I have a 2 pole circuit breaker in the panel with a 2/12 cable connected to it. Black wire is connected to one pole, white wire is connected to other pole and bare wire is connected to ground bar. This cable runs to a wall mounted box where a 4 wire 240V dpst thermostat will be installed.

Testing, at wall box, black wire is 120V and white wire is 120V. The thermostat has two load wires and two line wires and it seems that connecting the 120V black wire to one of the line wires and the 120V white wire to the other line wire is how 240V gets to the baseboard heater via the two load wires. Is that correct?

  • Black wire to white wire is 240v. – Tyson Oct 6 '18 at 16:14
  • Thanks Tyson. I had checked black to white and got 240v. Not being an electrician, I wanted to be sure that connecting two 120v wires to the thermostat line wires resulted in 240v going through thermostat to heater. – henrylr Oct 6 '18 at 16:32
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    FYI... you should wrap black tape around the end of all the white wire ends in this circuit to signify to anyone in the future the wire is being used as hot and not a neutral. – Tyson Oct 6 '18 at 18:22

Yes, in North America our 240V power is center grounded, with neutral added. That means it is 120V from any pole to ground or neutral, and 240V across the two poles.

Now sometimes in your situation, someone confuses a 2-pole breaker for a twin/duplex/tandem/double-stuff. Those only have access to one pole, so in that case both wires will be the same pole. That will provide 120V from either pole to ground, and 0V between them. This won't work for 240V loads. The giveaway is that the double-stuff can be switched separately, but the 2-pole is ganged.

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