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Re-wiring to add a C-wire? Read this!

If you’re re-wiring to get a C-wire, pull new thermostat wire with at least 7 or 8 conductors.

Running completely new thermostat wire to get a C-wire is a great way to upgrade your home’s wiring and add some future-proofing. If you’re going to go through all the hassle of pulling out the new wires and threading new ones through your walls and floors, do everyone a favor and choose a new thermostat wire bundle with at least 7 or 8 conductors. Conductors are the little colored wires inside the thermostat wire bundle. The more you have, the more things you can hook up now or in the future. 

Things you (or your home’s future owners) might want to add to your HVAC setup include:

  •  heat pump 
  • dehumidifier
  • multi-stage furnace 
  • air conditioner
  • individual fan control 
  • smart thermostat that requires a c-wire

How to tell how many conductors are in a bundle of thermostat wire

The number of conductors in a bundle of thermostat wire are indicated by the second number in a fraction that looks like this:

  • 18/3 – 3 conductors inside this wire
  • 18/5 – 5 conductors inside this wire
  • 18/7 – 7 conductors inside this wire


It’s fine to run more conductors than you need right now. Tuck the extras into the wall behind the thermostat, and simply leave them unhooked up in the furnace.

Where to buy thermostat wire

It might not be easy to find this many conductors or in a length under 250 feet if you’re shopping locally, so look online for your new thermostat wire. We recommend shopping on for thermostat wires. Amazon offers a greater variety of conductors and lengths, as well as fast shipping and easy returns. 

How to buy thermostat wire for your 24V HVAC system

You can buy thermostat wire for your 24V system from all major home improvement retailers and

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: