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 a

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.