What Is Thermostat C Wire And What Color Is It Usually?

One common problem when installing a smart thermostat is the lack of a c-wire in your home’s wiring. As more consumers install the device themselves, we suddenly need to become electrical experts. This quick guide will explain what a “c wire” is, what color it could be and how best to install one if needed.

thermostat wiring photo

Photo by gpshead

What Is A C-Wire And Why Do You Need One?

As a rule, whenever you are checking or changing your home wiring, ensure you have turned the circuit breaker off to avoid electrocution.

Since the arrival of the Nest device, smart thermostats have been a hot item for homeowners. They help heat a home efficiently and can use clever features to control the temperature from anywhere and save money through smart budgeting.

Traditional HVAC wiring in homes uses four or more wires, one of which is called the “common” wire or c-wire. The basics layout of a typical home wiring system is:

  •       R wire (red, for power).
  •       G wire (green, for a fan).
  •       Y wire (yellow, for air conditioning).
  •       W wire (white, for heat).
  •       C wire (blue or black, common for 24-volt power to the thermostat).
  •       Note that colors may vary and the labeling on systems may be inconsistent.

The trouble is, the c-wire is not that common in many (older) homes, as a traditional thermostat didn’t require much power. Today’s thermostats come with Wi-Fi connections and high-resolution color touchscreens, demanding more power than older models.

nest thermostat is installed photo

Photo by grantsewell

They are more likely to require one for this purpose, either to complete the circuit that helps power the smart thermostat or to charge its battery.

Good To Know
For reference, the common wire functions as a return path wire to complete the circuit fed by the power wire.

Even so, it can be used by installers to provide power to the thermostat. With some smart thermostats not requiring a C wire, this is confusing for people.

Those who find their wiring is missing the c wire can use one of the several solutions to power their smart thermostat. Since most brands sell their smart technology without mentioning this on the box, it is easy to find out after you have purchased one.

Which Smart Thermostats Don’t Need A Common Wire

The smart home thermostat market is booming with many new models available. The majority of them need a common wire to operate. Some of the leading models on the market can work without a c wire, but only on some HVAC systems, not all of them. These include the:

  • Nest Learning Thermostat
  • Nest Thermostat E
  • Honeywell Lyric Round 2.0. (but requires a c-wire for Apple HomeKit functionality)
  • ecobee3
  • ecobee3 lite
  • ecobee4
  • Sensi Wi-Fi Programmable Thermostat
  • Sensi Wi-Fi Thermostat (but requires a c-wire for Apple HomeKit functionality)

Even then, the Nest uses a battery that may degrade over time.

Good To Know
However, some heating and cooling systems and situations exist where the Nest Thermostat may need a common wire to bring power to the thermostat.

The Sensi recommends connecting to a common wire, to improve battery life and performance. With the ecobee, the company provides a common wire fix, offering a Power Extender Kit that is included in the package.

How To Check If You Have A Common Wire

The simplest way to check if you have a common wire within your existing wiring is to open up the installed thermostat and take a look. Check for a terminal that has a “c” next to it. If this terminal is connected to a wire, then you have a common wire.

In some rare cases, if you can’t see the connections, then you may need to remove the thermostat behind the wall. Check there for a wire running to the “c” terminal to confirm if you have an existing c wire.

What To Do If You Don’t Have A C-Wire

For homes that lack a common wire, a range of solutions are available, ranging from the practical to the rather risky.

Adding A C-Wire

The best all-round solution is to add a c wire to your home. When it comes to adding a c wire, you should use 18/5 gauge wire. If you only use the heating circuit, then 18/3 will do, but you might as well use 18/5 to cover yourself for future changes.

Use An Add-a-wire Kit

You can buy an add-on kit. Venstar is one vendor that offers a smart kit.

This requires you to switch everything off at the circuit breaker for safety. Then, connect the existing wires in the HVAC system to the Venstar box.

Having opened the thermostat, use the wire jumper in the kit to duplicate the wire suggested in the guide, allowing you to add a new wire with relative ease. If this sounds worrying, then a professional can install one for you quickly and easily.

Get A Thermostat That Works Without C Wire

This option is perhaps the simplest. Buy a smart thermostat that doesn’t require a common wire. The Nest manages quite well without one in typical homes, another example is the Honeywell Lyric Round 2.0. While the ecobee and Sensi recommend them, you can get by without one.

A Tricky Solution Involves Power Stealing

Other solutions include power stealing. This is where the thermostat leaches power from another wire when that’s not being used by the HVAC system and stores it in a battery for the thermostat to use.

Good To Know
This is not what the HVAC system was designed for and the tolerances or limited power supply may not be enough to power the device properly.

The problem is that too much power stealing can damage the HVAC system, while if you steal too little, this will damage the battery and thermostat, making it a highly risky strategy.

Issues with poor battery stealing may show up through poor WiFi reception or regular loss of signal, or failure to switch on. This is not something you would expect from a smart device, so don’t risk it.

Using The Fan Wire 

Finally, if you have a HVAC system that doesn’t use a fan, then you can borrow the G wire from the fan connection and use that as your new wire. In the video below you’ll find instructions how to do this.

However, if you ever decide to add a fan to the system, or someone else in the property in future does, then that does leave the system unable to operate such a feature. Considering that, it is probably best to get an expert in or use an adapter to make a more sensible change.


Building a smart home around an old property’s wiring can cause several issues, with this as one of the most common. As a rule, for your smart thermostat to function properly, they definitely require a common wire. Relying on a battery has proven troublesome to users in the past, and dubious workarounds are just that.

Use the best way to solve this problem based on your technical skills or get an expert to do it. That way your smart thermostat will have sufficient power to do its job and you can enjoy, smartly controlled, efficient heat and cooling when you need it.

About the Author David Waller

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