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It sounds like light switches and lights are usually wired with 14/2 cable. Is it okay to use 12/2 cable instead? Would I need special switches?

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As long as you have a 15A circuit breaker on that circuit, it's OK to use 12/2 NM cable for your lights. Since the maximum current on the circuit will be 15A, you won't need special switches. You should label the wire that it is 15A, not 20A, so that someone else doesn't come along and treat it as a 20A circuit.

  • Pro: 12/2 cable is a slightly better conductor of electricity, so you'll have slightly less loss between the service panel and your fixtures.
  • Cons: it's more expensive, harder to work with than 14/2 cable, and you may need to use larger junction boxes because the NEC allows fewer 12 gauge conductors than 14 gauge in a given volume box.

You'll probably also find that you won't be able to use the insertion fittings on the back of the switches (but that's OK, IMO, because I always use the screws), and you definitely won't be able to use the electrical nuts that come with your lights, so you'll have to get your own (and making a secure connection might be trickier with 12 gauge).

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The problem of confusing someone who comes along later, alone, would put me off doing it. –  flamingpenguin Feb 21 '11 at 13:28
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Niall C. touched on this a little but it will be much more difficult to work with the 12/2 wire than the 14/2. 12/2 is significantly stiffer and harder to pull through walls or other holes. And then it gets hard to bend and fit the 12/2 wire into a box. If you do not need to use 12/2 then don't... it will make the project take much longer. With that said, if you follow @Niall C's advice in his answer then you can use 12/2 (but I would not recommend it). –  Jeff Widmer Feb 21 '11 at 15:30
    
Well, I ask because it's already wired with 12/2, and I'm adding one more light on the circuit and noticed this. So I just wanted to make sure it's okay. In this case it'll be easier to just add one and rewire the whole circuit. I already ran the new wire anyhow, so that part is done. Thanks! –  99miles Feb 21 '11 at 19:33
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Not to mention, (at least today, at retail) 12/2 is about 50% more expensive than 14/2. If you have some 12/2 laying around that's one thing, but if you're just trying to buy one type of wire because you need some 12/2 anyways, I would buy what I needed of both, and save the cash and hassle of oversized wire. –  gregmac Feb 21 '11 at 19:35
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I just spent all weekend replacing every switch and socket in my mother's house and trust me, any opportunity you get to avoid working with 12/2 conductors in a tiny single gang box - take it. It basically doubles the amount of time it takes me to make up a box. And when you're running 2 or 3 conductors in/out of a box... UGH –  kkeilman Feb 23 '11 at 2:18
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