How to Connect an Unused C Wire (Common Wire) (2023)


How to connect an unused c wire in your furnace compartment. Firstly make sure you have a C wire. In order to see if you have one, refer to this video:
Once you follow the steps in this guide and have a C wire that is connected you can now install your choice of smart thermostat! Some thermostats even allow the use of without a c wire, by using an adapter or wire extender kit.
My Top Smart Thermostats:


Hey, there guys it's me Rob thanks for tuning into the Robson outlet YouTube channel.

So today in this video we're gonna be going back to the thermostat, and showing you guys how to connect that unused SI wire that's back there that blue coiled up wire how to connect that on the inside of your furnace, because connecting it to the thermos new thermostat is easy it's just putting it in a little wire terminal.

But we need to go ahead and get into the furnace compartment and connect that up so it will actually function so let's get over to there already did this step but I just want to remind you to make sure you are, shutting, off the power to any of your heating or cooling systems as you can see up here.

I have the AC shut off and the furnace.

Shut off as those are the two heating.

Cooling systems that I have makes you shut off.

All the power to be safe.

Also one way to be sure the power is cut is to just turn it to the heat, which here is to the right, and then just set the temperature higher than the temperature in the room.

So that way it would normally kick on, and then just wait a minute or so because my normally takes about 30 seconds to kick on.

And if it doesn't kick on, then, you know, the power is cut to it.

And obviously you want to make sure you do that in the breaker box first -.

And here we're going to be getting up in this little furnace compartment up here, we're gonna go ahead and have - this is in my little washer/dryer room.

So we're gonna go ahead and I have to put this tripod up on here.

So give me a minute and I will be back when we have when you go ahead and take that furnace compartment door off and get my tripod set up and I'll be right back with you.

So now we have that cover off and we're gonna get our tripod mounted up to get a closer we'll.

Look at some of these wires, okay.

So now that we've climbed up here, we got a nice close-up looky here, here's, the whole furnace unit.

And so what we're gonna do is to find our C wire since we know we have one present always need to find where it comes to and see if it's hooked up or not and we're gonna find the two wires that come in through the wall.

One of these is gonna be Diggs wires.

They go to the thermostat.

One of these is gonna be the wires that go to the AC compressor, and they come in through the top here, and they just come down right here through this hole, and they just loop around.

And then here they are.

They come to a bunch of wire junctions.

And those wire junctions come from a lot of leads off of this board.

So first thing we're going to do is find our c-- wire.

Now all of these are connected except for there is a loose one which is a blue one which is the typical color of a c-- wire though not always if the Installer did not follow the correct wire color coding or the standard wire color coding and that's why you should always double-check but I already know this is the C wire this is the unused one on the thermostat that is tucked back into the wall and this is a typical color so this is then going to be our C wire so they just have it wrapped up unused around the other you know the big cable which is fine we're gonna unwrap that and we're just gonna go ahead and get that stripped I need to go grab my wire strippers so I will do that now okay my camera died for a second but I got it stripped here and what we're gonna do now normally on most things that I've seen you would typically connect this a/c wire to be the C terminal on the board on the control board on in the, furnace, but the C terminal here is already taken up and one of these junctions actually comes off that C connection.

So what I found out you can do is just put that in with one of these junctions, you just have to find out what is the correct Junction.

So I'm gonna tell you how I found that.

And now this is for my system.

It should be similar for other systems, but I would go ahead and just do a little bit of light research online with some googling, just to double-check before trying.

So here we're going to be looking for is we're looking at the two bundles of wires that we discussed coming in from the wall earlier, the thermostat, bundle and the AC compressor bundle.

So if we look at the two-wire, the low-voltage two wire that comes from the AC compressor.

And you see, in this case, the white wire, which is connected to the yellow from the thermostat.

Now, the yellow from the thermostat is the call for cooling.

So if that's connected to the white wire, then you know, the other wire in this two wire cable right here has to be connected to this C for the common wire or the ground path for the 24 volts to return to.

And so that is actually where we're going to go ahead and hook our C wire into underneath this wire nut.

So again, with all power off and everything make sure you don't lose your wire nut and just put this sorry, I went off camera.

And this is the C wires, a thick wire compared to this little red one, but we're gonna put this in here with these other two we're, gonna go ahead and twist these together a little bit if we can just like so, and then just go ahead and put your wire nut on there, nice and snug, make sure it's a good connection and you're good to go.

Go ahead and seal, all your wires back into your furnace and I'm gonna go ahead and put this cover back on and then you're good to go to go ahead and start installing the thermostat and that's all we're gonna show you in this video.

But if you want to see the thermostat install video or how to locate, if you have a C wire in the first place, all those videos are down in the description below there also be in the top right and the info cards and also on the end screens on-screen.

Now, if this helped you out guys, please, please, please smack that thumbs up button.

It helps out the video so much.

It helps on my channel so much.

Thanks for watching guys.

I'll, see you in the next video peace out.


How to Connect an Unused C Wire (Common Wire)? ›

An extra unused wire in the wall can be used as the C wire for your new thermostat. The extra wire will also need to be connected to the C (common) terminal at the heating and cooling system. This will provide power to your new thermostat.

Can I use unused wire as C wire? ›

An extra unused wire in the wall can be used as the C wire for your new thermostat. The extra wire will also need to be connected to the C (common) terminal at the heating and cooling system. This will provide power to your new thermostat.

Is the C wire a common wire? ›

Blue wires are also called “C” wires because they are the Common wire. C wires are necessary for any “smart” thermostat that needs to be connected to a power source 24/7, regardless of your heat pump type.

Does my C wire have power? ›

But sometimes, you may need to connect a common wire (C wire) or power accessory, like the Nest Power Connector, to make your system compatible. Unlike other wires connected to your thermostat, a C wire doesn't control heating or cooling functions. It only provides constant power to your thermostat.

Can the C wire be connected to ground? ›

In my Goodman furnace, the C wire is connected to the 24v secondary/chassis or frame ground of the unit. It is also common with the 110~120 volt system ground.

Where does the C wire get its power? ›

The C wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat. Technically speaking, power flows from the R (red) wire, but not continuously (not on its own, anyway). To make it continuous requires a common wire to complete the circuit.

What color is common C wire? ›

The C, or common wire, provides smart thermostats with continual power. It's usually blue, but it may also be black, brown or purple.

How many wires are in a C wire? ›

Technically, there should only be one wire connected to the screw terminal.

What color wire goes in C? ›

The blue wire—also called the C, or “common,” wire—is required to power any thermostat and ensure proper system function. Your HVAC system and thermostat configuration will determine what color the C wire will be. The C wire in homes with a heat pump is always blue.

Can you leave unused wire in wall? ›

It's acceptable to leave wire in the walls. The only thing you need to do is leave the ends exposed in boxes and wire nut and tape the to legs together. That will indicate to an electrician what's going on, and if someone does try to tie into them in the future it will just pop the breaker.

Is it safe to cut unused cable? ›

A: Cable and phone wires don't carry current, so it's safe to remove them. However, it's critical not to cut into a power line, as you know.

What can I use instead of C wire on thermostat? ›

If there is no hidden C-wire, you can use a G-wire instead. However, you will not be able to use your fan independently when the heating or cooling isn't running. Also, many HVAC systems are incompatible with this solution, including: HVAC systems using electric heat.

Does unused copper wire go bad? ›

Although most copper wiring can last up to 100 years, it's the wiring insulation that can deteriorate. After about 25 years, house wiring insulation can become cracked and damaged, which can lead to exposed wire.

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