2009 Issue

38 IS IT LABELED? — continued from page 34 50. The circuit should be designed for 44 amps with a maximum breaker or fuse size of 50. The circuit is not designed as if it were a motor. The manufacturer’s mark drives the design. The difference between appliances, RTUs and motors are clarified in Article 422, Article 440 and Article 430. Is there an answer to the question regarding appliances with motors? Yes, the answer is in NEC, right where we have been looking, Article 422 and Article 430. Article 422.3 of the NEC states motor-operated appliances shall meet the requirements described in Article 430 and hermetic refrigerant motor-compressors (RTUs) shall meet the requirements of Article 440 and Article 430.6 (A) (1) has an exception (Exception No. 3) which says the full-load current shall be used to determine the amperage rating of the conductors, switches, branch-circuit short-circuit and ground-fault protection on a motor-operated appliance that is marked with both motor horsepower and full-load current. Also Article 430 .7 (D) (1) states factory-wired multimotor or combined-load equipment shall be provided with a nameplate marked with the voltage, number of phases and MCA, and the maximum ampere rating of the circuit’s short-circuit and ground-fault protective device (MOCP). For ex- ample, a roof-top unit has a nameplate that shows a 3 phase, 208 V, an MCA of 35 A and a MOCP of 40A. This means the conductors should be rated for 35A and the over current protection should be rated for 40 A not values found in Table 430.250. Therefore, any circuit supplying a RTU or appliance, either with or without a motor, should be designed by the minimum supply circuit con- ductor amperage rating, and the maximum ampere rating of the circuit’s short-circuit and ground-fault protective device on the manufacturer’s nameplate. Motor circuits should be designed by the horsepower as stated on the manufacture’s nameplate and determined using the tables in Article 430. Since the information on the manufacturer’s nameplate dictates the electrical design and most RTUs, appliances and motors are usually specified by mechanical engineers, communication is vital for an efficient and successful design. This means it is critical for the mechanical engineer to inform the electrical engineer of the manufacturer’s specification regarding phase, voltage, horsepower, MCA and MOCP. Even preliminary information can be helpful to an electrical engineer for rough estimations and starting points for power load calculations and circuit designs. The electrical engineer has the responsibility of informing the mechanical engineer about voltage and phase availability for the mechanical equipment. Basically when communicating for the design of mechanical equipment, motors and appliances more is more. Good communication is vital for a strong, efficient and successful design process. So cool your head by loosening your collar, breaking open theNEC codebook, and popping the cap on the highlighter as you prepare for the next code confrontation. Also have your phone, computer and fax ready for communication and remember the old G.I. Joe cartoon sign-off, Knowing is half the battle! Mr. Johns is a licensed professional engineer, and a principal in charge of electrical systems design at Von Boerum & Frank and Associates. Mr. Johns has twelve years engineering and design experience in commercial and light industrial power systems, computer/telephone network systems and automation/controls systems. Mr. Wilson at the time of preparation of this article was affiliated with Van Boerum & Frank and Associates. Member Societies www.aaee.net www.acecutah.org www.aiaa.org www.asce.org www.utahashrae.org www.asme.org www.ewh.ieee.org/r6/utah www.iesutah.org www.ite.org www.same.org www.seau.org www.swe.org www.ucls.org www.uspeonline.com