Published In September 2000
The NECA convention products showcase and various industry magazines demonstrate that many preassembled components, tools, and methods can be incorporated into most projects. Often, preassemblies improve productivity without compromising project quality. Manufacturers whose work forces are subject to collective bargaining produce most pre-assembled components or services. Additional approval should be via the recognized testing laboratory listing. While most people recognize the Underwriters Laboratories (UL) label, 17 other laboratories (www.osha-slc.gov/dts/otpca/nrtl/nrtllist.html) are recognized as meeting OSHA approval. For switchboards, the estimator should determine how multi-section components will be delivered. If delivered individually, or as portions of the overall board, then sufficient labor is needed to unload, place, level, fasten, and assemble the sections. Most manufacturers follow National Electrical Manufacturers Association (NEMA) standards. There are basically two classes, each with three variations, which are further divided into those units that have been prewired and those that are just the components. Classes NEMA Class I is very basic. Only the basic components are installed into the motor control center. Further, only the wiring diagrams for the components are supplied. While saving the labor of actually mounting the various components, this version still requires the installer to determine the connections, making a schematic diagram, installing all control and line wiring, and providing the diagrams called out in the specifications and operations manual. NEMA Class II is used when the motor control center is to be used for system installations involving interconnected and interlocked units, as well as the connection provisions to remotely mounted devices. The manufacturer must be given a complete schematic diagram. For a built-up panel for an air conditioning system, the consulting engineers’ mechanical/electrical (M/E) drawings provide such information. Since the manufacturer provides both schematic and wiring diagrams, verifying the final connections prior to approval saves delays and extra costs near the project’s end. NEMA types A, B, and C are wiring method variations. Type A costs the least initially, but requires more labor. The load and control wiring must be connected to the individual components, and only the required component wiring is pre-installed. In Type B the component wiring is brought to a vertical terminal block in a side gutter wiring space. Optionally, the wiring may be brought to terminal blocks within the starter enclosure, thus providing more space in the side gutter for installing wiring that enters the enclosure at either the top or bottom sides. Type C provides the best access to the wiring. The terminal blocks for each section of the motor control center are mounted at either the top or bottom conduit entries. In addition, the terminals are distinctly marked. This is ideal for more complicated control schemes. While more costly at the outset, the labor savings may compensate. Prefabrication’s advantages include: greater concentration and supervision to the item being assembled; more stable crew sizes; better control of material purchasing; well-timed delivery to the job site; less theft and pilferage; shallower learning curve; the fact that weather conditions won’t interfere with pre-assembly; and fewer required tools and jigs. The grandest scheme is the lighting system that is connected by plugging the fixtures into a loop, and connecting the circuit to a previously installed junction box. Other prefabricated aides for lighting systems include “whips,” or the flexible conduit system that feeds the fixture from an accessible junction box. Manufactured and listed whips are useful in Code-approved reduced wiretaps that contractors are not allowed to do in field installations. It’s simpler to purchase fixtures that allow for wiring from the top, eliminating the need to remove and reinstall lenses, lamps, and ballast covers. Fixtures can also be ordered pre-lamped with the lenses installed. Row-mounted fixtures should also be considered, where generally left and right sides are required, as are the intermediate fixtures that will be placed in between the ends. If they are taken off so they can be ordered, marked, and delivered as needed, this can be very cost effective. Estimating should be flexible enough to recognize that labor savings exceeds those dollars spent for prefabricated items. DAVID is a professor of electrical technology at Long Beach City College, Calif., a consultant, and an expert witness. He can be reached at (562) 597-1877or by e-mail at email@example.com.