Q. A construction site has 10 office trailers. Each unit is identical. The HVAC unit for each trailer has a motor-rated load current of 27A and a branch circuit selection current of 30A at 208VAC, 3-phase. What is the proper size for the ground-fault, short-circuit protective device for each HVAC unit?
As per 550.4(A), a mobile home not intended as a dwelling unit (i.e., contractor’s on-site offices) shall not be required to meet the provisions of Art. 550 pertaining to the number or capacity of the circuits required. If the supply voltage is something other than a 120V or 120/240VAC power supply system, Art. 550 does not apply.
Let’s run through the calculation required to determine the proper size of protective device for each HVAC unit in this example.
As per 440.22(A), “A protective device having a rating or setting not exceeding 175% of the motor-compressor rated load current or branch-circuit selection current, whichever is greater.” Therefore, 30A X 1.75 = 52.5A. Round down to 50A for the final answer.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at email@example.com.