Q. What are the Code requirements for transfer equipment of emergency systems?
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A. As noted in Sec. 700.4 of the 2014 NEC, an emergency power system must have adequate capacity to carry all emergency loads expected to operate simultaneously. If an alternate power supply has adequate capacity, it’s permitted to supply emergency loads [Art. 700], legally required standby loads [Art. 701], and optional standby system loads [Art. 702]. If the alternate power supply doesn’t have adequate capacity to carry the entire load, it must have automatic selective load pickup and load shedding to ensure adequate power in the following order of priority:
1) The emergency circuits,
2) The legally required standby circuits, and
3) The optional standby circuits.
A temporary alternate source of power must be available whenever the emergency generator is out of service for more than a few hours for maintenance or repair.
As noted in Sec. 700.5, transfer equipment must be automatic, identified for emergency use, and approved by the authority having jurisdiction (AHJ). Automatic transfer switches must be electrically operated, mechanically held, and listed for emergency power system use. Transfer equipment must supply only emergency loads. Note: Multiple transfer switches are required where a single generator is used to supply both emergency loads and other loads.
About the Author
Mike Holt
Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.