# Adding a C Wire on thermostat



## texwake (Aug 17, 2010)

Need help. Whats the easiest way to run a C wire to thermostat? Where can I connect easily to 24 Volts that is constant and wont short circuit my system?


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## On The Hook (Feb 24, 2009)

Not sure what a c wire is, but why not tie a whole new wire combination to the old thermostat wires and pull in the new wire with the old.


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## texwake (Aug 17, 2010)

C Wire is the Common wire that newer A/C systems have. Unfortunately old systems (like mine) do not have the capability to run a C wire which is 24 Volts. There used in thermostats that draw a little more power and cant run on just a battery for a long time (smart thermostats i.e. Nest, lyric) . I've been looking into transformers I could just add on but I don't have a outlet near by to plug it in.


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## OnedayScratch (May 23, 2012)

texwake said:


> C Wire is the Common wire that newer A/C systems have. Unfortunately old systems (like mine) do not have the capability to run a C wire which is 24 Volts. There used in thermostats that draw a little more power and cant run on just a battery for a long time (smart thermostats i.e. Nest, lyric) . I've been looking into transformers I could just add on but I don't have a outlet near by to plug it in.


Ask Mont. He'll know.


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## Tarr Balls (Feb 10, 2010)

I was able to use my door bell transformer for the 24 volt source.

Sent from my SCH-I545 using Tapatalk


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## psycho0819 (Aug 29, 2005)

There is a common leg available inside the air handler/furnace where the 24v that feeds your stat is. Both come off the same transformer in the unit. Most stats don't use that, as they simply switch power to different devices, so no need for a common wire at all. Some newer stats are battery powered, some aren't and require a common to work. 

Often, an installer will pull a wire that has extra conductors. A simple stat requires 4 conductors (24v, y-for cooling, w-for heat, and g-for fan), with a common (c) circuit it would need 5 conductors. 18/5 is the most commonly sold t-stat wire there is. So check your wire to see if the extra conductor is there and not used. I always wrap this wire around the rest of the wire when it's not use, so it can be used if needed in the future, a lot of other installers do this as well). If this is the case, then simply employ this extra conductor (it will often be blue) to serve as your new common wire. There will be a "C" terminal in the unit, or a common wire to attach to (the condensing unit will have a 24v common leg, you can tie into this wire). Then use the other end at the stat. 

If not enough conductors, I recommend pulling a t-stat wire that has enough (or a couple extra). As suggested above, disconnect the wire at the stat, tape new to it, and use old wire to pull new one. Watch your splice, as t-stat wires can be run through small holes, which makes using them to pull new wires difficult at times. Just depends on how the installer ran the wire (they usually aren't stapled to studs or anything like romex is). 

Using an external source for the common you need is not unheard of, but not the conventional way to go about things. Not saying it won't work, but not ideal either. What comes to mind is, if a service tech ever works on the system in the future, it shouldn't have any tricks/secrets/surprises. Having a separate transformer for just this purpose will just take added time to account for in future troubleshooting, time that could be costing you by the hour. I would do it right up front and avoid any issues later on.


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