4 Wire Thermostat Wiring Color Code

Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with wires connected.

If you’ve installed more than a few thermostats (t-stats), you likely noticed that many of them use four wires. See this shown below in the units we’ve wired. Typical 4 wire thermostat wiring color code examples follow.

4 Wire Thermostat Wiring Color Code

In the setups we’ve seen, the most common wire color code guide for single stage climate control thermostat follows.

The Red Wire

The red wire (R, RC, RH) connects the high side of the output of the control transformer to the t-stat.

The Blue, Brown, or Black Wire

The blue, brown, or black wire (C) connects the low side of the output of the furnace transformer. Note that in our case, we had to use the green wire for C, as discussed below. Why? Because there were only four wires from the furnace which included no blue, brown, or black wire. Also, these particular t-stats need a C wire. So we decided to do without the fan switching function that the G wire normally provides. We opted instead for the remotely programmable capabilities of a Wi-Fi smart thermostat that needs a C wire. So, we made the G wire into a C wire. But we hope later to run new thermostat cable with more conductors. With that, we’ll get the C as well as the G wires.

The Yellow Wire

The yellow wire controls the cooling (first stage). The t-stat connects this lead to the R wire whenever it calls for the air conditioning to come on.

The White Wire

The white wire controls the heating (first stage).

In the next two pictures, we’ve converted the G fan wire into the C common wire. This involved getting into the furnace at the other end of the wires. Then, we had to move the green wire from the G to the C terminal on the furnace wiring block.

A common thermostat wall plate, showing a typical 4-wire hookup.

Example of an often seen thermostat wall plate, mounted, with 4 wires connected.

Finally the next thermostat, the Honeywell T87N1026 big dial model, does not require a C wire. So the G wire actually does control the HVAC fan. Again, no blue wire appears. As in the previous cases above, this Honeywell round t-stat utilizes the same wiring color code guide. It does not have the G wire.

The wall plate for the Honeywell Large Dial Thermostat, model T87N1026.