Facts: old Eaton/Cutler Hammer panel with type BR breaker. Double pole 30 amp breaker, 240V, controls two wall heaters and two zone thermostats in two rooms. Heaters and thermostats and my brain got mixed up after 28 years….
Tested 120V at dining room wall thermostat wires in dining room so does it need a 120V or 240V heater for it?
In living room, on same circuit, its thermostat wiring also tested for 120 V. I am asking for advice on what voltage thermostat and heater to install in each room and an explanation of the 120V readings on a 240V circuit….
I vaguely recall that I might have had two underperforming 120V 1500W heaters on this circuit, but that was 28 years ago ……..So do I install 120v or 240v thermostats and 120 or 240 v heaters?
Best Answer
Ok this is probably a simple issue. You have a double pole circuit but the thermostat is only single pole and you read 120v not 240 as you expected.
With baseboard heaters it is common to only break one leg. With one leg open no current can flow so no heat. The heater is required to be grounded so measuring the voltage with the heater turned on providing heat at either terminal to ground will show ~120vac. With the heater heating the voltage from L1 to L2 (the heater connections) will show 240v.
Now here is what trips up non electricians. With the thermostat off each leg still reads 120v to ground the voltage coming through the heater has no voltage drop because there is no current with a normal high impedance meter. So a 240v heater can be controlled with a 120v line voltage thermostat.
To know if your heater is 120v or 240 measure the voltage L1 to L2 at the heater when the thermostat is turned on (the 2 wires connecting the power to the element) if you read 120 it’s 120 if you read 240 it’s 240.
You have found out you have a 30 amp circuit, this is quite common and #10 wire is normally used. A single pole thermostat can control 120 or 240v if when the thermostat is off you have voltage (120v to ground) at the heater this is another indication it is 240v.