Should I use 12/2 or 12/3 to wire a 220V electric baseboard heater and thermostat?

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?

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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 have to take issue with using 12/3. The heaters need 12/2wg. If 12/3 were used, convention requires that the black and red conductors are used to carry voltage 120/240vac and the white shall be neutral and bonded to the bare ground at the panel. When the white is used for a primary voltage leg (12/2wg) it must be painted or marked black to designate it as a primary. I totally disagree with using 12/3 and especially bonding red to ground, which is NEVER done. Your solution would certainly work OK, but not a good practice. Sorry down vote. – shirlock homes May 3 '11 at 10:21
@Luke: each hot wire is at 120V potential relative to the neutral wire. However, they are on opposite phases, so the potential between the two hots is 240V. You don't use the house neutral in this configuration. – Shimon Rura Jun 3 '11 at 2:16
I am an electrician. Red and Black are used to carry voltage and white for ground. If you are tempted to use red as a ground hire an electrician before you electrocute someone. – user18737 Dec 19 '13 at 19:31
Also, when @user18737says white is for ground he is mistaken. I hope it was an innocent mistake. White is typically used for the neutral, but can also be used as a hot when re-marked. It should never be used for ground. – Speedy Petey Sep 23 '14 at 23:12
cap off the white (unused neutral) wire at both ends of 12/3. use the black and red to carry the 240 volts. – Skaperen Jun 15 '15 at 11:06

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).

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I really don't like having wires that are un-connected at both ends. They act like an antenna, and can give you a shock if you touch it, particularly if they are long. That is why I always connect it to ground, or neutral. ( which one depends on the circumstances ) In this case I would hook it up like you normally would at the breaker, and just put a wirenut on the white wire in the junction box. Also if you replace your system with one that requires a neutral, you won't have to go into the breaker panel, to hook it up. – Brad Gilbert Jun 25 '11 at 6:05

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.

http://cdn.instructables.com/FJX/ZDUA/HAQ3A4EL/FJXZDUAHAQ3A4EL.LARGE.gif

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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.

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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.)

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