Consulting-Specifying Engineer - April 2008 - (Page 56) Codes & Standards disconnect typically be required (most generator breakers are not visible from the building). What size of generator breaker should feed a fire pump? The utility feeder powering the fire pump must be sized to indefinitely carry locked rotor current per NEC 695.4(B)(1). But does this apply to the generator? Confusion comes from the way NEC 695 interweaves the utility and generator requirements. As a result, generator suppliers receive many requests for magnetic-only breakers and breakers sized for six times rated amps. But what does the code actually request? The basic requirement is for the generator source and overcurrent protection to be sized for “normal starting and running” (NEC 695.4(B)(1)) and not indefinite locked rotor operation. The NEC Handbook states that “alternative source overcurrent protection device(s) for the electric-driven fire pump are not required to be sized for locked rotor current … the circuit components of the alternative source are permitted to be sized according to Article 430.” Confusion with respect to fire pumps is multipart. First, most people miss the phrase “remote power sources” when reading NEC 695.4(B)(1). This section, which requires overcurrent devices to be set for lock rotor current, only applies to remote power sources—the utility. Another point of confusion occurs around the topic of no overload protection. NEC 695.6(D) specifies: “Power circuits shall not have automatic protection against overloads. Branch circuits and feeder conductors shall be protected against short circuit only.” Many people mistakenly interpret this to mean a magnetic-only circuit breaker. The NEC defines overload protection in 430.32 and limits overload protection at 125% of rated running amps. Consequently, a breaker larger than 125% would provide short circuit protection only. This is the minimum size of a breaker feeding the fire pump from the generator source. The maximum size of the fire pump breaker typically will be limited to 250% of rated running amps: NEC 695.4(B) requires compliance with NEC 430.62. My interpretation is that the generator fire pump breaker(s) should be sized 125% to 250%, but local interpretation and norms may require a different solution to meet the AHJs approval. For more detailed information, visit the Web cast archive at csemag.com, and view the Web cast, “Understanding NEC Code for Standby Generators. Kirchner is an electrical engineer and sales training manager with Generac. With the Rugged SEL-2411 Programmable Automation Controller Easily program with powerful logic, math, and more Mix and match flexible I/O with SELECT I/O cards Calculate accurate ac electrical quantities Reduce analysis time with state changes reported to the millisecond Measure temperatures with RTD inputs or remote SEL-2600 Series RTD Module Choose from serial and Ethernet communications options and multiple protocols flexible I/O and control will improve your system at www.selindustrial.com/cse4 Electrostatic Shock (15 kV) Heat (+85°C) Vibration (15 g Shock) Cold (—40°C) The SEL-2411 withstands vibration, electrical surges, fast transients, and extreme temperatures, with over ten times the predicted MTBF of most PLCs. ® Input #230 at csemag.com/quickResponse 56 Consulting-Specifying Engineer • APRIL 2008 http://www.selindustrial.com/cse4 http://www.selindustrial.com http://csemag.com/quickResponse
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