Inside a machine, you are really outside of the NEC world. This might affect the listing of the machine but I have never actually seen that be brought up and we altered our machines all the time. As long as you are within the voltage rating and ampacity of the cable, with the proper O/C protection, you should be fine. I would also be looking at the exposure to damage. The biggest problem we saw with cables was flexing, chafing, pinching and other casualties.