Code Q&A: Grounding Requirements for Multiple Services

July 25, 2014

NEC grounding requirements for multiple services

Q. When a building has more than one service (as allowed by Sec. 230.2), is there a requirement to ground these multiple services to a common grounding electrode? (Answer based on 2011 NEC.)

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A. Where an AC system is connected to a grounding electrode in or at a building or structure, the same grounding electrode must be used. If separate services, feeders, or branch circuits supply a building, the same grounding electrode must be used [250.58]. Two or more grounding electrodes that are bonded together will be considered as a single grounding electrode system in this sense.

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Metal parts of the electrical installation are grounded (connected to the earth) to reduce induced voltage on the metal parts from lightning so as to prevent fires from a surface arc within the building/structure. Grounding electrical equipment doesn’t serve the purpose of providing a low-impedance fault current path to open the circuit overcurrent device in the event of a ground fault.

Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the electric utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.

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.