How-to guide
Baseboard Heater Thermostat Wiring: Single-Pole and Double-Pole
By Ismail Farooq · Updated 2026-10-30
Electric baseboard and wall convector heaters are common in Canada (especially Quebec), the Pacific Northwest and in additions, basements and condos across North America. Their thermostats switch the heater’s full current at 120 or 240 V, so they’re wired completely differently from a furnace thermostat. If you aren’t sure which kind you have, start with line voltage vs low voltage.
Line and load
A line-voltage thermostat sits in the middle of the heater circuit like a heavy-duty switch:
- Line wires come from the electrical panel. They’re live whenever the breaker is on. Thermostat leads or terminals for them are marked LINE or L1, L2.
- Load wires go to the heater. They’re only live when the thermostat calls for heat. Thermostat leads for them are marked LOAD or T1, T2 (some brands mark them “heater”).
Mixing them up is the most common mistake. The thermostat may seem to work, but many electronic and smart models lose power when they switch off the heater, or won’t power up at all. Before disconnecting the old thermostat, label which cable comes from the panel. If the labels have gone, a non-contact tester with the breaker on and the thermostat turned down shows which pair is still live (the line).
Single-pole vs double-pole
| Single-pole | Double-pole | |
|---|---|---|
| Wires at the thermostat | 2 (one line, one load) | 4 (two line, two load) |
| What it switches | One of the two hot wires | Both hot wires |
| Turned fully off | Heater stops, but one wire to it stays live | Heater fully disconnected (“off” position) |
| Wiring in the box | The other hot wire runs straight through, joined with a wire nut | Both cables connect to the thermostat |
Replace like with like. A double-pole thermostat can go into a 4-wire box. Putting a single-pole one there means joining one line and one load wire with a wire nut, which works but loses the true off position. Many smart line-voltage thermostats also need a particular setup, such as both hot wires or a neutral at 120 V, to power themselves, so check the product’s wiring requirements against what’s in your box before buying.
Check the load before you buy
- Add up the watts of every heater controlled by the thermostat. The rating is on a label on each heater, often under the end cover.
- Divide by the voltage to get amps. Example: a 1,500 W and a 1,000 W heater on 240 V draw 2,500 ÷ 240 ≈ 10.4 A.
- Pick a thermostat rated above that, in watts or amps at your voltage. A thermostat rated 3,000 W at 240 V is 12.5 A, which covers the example; one rated 2,000 W isn’t enough.
- Check the heater type. Some electronic thermostats aren’t approved for fan-forced heaters, radiant ceiling cable or heaters with a built-in thermostat. The compatibility list on the box says which.
Electric heating counts as a continuous load, so the circuit itself should carry no more than 80% of its breaker rating: about 3,840 W on a 20 A, 240 V circuit. If adding up the heaters comes close to that, have an electrician check the circuit before adding more heaters to it.
Wiring a double-pole thermostat
- Breaker off, and confirm with a tester on all four wires.
- Photograph and label the old connections: which cable is line (from the panel) and which is load (to the heater).
- Connect the line cable’s two wires to the thermostat’s two LINE leads, and the load cable’s two wires to the two LOAD leads. On 240 V either line wire can go to either line lead.
- Connect the grounds (bare or green) together and to the thermostat’s ground lead or screw, and to the box if it’s metal.
- Use connectors rated for the wire: baseboard circuits are often 12 AWG copper. If you see dull grey aluminum wire, stop and call an electrician; it needs connectors rated for aluminum.
- Fold the wires in, mount the thermostat, restore power and turn it up to check the heater warms.
For a single-pole thermostat, the thermostat’s two leads connect between one line wire and one load wire, and the remaining line and load wires are joined together with a wire nut.
Smart thermostats for baseboard heat
Smart line-voltage models (Mysa, Sinopé and others) add schedules, phone control and energy reports. Because electric heat is expensive to run, a schedule that lowers rooms you aren’t using is often where the savings are. Check three things: the voltage (120, 208 or 240 V), the wattage rating against your total, and whether it supports your wiring (single- or double-pole, neutral needed or not). Each room or heater group needs its own thermostat; one wall unit doesn’t control heaters on other circuits.
When to stop
Call an electrician if the box has scorched or brittle insulation, aluminum wire, more cables than line and load, or wires that are hot with the heater’s breaker off. Those mean the circuit isn’t what the thermostat’s diagram assumes.