Wire My ThermostatLine Voltage vs Low Voltage Thermostats: Check Before You Buy
Updated 2026-09-12
Line-voltage thermostats carry household current. Turn off the breaker and confirm with a voltage tester before touching the wires.
Nest, ecobee, Honeywell T-series and Amazon thermostats are low voltage (24 V). Connecting one to a line-voltage circuit destroys it instantly and is a shock and fire hazard.
Signs you have line voltage
- Electric baseboard or wall heaters with a thermostat on the wall.
- Thick wires (like lamp or outlet wire) joined with wire nuts instead of thin wires on small screws.
- Labels such as 120 V, 240 V, L1, L2, LINE, LOAD on the old thermostat.
- Only two to four heavy wires, usually black and red (and sometimes white).
Smart options for line voltage
Look for thermostats built for it, such as Mysa or Sinopé baseboard models, and match their amperage/wattage rating to your heaters.
Millivolt systems
Gas wall furnaces, floor furnaces and fireplaces that work during a power outage are often millivolt systems powered by the pilot light. They need a millivolt-rated thermostat; 24 V smart thermostats won’t work.