This is probably a 725 class 1 circuit and the short answer is you usually use chapter 3 wiring methods. There is no code issue with garden variety 14 ga THHN an an appropriate raceway or even tray cable (becoming popular for the control cable on HVAC mini splits). You could use smaller wire tho. 16ga seems popular for this stuff and my old company had purpose built cables using 300v 18 ga that was a sort of "junior" THHN (PVC with a slippery skin).
