 V power) and the call terminals (W, Y, G, O/B) to start heat, cooling, the fan or the reversing valve. Thermostat cable is multi-conductor, typically 18 AWG, with color-coded insulation. Installers usually match each color to a terminal letter, but no code requires a particular color. Always read the terminal letter at both ends.

How to identify

Note which terminal letter each wire lands on at the thermostat and at the equipment control board. Take a photo before disconnecting.
Count the conductors in the cable jacket (for example 18/5 or 18/8 means 18 AWG with 5 or 8 conductors). Unused conductors are often folded back.
On heat pumps, look for O/B and auxiliary heat (W2 / AUX / E) terminals.

Terminal and color chart

Terminal	Function	Common wire color
R	24 V power (common term for Rh and Rc)	Red
Rh / Rc	24 V power for heating / cooling transformer (two-transformer systems)	Red (jumper when one transformer)
C	24 V common (return), powers smart thermostats	Blue or black
W / W1	Heat, first stage	White
W2 / AUX	Heat, second stage or auxiliary heat	Brown or other spare color
E	Emergency heat (heat pumps)	Varies
Y / Y1	Cooling or compressor, first stage	Yellow
Y2	Compressor, second stage	Light blue or other spare color
G	Indoor fan	Green
O / B	Reversing valve (O = energize in cooling, B = energize in heating)	Orange (O) or dark blue (B)
L / X	Fault indicator or other functions (brand-specific)	Varies

Key points

O vs B depends on the brand of heat pump. Most brands energize the valve in cooling (O). Some energize it in heating (B). The thermostat setting must match the equipment.
Blue is used for both C and B in the field. Check the terminal, not the color.
Smart thermostats usually need a C wire. If there is no spare conductor, use the manufacturer's add-a-wire kit or run new cable. Do not repurpose G without understanding what that does to fan control.
Communicating systems (proprietary 2-, 3- or 4-wire buses) do not follow this chart. Use the manufacturer's diagram.
Thermostat cable is a power-limited Class 2 wiring method. Its installation and cable type follow the NEC power-limited circuit articles and the cable's listing.

Common mistakes

Assuming red is always R and blue is always C.
Wiring a heat pump thermostat to a conventional system, or the reverse, without changing its configuration.
Shorting R to C while the system is powered. This can blow the low-voltage fuse or damage the transformer.

Governing standards: NEC (NFPA 70) Article 725 and Article 722 (power-limited circuits and cables, depending on the code edition adopted); equipment and thermostat manufacturer wiring diagrams.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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