I have a new Stelpro RUH5-51T heater I'd like to 'safely' install in the garage. It's 240/208 v, 5000/3750 w, 50-60 Hz, 1 phase. The plate on the unit says 20.84 amps. Does this mean I can't use 12 gauge wire with 20 amp breaker? The run from the breaker box would be ~20-25 ft. Do I need to go with 10 ga with 25 or 30 amp breaker? The unit requires the green wire ground connection to the breaker box. Also, conduit protection, can I use flexible metal or do I have to use metal tube? THANKS!!!!!!