When I bought my baseboard heaters, thermostats and 220v breakers I was advised to get 12/3 wire. When I attempted to install the heaters I found that the thermostats, heaters, and even breakers have connections for only 2 wires. I have since read that 12/3 is only necessary for wiring something like a dryer that uses both 220v and 110v over the same wire. My question now is, should I get 12/2 instead of 12/3 to wire these heaters?
12/2 would be a less expensive and more obvious-to-future-electricians way to wire the heaters. However, you can use 12/3 and just ignore one of the wires. The safe way to do that is to bond the red wire to ground at every junction. That way, if it is ever wired to current somewhere in the system, the circuit will immediately short and your circuit breaker will break the circuit.
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I had a similar situation when I wanted to add 240v baseboard heaters to my attic renovation. I was initially told I needed to run 10/3 for a 240v line (I forget who, maybe my father-in-law or the guy at Home Depot), so I ran 10/3 from the basement through two stories and up into the attic. Later I found out all I needed was 2 conductors for the heaters so I ran the 10/3 into a junction box in an attic closet and then had 10/2 go from there to the heaters and thermostat. I put a wire nut on the unused conductor in the junction box as well as at the other end inside the electrical panel. I wish I had known that I only needed 2 conductor when I was fishing it from the basement as it would have been easier to deal with and cheaper.
Where are you located? My experience is in the USA and I am NOT an electrician (though it did pass inspection from the city).