What is C-Wire and Guide to the Thermostat C-Wire

If you have read some of my reviews on this website, you may notice that lots of wireless thermostats need a c-wire support in order to work properly. What is C-Wire? And what’s the different between C-Wire and normal wire?

Most people don’t know anything about C-wire, I have done some research in this respect, I will introduce what is C-Wire and give you a video tutorial about how to connect C-wire.


A c-wire is a low voltage power wire that many Wi-Fi thermostats require for work. The c wire provides 24vac power source. Since most thermostats have a big screen or Wifi module, they need additional power.

The wire of the thermostat connects it to the air conditioner or furnace, so that they can communicate with each other to keep the home at a steady comfortable state. Older wires of old thermostats have 2 or 4 strands, but latest thermostats need 5 strands, the fifth strand is known as the C-Wire. The C-Wire can provide 24vac power for the smart thermostat as it was connected to the air conditioner or other electronics.

If you have done some research before buy a home thermostat, you must want to install it by yourself. I will show you how to add a c-wire later in this article.

Usually the blue strand is the C-Wire, but that is not always right, some people don’t follow the recommended connections. So, make sure you label the wires. There are no standards for wire color!

Here are common uses for wire colors:

  • Blue or Black – C – Common wire, may be unused by your existing thermostat. Enables continuous power flow from the Red wire.
  • Red – R – 24VAC power from the furnace’s transformer
  • Red – Rc – 24VAC (dedicated to heat call)
  • Red – Rc – 24VAC (dedicated to cooling call)
  • Green – G – Fan
  • White – W – Heat
  • Yellow – Y – Air conditioner

Do I Really Need a C Wire?

Yes, you should install a C-wire to avoid any future problems. Nearly all modern thermostats, smart or not, require a power source so that a common wire is needed. But Sensi and ecobee 3 now don’t need a C-Wire.

How do I get a C wire?

Some thermostat provide C-Wire in the box with it, you can ask the manufacturers if they provide the C-Wire, What the length is.

If you have heat and AC, an 18/5 cable is needed. If you don’t have AC only heat 18/3 cable is enough, but it is better to pull 18/5 for future need.

How to connect the C-Wire?

The best way to show how to connect the C-Wire is a video guide. So, here is a YouTube video about how to connect the C-wire.

Honeywell Wi-Fi Thermostat- ‘C’ wire install:


The easiest way is choose a Ecobee 3 or Sensi Wifi thermostat. You can also run a new wire. Thermostat wire can be found pretty cheap on amazon. or you can buy a 24vac transformer(click here for more information on Amazon), or venstar add-a-wire adapter. But if you find it difficult to do all the works, you’d better hire a pro to do that works for you.

Here is a video about how to power the thermostat via an additional transformer.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top