Thermostat wire is rated by gauge and the number of conductors inside. If you see “18/5” on a bundle of thermostat wire, the first number is the gauge (almost always 18), and the second number to the quantity of individual copper conductors inside the bundle.

Before you buy, you should know the following:

• How much length you need
• How many individual conductors you need

Length is pretty easy to figure out: measure the distance your current wiring runs between your furnace and your thermostat. If your furnace is in the basement, the wiring probably runs up into the ceiling and through the floor of the first floor. For most homes, 50 feet seems to be sufficient. 

As for individual conductors (that’s the little colored wires inside the bigger sheath), pull at least 7 conductors. If you have (or might someday have) a heat pump, pull at least 8. Your future self or the future owners of your home will thank you. 

You will need the additional wires for modern 2-stage furnaces and A/C units, even if you don’t need it now. There is no harm in running more conductors than you need. If you ever upgrade your furnace, add a heat pump, a dehumidifier, or upgrade your thermostat, you may find yourself in need of additional wires. You can tuck the extras into the wall behind the thermostat, and simply leave them unhooked up in the furnace.

Shop for thermostat wire online

We recommend going with at least 7 or 8 conductors if you are pulling new wires. Here are two well-rated brands:

• Southwire 18/7 Thermostat Wire (50 feet)
• Coleman Cable 18/8 Thermostat Wire (50 feet)