Code Q&A: Grounding Service-Supplied AC Systems

Oct. 8, 2015
NEC rules for grounding and bonding service equipment.

Q. What are the rules for grounding and bonding service equipment?

Find the Answer

A. Service equipment supplied from a grounded system must have the grounding electrode conductor (GEC) terminate in accordance with Sec. 250.24(A)(1) through (5). Let’s see what parts 1, 4 and 5 of this section have to say on the subject.

1) A GEC must connect the service neutral conductor to the grounding electrode at any accessible location, from the load end of the overhead service conductors, service drop, underground service conductors, or service lateral, up to and including the service disconnecting means.

Some inspectors require the service neutral conductor to be grounded (connected to the earth) from the meter socket enclosure, while other inspectors insist that it be grounded (connected to the earth) only from the service disconnect. Grounding at either location complies with this rule.

4) When the service neutral conductor is connected to the service disconnecting means [250.24(B)] by a wire or busbar [250.28], the GEC is permitted to terminate to either the neutral terminal or the equipment grounding terminal within the service disconnect.

5) A neutral-to-case connection isn’t permitted on the load side of service equipment, except as permitted by Sec. 250.142(B).

If a neutral-to-case connection is made on the load side of service equipment, dangerous objectionable neutral current will flow on conductive metal parts of electrical equipment [250.6(A)]. Objectionable neutral current on metal parts of electrical equipment can cause electric shock and even death from ventricular fibrillation, as well as a fire.

 A main bonding jumper [250.28] is required to connect the neutral conductor to the equipment grounding conductor within the service disconnecting means [250.24(B)].

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.

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