There is no official standard for wiring colors, or wiring terminals. To be safe it’s always best to refer to your equipment manufactures wiring diagram. Don’t trust the wire colors. The installer can use any color to complete any circuit. Verify each wire first.
In the following table is a note for each terminal and its common function. The normal color wire is listed. DO NOT ASSUME these are what you have.
Terminal Purpose Normal Color Notes
R 24 V Power Red
Rh 24 Vac Heating Power Red
Rc 24 V Cooling Power Red
C 24 V Common Black
Y Cooling /Compressor Yellow Cool or 1st stage ht heat pump
Y2 Second stage Cooling Blue or Orange
W Heat White First stage heat could require jumper to Y for heat pump. Could be second stage heat
W2 Second Stage Heat Various First stage Aux heat for heat pump
G Fan Green Fan switch or call for cooling or heat pump
E Emergency Heat Relay Various Disable the heat pump or make Aux heat first stage
O Reversing Valve for Cooling Various
X See notes
X2 Second stage heat, indicator light for some thermostats Various See notes
T Outdoor anticipator Various
L Service Light Various Used to notify you of a problem with the system
Wires that cause greatest confusion are B and X
B terminal NEMA standard use B terminal for heating changeover valve.
Trane and older Honeywell common side of transformer.
“X” standard for common. some use “X” or “X2” for emergency heat relay.