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Thermostat Wiring Chart
There is no official standard for wiring colors, or even terminals for that matter. To be safe itís 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
More information on the B and X terminals
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.
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