Code question for 240V line

I currently have a receptacle that has two 125V lines (both connect to the same breaker). I would like to convert this to 240V. My plan is to replace the single pole breaker with a 240V breaker and replace the receptacle with the proper 240V one. My questions are:
1) Does NEC allow 240V and 120V in the same conduit? The current conduit is fed from a sub panel. The existing wire feeds a single receptacle located on a dock.
2) The current wire is 12 awg (black, red, white, ground). I know code allows for 20 amps at 120V. Am I correct that I can use this for 20 amp at 240V.
Thanks.
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On Fri, 20 Aug 2010 20:21:43 -0700 (PDT), noname87

Yes and Yes, In fact you could split this out in a multiwire circuit and have two 120s and a 240 out there. You just need to put GFCIs on the 120s after you split them out.
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How do you think you get 240 V? It is two 120 V circuits with the 120V in one circuit 180 degrees out of phase with the 120V in the other circuit. Look at it as a 240 V circuit center-tapped, with the center tap being the neutral/at ground potential.
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I have 240V at the sub panel. By using the right breaker, I can access both 120V lines to get 240V.
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