Trying to connect my smart thermostat up, but the C wire (blue wire) is not connected to anything, it’s just there capped off . (Bottom left)

Anyone know where I should connect the blue wire to so it can be active and so I can power my smart thermostat

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2 Answers 2


The transformer (the thing labeled TT104 in your picture) has a blue wire and a green wire coming out. One of those will be connected to at least one wire already going to your thermostat. That is your 24 volt wire. The other is the common wire.

  • So I need to remove the transformer & wire up the blue wire to blue wire from the back ?
    – Theman81
    Commented Sep 22, 2023 at 0:20
  • Can I just cut the blue wire from the transformer and bridge them together & add the little capped off blue wire together . So it’s all 3
    – Theman81
    Commented Sep 22, 2023 at 0:36
  • @Theman81 -- yeah, cut and add works Commented Sep 22, 2023 at 1:29
  • Can't tell whether or not the blue wire has a male or female spade connector, but you should be able to just use a splitter cable too, with the appropriate gauge and ends (instead of cutting).
    – Huesmann
    Commented Sep 22, 2023 at 13:38

At the transformer you have the primary side with 120 Volt (2 wires) and the secondary side also with 2 wires delivering 24 volt. On one side it is the 24 volt (usually red), on the other side is the C terminal.

find out if you have spare wire in the cable going to the thermostat and use it. Color does not matter, just mark it as C at the thermostat.

You need minimum 5 wires at the thermostat. The 24 volt (R+C), a cooling wire, a fan wire and the heating wire.

  • I’m confused on what you mean? Check picture ! I’ll post more pics
    – Theman81
    Commented Sep 21, 2023 at 22:45
  • @Theman81 how many wires do you have at Thermostat ?? connected and not connected ??
    – Traveler
    Commented Sep 21, 2023 at 23:21
  • 5 wires total . I took off the old thermostat but there was 4 connected , the blue one wasn’t
    – Theman81
    Commented Sep 22, 2023 at 0:35
  • Then connect the blue to C and connect it to the C terminal on the transformer.
    – Traveler
    Commented Sep 22, 2023 at 0:44
  • Don’t think there is a C terminal on the transformer. What if I cut the blue wire in half and reattached it back but this time added the C wire inbetween .
    – Theman81
    Commented Sep 22, 2023 at 4:01

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