# need advice, running control wiring for a simple ac unit



## realworldelectric (Dec 17, 2010)

The AC unit is 208 v 30 A, i have a 2 wire 18AWG running from the unit to the furnace, connected to the Y an C terminals on the control board.

From the furnace i have regular heating control wiring R,G,W running to the digital thermistat, am i correct to run another 2 wire from the terminals on y and c from the furnace and connect them to the y an c on the thermistat ? 

Thanks for the help guys, i dont have much experance running control wiring, i dont believe i would need a relay for this operation considering the thermistat itself has a manual switch to switch from cooling to heating. 

Thanks


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## oldtimer (Jun 10, 2010)

realworldelectric said:


> The AC unit is 208 v 30 A, i have a 2 wire 18AWG running from the unit to the furnace, connected to the Y an C terminals on the control board.
> 
> From the furnace i have regular heating control wiring R,G,W running to the digital thermistat, am i correct to run another 2 wire from the terminals on y and c from the furnace and connect them to the y an c on the thermistat ?
> 
> ...


In the *real world*, why don't you use 5 conductor?

When all else fails.... read the instructions!:jester:


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## janagyjr (Feb 7, 2011)

oldtimer said:


> In the *real world*, why don't you use 5 conductor?
> 
> When all else fails.... *read the instructions!*:jester:


I generally try that from the start.


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## wcord (Jan 23, 2011)

realworldelectric said:


> The AC unit is 208 v 30 A, i have a 2 wire 18AWG running from the unit to the furnace, connected to the Y an C terminals on the control board.
> 
> From the furnace i have regular heating control wiring R,G,W running to the digital thermistat, am i correct to run another 2 wire from the terminals on y and c from the furnace and connect them to the y an c on the thermistat ?
> 
> ...



What you are describing will work. As long as the furnace has a Y terminal, there is no need to add a relay. Most thermostats do not need the C wire, but it is good practice to connect it if possible.
As you don't do AC units often, it is probably a safe bet that you don't have 5 wire. That is the preferred method. 5 different colours make it easier for terminating and troubleshooting. 
Your 2 wire will do the same job, just don't mix up any wires.


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## realworldelectric (Dec 17, 2010)

*Actually i did run a 5 wire*

I did run a 5 wire from the furnace to the t-stat, just for simplicity to explain what i wanted to do, i used that method of descripion, wouldnt want some of the guys on here to get confused  

Thanks for the help


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## wcord (Jan 23, 2011)

realworldelectric said:


> I did run a 5 wire from the furnace to the t-stat, just for simplicity to explain what i wanted to do, i used that method of descripion, wouldnt want some of the guys on here to get confused
> 
> Thanks for the help


At the risk of being shot, simple here is sometimes painting a target on yourself. 
BTW 
IF you find yourself hooking up a really old furnace which doesn't have the Y or G terminals, than you will have to install a fan control center ( WR 90-112 for example). It is a relay and transformer all rolled up into 1 package.


Edit- should have said G terminal. Y terminal for the old furnaces serves no purpose on them. ( just so used to connecting RWGYC that it is a habit)


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## mgraw (Jan 14, 2011)

Why would you need a relay if the furnace doesn't have a Y terminal?


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## RIVETER (Sep 26, 2009)

realworldelectric said:


> The AC unit is 208 v 30 A, i have a 2 wire 18AWG running from the unit to the furnace, connected to the Y an C terminals on the control board.
> 
> From the furnace i have regular heating control wiring R,G,W running to the digital thermistat, am i correct to run another 2 wire from the terminals on y and c from the furnace and connect them to the y an c on the thermistat ?
> 
> ...


The R is the 24volt hot, the G is the fan relay, the W is the heat, or gas valve, and Y is the cooling stage which would bring in the contactor at the condenser unit. However, it has been a long time.


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## johnsmithabe (May 3, 2011)

*Thanks*

thanks for the support.


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