Thermostat Wire – Thermostat wire comes in a bundle with the outer jacket of the thermostat wire being either brown or white. The bundle can have anywhere from 2 conductors inside the bundle or outer jacket and all the way up 8 or 10 conductors inside the outer brown or white jacket. The wire inside the bundle for typical thermostat wire is 18 gauge solid copper conductors and each has its own outer jacket that is a different color than any of the other conductors in the bundle. These multi-color wires are colored different for identification from one end of the bundle to the other end of the bundle. Thermostat wire can be purchased in a bundle as follows:

All About Thermostat Wire – HVAC Control

All About Thermostat Wire

  • 18-2 – 18 gauge solid copper – two wires in bundle – red and white colors with brown/white jacket
  • 18-4 – 18 gauge solid copper – four wires in bundle – various colors with brown/white jacket
  • 18-5 – 18 gauge solid copper – five wires in bundle – various colors with brown/white jacket
  • 18-6 – 18 gauge solid copper – six wires in bundle – various colors with brown/white jacket
  • 18-8 – 18 gauge solid copper – eight wires in bundle – various colors with brown/white jacket

The above list represents the most common available combinations of wire available for air conditioning and heating thermostat wires. Other combinations of wire may be available.

All About Thermostat Wire – Composition

18 Guage Thermostat Wire Bundle in an Outer Brown Jacket

Solid copper wire is recommended for use in HVAC applications. Stranded copper wire leaves open the possibility that a hair (part of a strand in stranded wire) can cause serious problems. Stranded wire increases the possibility a malfunction can occur in the system. Using a solid conductor ensures a hair from a strand will not cause problems.

All About Thermostat Wire – Why does the outer jacket make a difference? It does not make a difference unless the wire is run in a plenum. Inside a plenum or a place where the air will be used for the air inside the structure. This is an NFPA Code (National Fire Protection Association). The code requires any wire run in a plenum is required to be plenum rated wire. This refers to the outer jacket and what happens to the outer jacket of the wire if it catches fire.

Plenum rated wire has an outer jacket that is not toxic when exposed to heat or flames. Plenum rated wire increases the chances of people surviving a fire if the structure has a fire in the future. If the thermostat wire is run inside the wall then there are no codes about using any special wire. That is as long as the wire is inside the walls and not exposed to conditioned air or spaces.

High Performance HVACAll About Thermostat Wire

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