A furnace has a power cable supplied by the manufacturer to feed the fan on the roof. The fan is 120 volts protected by a 5-amp fuse in the furnace. The cable has a molded plug to attach to the fan. The manufacturer’s instructions show running the cable through the walls to the roof. The cable is CMG rated 600 volts size AWG 18. It is my understanding that CMG cable is allowed for class II or class III wiring only. If the cable is considered a class I power source, then it still would not be allowed if I read NEC 725 correctly as class one power sources need to meet chapter 3 wiring methods. Since it was supplied by the manufacturer and is shown in the manufacturer’s instructions, is it acceptable to run the cable to the roof fan?