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idiot DIY'er here :P. I recently installed a smart thermostat and long story short; I removed the two wires from my furnace's board for the external air conditioning unit in the process. I was going to put them back today, but found that the Y and C wires that are coming from it are not standard colors, so I am absolutely unsure which is which. I tried to look up some information on which is which before just connecting them and I found a few stack exchange postings on which to connect where, but no concrete "this goes there" info.

I have a few pictures of the connector from the outside unit with the corresponding wires which i am trying to identify and I think that the combination that I am looking at is as follows (connector->low voltage wire):

Y wire = Yellow/Orange->White C wire = Black->Red

Can someone just confirm that I am correct with this based on the pictures posted?

Wiring transfer Connector In the furnace

  • Can you post a photo of the disconnected cable at the air handler end? – ThreePhaseEel May 18 at 21:39
  • Editing the post right now – AADJ May 18 at 21:52
  • added the connector pictured – AADJ May 18 at 22:27
  • Installers don't care much about color codes . The A/C unit only needs 2-wire thermostat cable for the run from furnace to A/C, so they just used /3 thermostat wire (red white green) because they had that on-hand. The wires going from air handler to A/C are are call-for-AC (Y) and common (C). I suggest colored tape on both ends of the thermostat cable to color-code them meaningfully. – Harper May 18 at 23:32
  • @Harper I was going to label them after I figure out which was which. Before that though I need to confirm which wire is C and which is Y. Do you happen to know which would be which in this instance? – AADJ May 18 at 23:50
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Connect one to Y and the other to C -- it actually doesn't matter which one is which

On an ordinary air conditioner like yours, the Y and C wires going off to the outdoor unit connect directly to the coil of the compressor contactor (basically, a big switch that is turned on and off by an electromagnet). Since they carry 24VAC to the coil to turn it on when the thermostat calls for cooling, that coil is polarity-insensitive (it has to be to operate on AC, after all).

As a result of this, it does not matter which wire in the air conditioner cable goes to Y and which goes to C, as long as one goes to Y and the other goes to C, and they're both hooked to the contactor coil on the other end. So, go with your preference, or even flip a coin, when it comes to labeling!

  • Thanks a ton!!! I was wondering if this was the case since I couldn't find anything on the subject! – AADJ May 19 at 2:18

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