Thermostat Cable Explained

Portable Lighting from Vericom Global Solutions


Thermostat cabling consists of multiple strands of solid bare copper wire, each wrapped individually with colored insulation and encased together inside a protective jacket. During the installation process, the wires are connected to a board that transmits data via each wire to a thermostat or other low voltage device. If the wires are incorrectly connected, the system will not operate properly.

Thermostat cabling is color coded to easily identify which cable goes to which port on each board. Keep in mind, the color of the wire doesn't always signify a connection to the same color on the board. Examples of this include when wires are swapped on both ends or if there's a splice in the wire that was not matched up to the correct color during installation.

R Red Power, 24 VAC Transformer
Rc Red Power from Cooling, 24 VAC Transformer
Rh  Red Power from Heat, 24 VAC Transformer
Black/Blue Common of 24 VAC Transformer
White Primary Heat Cell Relay
Brown Emergency Heat Relay
Yellow Primary Cool Call Relay
Y2  Yellow Secondary Cool Call Relay
Green Fan Relay
Orange Change-Over Relay to Cool Relay
Blue Change-Over Relay to Heat Relay
  Heat and Cool Active to any Relay 
L   Emergency Heat Active Lamp 
  System Malfunction Lamp 
  Defrost Control Lamp 


Thermostat cabling comes in a variety of types, with the main difference being the number of conductors in the cabling. Systems with a higher number of available functions need more conductors to communicate to and from the board and thermostat/low voltage device.

The most common thermostat cabling is 18/5 (18 gauge, 5 conductor), because 5 conductors typically offer enough functions for a modern thermostat/low voltage device. Additionally, a 5 conductor thermostat cable is equipped with a common wire (C wire), which provides a 24 VAC power source needed for most smart thermostats/low voltage devices.

A 2 conductor thermostat cable is excellent for simple, straightforward applications. However, multi-stage systems may require up to a 10 conductor thermostat cable to successfully accomodate communication between components.


Vericom Global Solutions proudly offers 18 and 20 gauge thermostat cable in a wide variety of conductor variants. Containing solid bare copper conductors for maximum conductivity, our cable is perfect for thermostats, HVAC controls, heat pumps, intercom systems, remote control units, doorbells, alarms, and other low voltage applications. 

These thermostat cabling options are available now for purchase. Contact us today to place your order.

Vericom Global Solutions
Phone: (865) 671-4455