What is a thermostat c wire or common wire?

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If you’re wondering what a common or c wire or is for a thermostat, you’re not alone. A lot of people don’t know what this wire does or why they need it. In this article, I will explain what the common wire is and why you need it in your thermostat installation. Keep reading to learn more!


Table of Contents

What is a c wire

A common wire, or c wire, is a wire that is used to complete the circuit from the power supply transformer in your heater or air conditioner to supply power to your thermostat. Almost all thermostats will already have 1 wire from the transformer running to it. This wire will be connected to the R, Rc or Rh terminal. Some thermostats will also have a wire hooked up to the C terminal.

Is a C wire necessary?

Most wifi or smart thermostats need the 24v power from your heater or air conditioner to operate.

There are some programable and non programable thermostats out there that can run with or without a C wire, those thermostats will also have batteries.

If you have an older heating or cooling system that has a standard mechanical or non-programmable thermostat you will have to determine if you have this wire.

The common wire completes the circuit from the transformer in your HVAC system and allows your thermostat to use the power from the transformer to operate. This is typically 24V AC for most heating and cooling systems. Most new WIFI or smart thermostats need this power supply to work.

How do you identify C wire?


If you’re not sure whether or not your thermostat has a c wire, don’t worry – it’s easy to check. First take a look at the terminals on your thermostat, If there is a wire connected to the C terminal you are in luck and already have the common wire. If there is no wire on the C terminal, just take a look behind your thermostat and see if there is a disconnected wire. The c wire may be pushed back in the wall or wrapped around the cable, but it should be easy to spot.

Not all thermostats look exactly the same but they will all have wire terminals. These terminals should be labeled with letters just as shown in the image above.

Once you’ve determined that your thermostat does have a c wire, you can start thinking about how to use it. C wires are able to provide power to devices like Nest Thermostats and Ecobee Thermostats, which can save you money on your energy bill.

What color is the c wire?

There is no specific color for the c wire. There are no industry standards for thermostat wire colors. However, most installations will have the c wire as the blue wire or sometimes it can be black. The best practice is to write down the colors and what terminals they go to before removing your old thermostat. Then you can reference your notes to make sure the correct wire goes to the correct terminal when installing your new smart thermostat. You can also go to your heating or cooling equipment and look inside at the control panel to see what colors are hooked up to what terminals. You will find a terminal strip with the same letter designations on it in the heater or air conditioner. Please make sure to turn the power off to your heating system or cooling system before removing any covers or panels. Once you remove the access door look for the control board. This is where you’ll discover the current wires that link to your thermostat. This will also show you if there is a existing c wire connected at the control pannel.

Is the common wire the live wire?

Depending on how your system is wired the common wire should be 1 leg of the 24vac transformer, it is live but with low voltage. With a AC transformer, you will have 2 live legs in order top provide constant power to flow you need to have your thermostat connected to both legs of the transformer.

What to do if you don’t have a common wire and need to install a new thermostat?

If you’re installing a new thermostat that needs a common wire and find that you don’t have this wire, there are a few different ways to solve the problem.

The first option is to install new thermostat wiring that includes the c wire.

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You can pull new wire from your heater or ac unit control board if it has a c wire terminal. This might mean that you’ll have to send new wires through your walls until they reach where your thermostat is located. The outcome could be good or bad: on the one hand, you would be future-proofing your system; if you need to relocate your thermostat, this would be a good time to do it. The downside is that you need to work with wires in your walls–and if you’re not experienced at this, it can pose a challenge (or even become impossible), depending on how far away the wall is and how easy it will be for you gain access to it.

your second option is to install a c wire add-on kit.

What is a C wire adapter?

Venstar add a wire module

A c wire adapter or also known as a common wire adapter or ad a wire kit, consists of a small electronic module that uses your existing wiring and combines the function of 2 wires into 1 this gives you the spare wire to supply the common connection to your thermostat. this is a good option for many situations that don’t have a c wire. It is also much easier to install than running a new wire to your thermostat. Most people who can follow instructions and have basic skills can install one of these kits.

There are several kits and some manufacturers include or offer a kit to go with their thermostat. These kits use the existing wires and add a module to provide constant power for thermostats that need a c wire.

If you are not comfortable with changing or modifying your thermostat wiring you should call a professional. And while the add on kits have instructions if you are not comfortable with working around electricity or don’t want to take a chance at damaging your new thermostat or even worse your heating system or air conditioning system, please call a professional to come to install the kit.

Here is a list of add a wire kits I recommend:

  1. Honeywell Home C-Wire Adapter for Wi-Fi Thermostats THP9045A1098

  2. Google Nest Power Connector – Nest Thermostat C Wire Adapter Works with nest thermostats.

  3. Venstar ACC0436 2-Wire Kit for all 24VAC Thermostats This is a great kit if you have a old 2 wire system

  4. Venstar ACC0410 Add-A-Wire Accessory for 24 VAC Thermostats (4 to 5 Wires)

  5. Briidea Common Wire Kit for All 24VAC Thermostats (4 to 5 wires)

What to do if you don’t have a C terminal in your heater or air conditioner?

You can use a stand alone transformer.

A/C adapter for thermostat c wire

If your system doesn’t have a c wire terminal, or your system is older and has a 2 wire thermostat this might be the best option for you. This will provide the power that your new thermostat needs without requiring a c wire from your HVAC system. Again there are several kits and power supplies that are available for this. You should only need to go this option if your system is very old or you have no way to access the control panel, or existing wiring. Adding a transformer will provide a constant supply of power to your thermostat. The downside is most of these kits use a plug-in transformer that will need a wall receptacle near your thermostat and will also have a wire running from it up to your thermostat. I would consider this a last resort but it will work and in some situations may be the only option to get power to your smart or wifi thermostat.

Here are some power supply kits:

1.  24 Volt Transformer, C Wire Adapter for thermostat.

2. 24 Volt Transformer, C Wire Adapter

3. Xwartre UL Certified Thermostat C Wire 24V Transformer

If you’re not sure which solution is right for you, please consult a professional HVAC technician. They will be able to help you figure out the best way to install your new thermostat and get power to it.

How to connect the common wire if you have one

If you’re looking to install a new thermostat, one of the things you’ll need to consider is how to connect the common wire if you have one.

If your old thermostat had a c wire connected, just take a picture of the old wiring connections before you disconnect the thermostat and transfer them over to your new one. If you have a c wire but it is not connected, simply connect the wire to your c terminal on your thermostat. You might also need to check inside your heat or AC unit to make sure it is connected correctly on that end. The c wire can be any color, so if there is a loose wire and you are unsure what its function is, look at the control board in your heater or AC unit. Look to see what color wire is connected to the c terminal and make sure it matches the color at the thermostat. Always turn the power off to your system before removing any covers or panels to look inside at wire connections. 

 So, what is a c wire? The c wire, or common wire, is a low-voltage power line that completes the circuit from the furnace to the thermostat. This wire is necessary for many of today’s high-tech thermostats. There are several options if you don’t have a c wire in your home and need to install a new thermostat. You can run new wires from your furnace or ac, or you can purchase an adapter that will allow you to use your current wiring to provide power to the thermostat. In rare cases, you can purchase a transformer kit to supply constant power to your thermostat.

Trane system differences, B wire as the common.

If you have a Trane system and you don’t have a C wire connected to your thermostat and need one some Trane systems use the B terminal in the air handler as the common wire. If you open your unit looking for the c connection and only have a B terminal and no C terminal you have one of these systems. If you are not sure please contact a HVAC professional. 

If you are looking for a new thermostat be sure to check out our full guide here:

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