wiring for 'fogless mirror' in shower?

I have to wire for a customer's fogless mirror in a shower. The UL listed item says to run minimum of 18 gauge wire from a 24 volt transformer into the box in the shower wall behind the mirror. Is 18 gauge thermostat wire ok? Or a different type of wire? I called the manufacturer, they didn't know. 50 watts heated mirror. Thank you
 
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