Simplifying motor coordination studies

Low-voltage motor protection is normally the starting place for coordination studies of new industrial facilities. Also, improper low-voltage motor coordination is the source of many existing facility problems. Some engineers may frequently do coordination studies, but many engineers only occasionally do coordination studies. Trying to recall the applicable codes, standards, and references is difficult. Funds may not be available to hire “experts” for this type of study. The flow chart developed is to guide the “occasional” coordination engineer and to assist coordination “experts” by providing a reliable method for performing low-voltage three-phase motor coordination studies. The flow chart methodology is suggested for simplifying low-voltage overcurrent device coordination studies for new and existing three-phase motor installations. The flow chart provides references to codes and standards, highlights benchmarks for coordination, provides guides to device selection, and notes the overcurrent values of interest. The text elaborates the use of the National Electrical Code (NEC) for complying with legal requirements, and notes some significant changes in the 1999 NEC. The text also discusses some device selection criteria and expands on the decision points of the flow charts. Other flow charts for upstream devices and medium voltage equipment are being developed for future publication