There is no official standard for wiring colors, or even terminals for that matter. To be safe it’s always best to refer to your equipment manufacturers wiring diagram to see what their terminals are meant for.

The following table lists each terminal, what it commonly does, and what the typical wire colors are. Do not assume that your wiring matches the table below. Verify your wiring by following it to each piece of equipment. Only then will you be able to match the equipment with the terminal.

Terminal	Proposed NEMA Standard	Normal Color	Notes
R or V	24 Vac Power	Red
Rh or 4	24 Vac Heating Power	Red
Rc	24 Vac Cooling Power	Red
C	24 Vac Common	Black
Y	Cooling/Compressor Contactor	Yellow	Cooling or 1st stage heating on a heat pump
Y2	2nd Stage Cooling	Blue or Orange
W	Heating	White	1st stage heating; may require a jumper to Y on a heat pump. On some home systems this may be 2nd stage heating.
W2	2nd Stage Heating	Various	1st stage aux. heating on a heat pump
G	Fan	Green	Fan switch on thermostat or on a call for cooling or heat pump
E	Emergency Heat Relay	Various	Used to disable the heat pump and make aux. heating 1st stage
O	Reversing Valve Cooling	Various
B	See notes below	Various
X	See notes below	Various
X2	2nd stage heating or indicator lights on some thermostats	Various
T	Outdoor anticipator	Various
L	Service light	Various	Used to notify you of a problem with the system

The wires that appear to cause the greatest amount of confusion are the B and X wires. The NEMA standard is to use the B terminal for the heating changeover valve; however, this is not always the case. Trane and some other manufacturers (including older Honeywell thermostats) use it for the common side of the transformer. The X terminal, on the other hand, is the standard for common, but again like the B terminal some manufacturers use X or X2 as the emergency heat relay.