Ecmweb 25195 Nec Code Quandaries 0719 Pr
Ecmweb 25195 Nec Code Quandaries 0719 Pr
Ecmweb 25195 Nec Code Quandaries 0719 Pr
Ecmweb 25195 Nec Code Quandaries 0719 Pr
Ecmweb 25195 Nec Code Quandaries 0719 Pr

Stumped by the Code? Connecting Electrical Systems to the Earth

July 17, 2019
NEC Code Quandaries

All questions and answers are based on the 2017 NEC.

Underlined text indicates a Code change.

Q. What is the reason stated in the NEC for connecting electrical systems to the earth (grounding)?

A. Electrical power systems are grounded (connected to the earth) to limit the voltage induced by lightning, line surges, or unintentional contact by higher‑voltage lines and stabilize the system voltage to earth under normal operation [Sec. 250.4(A)(1)], as shown in the Figure.

System grounding helps reduce fires in buildings as well as voltage stress on electrical insulation, thereby ensuring longer insulation life for motors, transformers, and other system components.

Note 1: To limit imposed voltage, the grounding electrode conductors shouldn’t be any longer than necessary, and unnecessary bends and loops should be avoided.

Note 2: See NFPA 780, Standard for the Installation of Lightning Protection Systems, for grounding and bonding of lightning protection systems.

Q. Which alternating-current systems are required to be grounded by the NEC?    

A. Systems operating below 50V aren’t required to be grounded or bonded in accordance with Sec. 250.30 unless the transformer’s primary supply is from [Sec. 250.20(A)]:

(1) A 277V or 480V system.

(2) An ungrounded system.

The following 50V to 1,000V systems must be grounded (connected to the earth) [Sec. 250.20(B)]:

(1) Single-phase systems where the neutral conductor is used as a circuit conductor.

(2) Three-phase, wye-connected systems where the neutral conductor is used as a circuit conductor.

(3) Three-phase, high-leg delta-connected systems where the neutral conductor is used as a circuit conductor.

Q. What are the Code permitted methods for the termination of the grounding electrode conductor (GEC) for a grounded system?

A. Service equipment supplied from a grounded system must have the GEC terminate in accordance with Sec. 250.24(A).

Grounding Location. 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 disconnect [Sec. 250.24(A)(1)]. 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.

Grounding Termination. When the service neutral conductor is connected to the service disconnect [Sec. 250.24(B)] by a wire or bus bar [Sec. 250.28], the GEC can terminate to either the neutral terminal or the equipment grounding terminal within the service disconnect [Sec. 250.24(C)(4)].

Neutral‑to‑Case Connection. A neutral‑to‑case connection isn’t permitted on the load side of service equipment, except as permitted by Sec. 250.142(B) [Sec. 250.24(A)(5)]. 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 [Sec. 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.    

These materials are provided to us by Mike Holt Enterprises of Leesburg, Fla. To view additional Code training materials offered by this company, visit www.mikeholt.com
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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