I prefer #10 romex for 12vdc circuits, but #12 is OK if you don't run too much power thru it. A couple efficient light bulbs will be fine. Wooferhounds answer is a good'n too. Technically be code, any 12v lines must be separated by something like 6 inches from any AC lines, but i cheated on that in my house with no problems -- just got to be sure you know positively which romex is AC and which is DC!
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