2009 Issue

34 T HE BIBLE OF ELECTRICAL ENGINEERING is the National Electrical Code (NEC), which is meant to direct design in a universally cali- bratedmanner. As with any large document, the NEC requires professionals to do some inter- pretation. The most fascinating thing about code debate among electrical engineers is watching typically cool heads become HOT. Voices rise in pitch and timber, faces turn red, and collars are unbuttoned. However, the end result is generally heads buried in books looking for the exact word and phrase that proves one interpretation over another. When the engineers must communicate with each other on a design, intense discussions may occur. Sure EE’s are happy to discuss cir- cuit diagrams and load calculations but leakage and heat dissipation from a building are not in their happy zone. Conversely the opposite is true for ME’s which only is magnified when the two disciplines must interact on a design. However, as we shall see, communication between the two is vital for an efficient and successful design. One such debate that has been occurring involves specifying branch-circuit ratings, conductor sizes and over-current protection for appliances, motors and packaged HVAC Units. Some common questions are; What are appli- ances as defined by the NEC? What is the dif- ference between remote telemetry unit [RTU], motor and appliance? How do the differences for each direct the design of the over-current protection and conductor sizing? If the appli- ance has a motor is it treated as an appliance or a motor with relation to NEC? Finally how does this relate to design and what is the role of mechanical engineers as it pertains to elec- trical design of RTUs, appliances and motors? In this article we discuss these questions and assist you in winning this debate. Any good code debate usually starts with definitions. Unfortunately, the NEC gives no definition of a motor in Article 100, instead drive systems and control systems for mo- tors are defined in Article 430 Motors, Motor Circuits, and Controllers. Article 430.6 (A) (1) states that the horsepower marked on the motor’s nameplate shall be used to determine the full-load current in amperes from Tables 430.247,430.248, 430.249, and 430.250 which dictates the amperage rating of the conduc- tors, switches, branch-circuit short-circuit and ground-fault protection. Even if the actual cur- rent rating is marked on the motor nameplate (this is of course excluding low speed motors, high torque motors or motors for multiple speeds) this section directs us to the full-load current tables for sizing everything related to the circuit supplying power. For example, a motor’s nameplate has the following information: 3 phase, 208 volts [V], 11 ampere [A] FLA (full-load amps), and 10 hp. According to Article 430.6 (A) (1) ampere rat- ing of the conductors, switches, branch-circuit short-circuit and ground-fault protection would Is It Labeled? STAN JOHNS, P.E. AND COLBY WILSON, VAN BOERUM AND FRANK ASSOCIATES INC. Mechanical Engineers (ME’s) and Electrical Engineers (EE’s) are known for their cool, calculated and precise approach to any array of problems. Engineering mostly involves the brain, but code interpretation sometimes requires a strong heart. or connected as a unit to perform one or more function such as clothes washing, air conditioning, food mixing, deep frying, and so forth. Utilization equipment is also defined in NEC Article 100 as: Equipment that utilizes electrical en- ergy for electronic, electromechanical, chemical, heating, lighting, or similar purposes. This informs that appliances are any piece of equipment that uses electricity, and performs some function and is generally not industrial. So, does this imply that if the appliance is used in a commercial or industrial setting, that the term appliance does not apply? No, because the definition of appliance uses the word ‘generally’, which allows for the application of this definition in commercial and industrial set- tings. This clarification is confirmed in the NEC Article 422.1, which details the scope of the article as covering electrical appliances used in any occupancy. Even though the definition seems to exclude commercial and industrial settings, the NEC Article 422 which covers ap- pliances, makes no such differentiation. The ratings of the conductors for appliances and RTUs are determined by the marked rat- ings according to NEC Article 422.10 (A) and continued on page 38 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. need to be rated for 30.8 A as given in Table 430.250, even though the nameplate stated the full-load amperage at 11 A. This is markedly different when the design for appliances and RTUs is investigated. Again, back to the definitions, but this time that of appliances. At first glance the definition of appliances, RTUs and motors appear obvious, but the NEC has specific definitions that are not as common upon inspection. Article 100 of the NEC defines an appliance as: Utilization equipment, generally other than industrial, that is normally built in standardized sizes or types and is installed NEC Article 110.3(B) with NEC Article 422.11 (A) stating the overcurrent protection not to exceed the ratings marked on the appliance. Therefore the circuit design for appliances and RTUs is dictated by the manufacturers’ mark- ings while motor’s designs are determined by tables in the NEC Article 430. For example, a fryer has a manufacturer’s marking with 115V and 10 A. The circuit should be designed for 10 amps, no extra tables or calculations are needed. Equally, an RTU is labeled with a minimum circuit amperage [MCA] of 44 and a maximum over current protection [MOCP] of