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?
I'd use 12/3 -- that way, the neutral is there if you need it (for built-in outlets for instance), and you can use standard color codes. (Just about ALL /2 NM is black/white/bare.)
You should use 12/2 if the 240v appliance requires two hot plus ground. If you can't find 12/2 wire that has black/red/ground... you can use the more common wire that has black/white/ground where both black and white are hot. If doing this you MUST identify the white wire as hot at the appliance (or outlet if used) and at the panel. I always use colored heat shrink tubing because electrical tape almost always falls off after many years and is not reliable. Also make sure that you're using a proper 2-pole breaker at your electrical panel and if using an outlet, use the correct type to match the circuit's amp rating.
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).
Refer to the total amps for the heater. Base the wire selection on this number. The wire size should be xx/2 WITH GROUND. Try to get wire with black and red conductors for 220 volts. If the wires are black and white the white wire MUST be pemantly marked red or black. Black and white conductors ar ok for 120 volt.
I wire both of my 240 heaters on the same circuit with 12/2 wire identifying the white with black electrical tape.A 240 circuit is alot cheaper than a 120 volt.
I like the idea of using 12-2 with ground, and marking the white wire red. But, my inspector said it is not allowed to mark white to red for small wires. I don't recall the cut-off AWG.
EDIT: As comments have noted, this isn't a good answer. Thanks for the lesson! Look at the other answers on this page for wiser advice.
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.