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 wire and each wire 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:
- 18-2 – 18 gauge solid copper with two wires in the bundle – red and white colors with a brown or white jacket
- 18-4 – 18 gauge solid copper with four wires in the bundle – various colors with a brown or white jacket
- 18-5 – 18 gauge solid copper with five wires in the bundle – various colors with a brown or white jacket
- 18-6 – 18 gauge solid copper with six wires in the bundle – various colors with a brown or white jacket
- 18-8 – 18 gauge solid copper with eight wires in the bundle – various colors with a brown or 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.
Solid copper wire is recommended for use in HVAC applications for simply because stranded copper wire leaves open the possibility that a hair (part of a strand in stranded wire) can cause serious problems and possibly a malfunction in the systems. Using solid wire ensures a hair from a strand will not cause problems.
Why does the outer jacket make a difference? It does not make a difference unless the wire is run in 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) that requires any wire run in a plenum or that is exposed to the conditioned air 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 and using 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 and inside walls are using not exposed to conditioned air or spaces.