How To Use 18 AWG Thermostat Wire

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How To Use 18 AWG Thermostat Wire

A thermostat cable is a perfect wiring solution for your HVAC system. In addition, a quality 18 AWG thermostat wire also has its applications for different alarm systems and green compliant RoHS. It comes in a bundle of white or brown color, which can have 2 to 10 conductors in it. Learn in detail about a thermostat wire and methods to use 18 and 20 gauge wire.    

In the world of cables, a thermostat wire holds a special place due to its applications and appearance. It comes as a bundle of 18 or 20 gauge copper conductors, and each of these conductors is shielded with a different color of coating. The conductors are made of solid copper wire. The outer jacket of the bundle is usually brown or white. 

During the time of installation, these different-colored wires are connected to a board to send signals towards the thermostat. If these cables aren’t installed in the right way, the system doesn’t work. 

How is Thermostat Cable Connected to a Board

A bundle of thermostat cable can have 2 to 10 conductors in colors different from each other. These colors are given to make it easier for an electrician to figure out which wire can be connected to which part of the HVAC system. Here, it is necessary to note that a wire’s color doesn’t always need to match with the right wire on the board.

In the case of a smart or wifi thermostat, here is information on colors to consider:

Wire ColorMarkingMeaning
Black or BlueCCommon of 24 Vac transformer
Red RPower
RedRcPower from cooling
RedRhPower from heat
YellowYPrimary cool call relay
YellowY2Secondary cool call relay
BrownEEmergency heat relay
WhiteWPrimary heat cell relay
BlueBChange-over relay to heat relay
OrangeOChange-over relay to cool relay

Apart from these colors, there can be wires with different color codes. Before using them, understand their purposes and positions on a board. 

What is an 18 AWG Thermostat Wire and Where to Use It

A thermostat wire is a bundle of either 18 or 20 gauge conductors. Although both types of copper conductors are coated properly, 18 gauge cables are thicker than the 20 gauge ones. While shopping for a thermostat cable, buyers get options like 18/8 CMR, 18/6 CMR, 18/4 CMR, 18/5 CMR, and others. Here, the number 18 refers to the gauge, while the second part represents the number of copper conductors in the bundle. 

When it comes to a significant reason to choose an 18 AWG thermostat wire, it is the distance. It makes an ideal solution for a distance of up to 250 feet. For a shorter distance, use 20 gauge. 

 A thermostat cable makes use of low voltage signals to send data over a network. When used over long distances, signals may become weaker to offer output. In such cases, an 18 AWG thermostat cable becomes the right solution to protect signals. There can be 18 AWG thermostat wires up to 500 feet. 


Thermostat wires are ideal cabling solutions to ensure a smooth transfer of data to an HVAC system. When connected in the right manner, they generate expected results over long distances. Use an 18 AWG thermostat wire for outputs over a network of more than 250 feet. Buy high-quality wires after considering their number of conductors to make sure they can be easily connected to a board.