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So, the spiffy new wi-fi thermostats want power on the C wire from thermostat, but the old thermostat doesn't have a C wire at it. What's the easiest way to add the C wire/24 volt to the thermostat location?

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Are there any extra wires in the cable? –  Tester101 Dec 6 '11 at 18:25
Sadly, no, just the 4 that are already terminated on the thermostat - Red, White, Green, Yellow –  Aaron Dec 14 '11 at 13:36
Can you run a jumper from one of the other wires? Like jump the red to the C –  user9801 Dec 29 '12 at 1:34
@EarlCook jumping the neutral with the hot is just going to result in a short. You'd need an unused wire, or one that you don't mind always being on (e.g. the fan). –  BMitch Dec 29 '12 at 1:53
@user9956 yes, all of the wires really don't do anything except where you have them plugged into. If you choose to completely rearrange them to suit your purpose, that's fine. –  Aaron Jan 7 '13 at 19:22

4 Answers 4

up vote 53 down vote accepted

More than you've ever wanted to know about the C wire:

Lets start by explaining what the C wire is, and why it's needed.

Ye olde thermostat

In the olden days thermostats were simple switch devices, that used Mercury Switches to complete the circuit and turn on the heat/AC.

Mercury switches were commonly used in bimetal thermostats. The weight of the movable mercury drop provided some hysteresis by moving the bimetal spring slightly beyond the point it would normally assume, thereby holding the thermostat off slightly longer before flipping to the on state and then holding the thermostat on slightly longer before flipping back to the off state. The mercury also provided a very positive on/off switching action and could withstand millions of cycles without degradation of the contacts. Source

Because of this, there was no reason to run a return wire to the thermostat. For example a thermostat that only controlled heat, would only require 2 wires.

Thermostat wiring without a C wire

Thermostat wiring without a C wire showing electricity

Welcome to the future

Newer thermostats that offer clocks, backlit displays, WiFi, etc. are controlled using circuit boards and integrated circuits. These new circuits require a path for electricity to return to the source, and so require an extra wire. This new wire is known as the C wire, or Common wire.

Thermostat wiring with a C wire

Thermostat wiring with a C wire showing electricity

How do I get a C wire?

If you're lucky when you upgrade to a newer thermostat that requires this connection, there will be an extra (unsused) wire in the cable at the thermostat. If you're not, you'll have to run new cable to the thermostat.

If you have heat and AC, you'll have to pull an 18/5 cable. If you have just heat you can get away with pulling 18/3 cable, but you may want to pull 18/5 anyway to make adding AC in the future easier.

enter image description here

What do all these wires do anyway?

There are no standards for wire color, so any wire could be used for any purpose. The most common color code would be (note: this is for forced air furnaces, heat pumps and other systems may be different).

  • Red - R - 24VAC
  • Red - Rh - 24VAC (dedicated to heat call)
  • Red - Rc - 24VAC (dedicated to cooling call)
  • Green - G - Fan on
  • White - W - Heat call
  • Yellow - Y - Cool call
  • Blue or Black - C - Common

Alternative Solution

This solution is illustrated in this video from Honeywell. With this solution you lose the ability to manually turn on the blower fan, but the fan will still run properly in the Auto position.

WARNING: This procedure involves modifying the wiring in the furnace, and may not be approved by every manufacturer. Check with the furnace manufacturer, and all local codes before attempting this procedure. Make sure the breaker for the furnace is OFF before you begin.

  1. Make sure the breaker for the furnace is OFF.
  2. Start by removing the access panel on the furnace, and locating the thermostat wires.
  3. Remove the wire from the G terminal, and connect it to the C terminal.
  4. Using a short piece of 18 AWG wire, make a jumper and connect it between the Y and G terminals (this is only required if you have both heat and central air).
  5. Replace the access panel.
  6. Remove the thermostat from the wall to gain access to the wiring.
  7. Remove the wire from the G terminal, and connect it to the C terminal.
  8. Replace the thermostat.
  9. Turn the furnace breaker back on.

A bit more about transformers and C wires.

A transformer uses coils of wire, magnetism, and a bit of magic to transfer energy from the primary side of the transformer to the secondary side of the transformer. Usually during the transfer, the voltage is either increased or decreased. In the case of our furnace we're likely talking about taking 120VAC, and transforming it into 24VAC. Once the voltage has been reduced, we can use the lower voltage and a thermostat to control the furnace.

Now that you know even less about transformers than you did before, lets look at a diagram.

Highlighted Furnace Wiring Diagram

This is an actual wiring diagram from a furnace, but you'll notice I've highlighted a few things. First, in red I've highlighted the 120V primary side of the transformer. I've also highlighted the secondary side of the transformer in a couple shades of blue. This was done to illustrate that one side of the transformers secondary winding (light blue), is attached to the R or power terminal. While the other side of the secondary winding (dark blue), is attached to the C or "neutral" terminal.

Locating transformers

On a schematic

On a schematic or wiring diagram, a transformer will look something like this.

Transformer Symbol

Often you'll see a number written on each side, which denotes the expected voltages on each side of the transformer. Notice in the schematic above that the top side lists 120V (120 volts), while the bottom lists 24V (24 volts).

In the real world

When actually digging through HVAC equipment, a transformer will look something like this.

Transformer Transformer

Notice the rectangular middle section, flanked by a bulge on each side. These are the typical physical characteristics of a transformer.


Transformers typically have a volt-ampere (VA) rating, which can be used to determine the amount of current that can safely flow through the transformers coil wires. To determine the maximum current, simply divide the VA value by the voltage.

For example, a 120V/24V 40 VA transformer would be capable of 1.66667 amperes on the secondary.

40VA / 24V = 1.66667A

and .3333 amperes on the primary

40VA / 120V = .3333A

Normally this is not a problem, since the only things supplied by the transformer are switches and relays. If you install a thermostat that draws more current than the transformer can carry, you're going run into problems. So in this case, you'll have to upgrade the transformer and any fuses that protect it (since the fuses are sized based on the VA rating).

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Use an Add-a-Wire Device

Assuming your thermostat and furnace are compatible and you're just short on conductors between them, you could use one of these add-a-wire devices.

This device won't work for C wires. However, it does allow one to combine two of the other relay wires down to 1, freeing up a dedicated wire for C. For example, in a standard 4-wire (R/W-heat/Y-cool/G-fan) thermostat, you could combine the Y and G wires through the Add-A-Wire device, and use the freed-up wire as C directly.

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Comment from an anonymous user: Above cannot be used for adding a common line, this is noted in the description on the page. –  Niall C. Dec 28 '12 at 20:14
Couldn't you repurpose one of the existing conductors to be common then "add-a-wire" to replace that conductor? –  alx9r Dec 31 '12 at 2:30
Suggested edit from anonymous: This won't work for C wires though, as it says on the website: Cannot be used as "R" power or "C" common wire –  Niall C. Feb 28 '13 at 18:40
Suggested edit from user868289: This device would, however, allow one to combine two of the other relay wires down to 1, freeing up a dedicated wire for C. For example, in a standard 4-wire (R/W-heat/Y-cool/G-fan) thermostat, you could combine the Y and G wires through the Add-A-Wire device, and use the freed-up wire as C directly. –  BMitch Jun 4 '13 at 1:28
@BMitch Incorporated the suggested edit. –  alx9r Jun 6 '13 at 15:47

If your furnace has a C terminal then you can run a new wire from the C terminal to your thermostat. You can connect as many wires to the C terminal as you'd like as long as you do it in parallel (ie: all wires must terminate on the terminal itself, do not "daisy chain" them). In the case when your furnace does not have a C terminal, you can utilize an external transformer to provide the neutral line that would otherwise be provided by the C wire. While most of the WiFi thermostats accept a range of voltages, my experience is thats best to use a 24V transformer. Make sure to verify the voltage and amperage requirements with your thermostat specs.

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Can you expand on this answer a bit, maybe explain how a transformer will help? –  Tester101 Dec 9 '11 at 14:41
Sure thing, but your answer is stil better :) –  Steven Dec 9 '11 at 20:05
Can you give a product that is a 24V transformer? I would want to make sure that I'm looking at the right piece of equipment. –  Aaron Dec 12 '11 at 18:31
There are two types really. Direct wire and plug-in. Direct wire usually attach to an electrical box: homedepot.com/h_d1/N-5yc1vZ12kxZ12ky/R-202278728/h_d2/… The plug-in type you'll recognize as those black boxes that come with most electronics: homedepot.com/h_d1/N-5yc1vZ12kxZ12ky/R-100201044/h_d2/… –  Steven Dec 12 '11 at 19:06
I have seen this answer, but don't understand how a 2nd transformer, connected with one side of the secondary connected at 'C' of the thermostat, will allow a current sink for the electronics (which are supplied with 'hot' from the furnace transformer (seems like spooky action at a distance)). –  HerrBag Jul 20 '13 at 13:41

Forget the wires. Use a wireless thermostat, placing the receiver in the basement at your furnace and the transmitter wherever you want it. The Venstar Add-A-Wireless Thermostat T1119RF might do the trick. If you musts have the wifi thermostat, put it in the basement, and add a remote sensor to the mix.

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