How-to guide
Thermostat Wire Gauge, Conductors and Running New Cable
Updated 2026-09-30
Thermostat wire is a bundle of thin solid-copper conductors in one jacket, carrying 24 V AC between the thermostat and the furnace or air handler. You may need new cable when the old one has too few wires for a C wire, when a wire is broken, or when you are moving the thermostat.
Reading the label: 18/5, 18/8
The first number is the wire gauge (AWG); the second is the number of conductors. 18/5 is 18-gauge with five wires; 18/8 has eight. 18 AWG is the standard for residential thermostats. 20 AWG is sometimes used for short runs and is thinner; for new cable, 18 AWG is the safe choice.
How many conductors you need
| System | Typical wires | Buy |
|---|---|---|
| Furnace with AC | R, C, W, Y, G | 18/5 (18/7 leaves spares) |
| Two-stage furnace or AC | R, C, W, W2, Y, Y2, G | 18/8 |
| Heat pump with aux heat | R, C, Y, G, O/B, Aux/E | 18/7 or 18/8 |
| Boiler, heat only | R, W (+ C) | 18/3 or more |
Always buy at least one or two more conductors than you need today. A spare wire is how you add a C wire later, or replace a broken one, without pulling new cable.
Cable rating
Look for thermostat cable marked CL2 (or CL2R/CL2P). That is the usual rating for low-voltage wiring inside walls in the US. Outdoor runs to the condenser need cable rated for outdoor or wet locations; ordinary indoor thermostat wire breaks down in sunlight. Never run thermostat wire in the same box or conduit as mains-voltage wiring.
Replacing an old cable through the wall
- Turn off power at the furnace or air handler.
- Photograph both ends: the thermostat base and the furnace board.
- Tie the new cable to the old one at the thermostat end. Strip the jacket back a few inches on both, twist the copper of the old cable through a loop in the new one, and wrap the joint tightly with electrical tape so it is no thicker than the cable.
- Pull from the furnace end, slowly, with someone feeding the new cable at the wall. If it catches on a staple, don’t yank it: the joint can break and leave nothing to pull with.
- Connect both ends using the same colours on the same letters. Colours aren’t standard, so write down what you used. See thermostat wire colors.
If the old cable is stapled inside the wall and won’t move, you will need to fish a new cable with a fish tape or glow rods from the basement or attic, which is harder. At that point a C-wire adapter may be the easier fix; see no C wire options.
Connecting the wires
- Strip about 6 mm (¼ inch) of insulation from each conductor. Longer ends can touch each other behind the thermostat.
- Straighten the ends so they push fully into push-in connectors. Bent ends are the most common cause of a thermostat not detecting a wire.
- Fold unused conductors back, and don’t strip them. Bare spare wires touching R can blow the furnace fuse.
- At the furnace, never connect a wire to a terminal just because the colour matches. Follow the letters.
Once it is wired, check each conductor with our multimeter guide, then tick your wires in the wiring tool.