If I only need a 20A circuit but I have 12/3 running from a 20A double pole breaker can I replace the double pole with a single 20A breaker and only use the black hot, the neutral and the ground? The 12/3 is buried deep in my stucco walls of my cement house and since it's there I would like to use it. Alternatively could I use both 20A circuits to power my 120V 20A fan/light/heater? I guess the heater could be on its own circuit, but that seems overkill.
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3Does this answer your question? [How should I secure an unused wire in my breaker panel?](https://diy.stackexchange.com/questions/138235/how-should-i-secure-an-unused-wire-in-my-breaker-panel) – isherwood Aug 10 '20 at 20:14
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You don't even need to change the breaker. You can just disconnect the red wire at both ends and done.
You're allowed to use half of a 2-pole breaker as a 1-pole breaker. It'll work fine.
There are ways to get additional capacity out of a /3 cable (as compared to a /2 cable) but they are rather complex and arcane, and there are many ways to screw them up and create a hazard. Google "MWBC" but bring your SCUBA tank - it's a deep dive.
Harper - Reinstate Monica
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You don't even need to change out the double pole breaker. If you use the black, white and ground, just cap the red with a wire nut in the box where the 12/3 ends. If your fan/light/heater is a combined unit, you don't want to separate the feeds unless the direction say to do so. If you're comfortable working inside your main panel, you can switch out the breaker to a single and cap the red wire in the panel and get a blank filler for the empty space in the cover or leave the double pole breaker in place, mark it as a spare and add the single breaker.
JACK
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Yes, you can leave a conductor unused. Just cap it at each end to prevent unintentional contact and indicate that the wire wasn't connected elsewhere.
isherwood
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