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I was replacing few of my old manual thermostats with smart thermostats. While installing the new ones, I noticed that some of the gang boxes have 4 wires (2 black, 2 red). Some of them have 6 wires (3 black, 3 red). All of my electric baseboard heaters are labelled as 208/240 V AC. My question is,

  • Are the baseboard heaters that have 4 wires in gangbox running on 120v?
  • Are the baseboard heaters that have 6 wires in gangbox running on 240v?
  • Or they all are running on 240V regardless the number of wires in gangbox?

I do not need to know the actual operating voltage precisely, just need to know which range it is falling in, 120v or 240v.


This is in Canada.

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Assuming you're in the US and standard NEC color codes were followed when this was installed, it's all 240V (or possibly 208V if you're in NYC or a large apartment complex). Both black and red are hot colors, and wiring from one hot to the other gets you 240V. Wiring from either hot to neutral (which should only be a white or grey wire) would be 120V, but since you don't mention any white wires in these boxes, that's a clear indication that it's 240V.

An even more reliable way to tell than looking into the junction box is to look at the breaker panel (you did turn the circuit off before you started working on it, didn't you?) If the circuit is controlled by a two pole breaker (not to be confused with a tandem breaker) then it's 240V. If it's controlled by a single pole breaker, it's 120V.

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    Excellent! Thanks for explain so nicely! Yes I did turn off the breaker panel just because i dont want to die so young :P and two switches moved together. I am in Canada and no white or gray wire was present in the gangbox. So all you are saying sort of matched with my scenario. Accepted! – Samiron Dec 2 at 22:21
  • Yep, it works the same in Canada -- NEC and CEC are almost identical, and the color coding is definitely the same. Glad you got it all sorted out. – Nate S - Reinstate Monica Dec 3 at 17:05
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Quick answer is that 240v only requires two wires (black and red). Four wires would be power coming in, and the switched power from the thermostat going out. If there are 6 wires, then power comes in, and the thermostat controls two pairs, or one pair is power out to another thermostat and one pair out is to a heater. How the old thermostats were hooked up should tell you that.

  • Thanks for your answer. It is indeed helpful. – Samiron Dec 2 at 22:21

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