Q. How do I size an optional standby generator when a manual or automatic transfer switch is installed? (Answer based on 2011 NEC.)
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A. The calculated load on the standby source must be in accordance with Art. 220 or by a method approved by the Authority Having Jurisdiction (AHJ) [702.4(B)].
When using manual transfer equipment, the optional standby power source must have adequate capacity for all equipment intended to operate at one time as determined by the user [702.4(B)(1)]. Note: When a manual transfer switch is used, the user of the optional standby system selects the loads to be connected to the system, which determines the system kVA/kW rating.
When using automatic transfer equipment, the optional standby power source must have adequate capacity to supply the full load transferred [702.4(B)(2)]. Note: For a new installation, the load is determined by Art. 220 or an alternate method approved by the AHJ. For existing facilities, the maximum demand data for one year — or the average power demand of a 15-min. period over a minimum of 30 days — can be used to size the power source [220.87].
A transfer switch is required for all fixed or portable optional power systems [702.5].
Exception: Temporary connection of a portable generator without transfer equipment is permitted where written safety procedures are in place and conditions of maintenance and supervision ensure that only qualified persons will service the installation — and where the normal supply is physically isolated by a lockable disconnecting means or by the disconnection of the normal supply conductors.
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.