Thermostat Wire - Thermostat wire comes in a bundle with the outer jacket 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 bundle for typical 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 differently for identification from one end of the bundle to the other end of the bundle. It can be purchased in a bundle as follows:

This article refers to conventional thermostat wire which is different than the wire you would have if you have a communicating thermostat.

All About Thermostat Wire - HVAC Control

All About Thermostat Wire

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

The above list represents the most common combinations of wires available for air conditioning and heating wires. Other combinations of wire may be available. Wiring a new thermostat is made simple with our handy instructions for thermostat wiring color codes. See the menu to the right for the page.

All About Thermostat Wire - Composition

Solid copper is recommended for use in HVAC applications. Stranded copper wire leaves open the possibility that a hair (part of a strand in a stranded bundle) can cause serious problems. Stranded conductors increase the possibility a malfunction can occur in the system. Using a solid conductor 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. Inside a plenum or a place where the air will be used for the air inside the structure.

thermostat wiring

18 Gauge Wiring Bundle in an Outer Brown Jacket

Thermostat Wiring and the Code

This is an NFPA Code (National Fire Protection Association). The code requires any wire run in a plenum is required to be plenum rated. This refers to the outer jacket and what happens to the outer jacket if it catches fire.

Plenum-rated wire has an outer jacket that is not toxic when exposed to heat or flames. Plenum-rated thermostat wire increases the chances of people surviving a fire if the structure has a fire in the future. If the conductors are 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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