Equipment and Structure: Grounding and Bonding at Separate Buildings or Structures

Multiple buildings or structures on the same property are supplied by a single, free-standing electric utility service, and the separate buildings on the property are supplied by feeders. The service could also directly supply one of the buildings, and feeders or branch circuits supply other buildings from that service equipment. Specific grounding and bonding rules apply to separate buildings or structures supplied by feeders or branch circuits. The requirements are located in Part III of National Electrical Code (NEC) Article 250—specifically, 250.32.

Section 250.32(A) requires a grounding electrode system in accordance with Part III of Article 250. This means the grounding electrode requirements in 250.50 must be applied to separate buildings or structures supplied by feeders or branch circuits. If any of the grounding electrodes in 250.52(A) are present (exist) at the building or structure served, they must be bonded together to form a grounding electrode system. This includes the water pipe electrodes, in-ground metal support structures that qualify as electrodes, ­concrete-encased electrodes, and so forth. If no electrodes are present, one must be installed. The exception to the grounding electrode requirement in 250.32(A) applies to a building or structure that is supplied by a single branch circuit, either an individual or multiwire, that includes an equipment grounding conductor.

For consistent application of the NEC, it is important to differentiate between a building, a structure and equipment. The definition of “structure” has been revised in the 2017 NEC as “that which is built or constructed, other than equipment.” The definitions clarify that all buildings are structures, while not all structures are buildings.For example, a house is a building, while a billboard sign is a structure.

The other important factor to understand is the difference between equipment and structure. For example, a motor or a ground-mounted air-conditioner are equipment covered by Chapter 4 of the NEC. When installed outdoors, they are typically anchored to a concrete pad or footing. The concrete footing is the structure to which the equipment is mounted. This does not render the combination of the equipment and footing—a structure—and thus trigger the requirements for a grounding electrode. Structures are constructed. Any grounding electrode at these types of equipment installations is likely an auxiliary grounding electrode and, therefore, is optional.

Another example is a parking lot luminaire installed on a pole. The luminaire and the pole are equipment covered by NEC Article 410, and the concrete base is a structure to which the equipment is mounted. The key difference between equipment and a structure is that structures are constructed, as indicated in the definition, while equipment typically is produced or manufactured in a facility and delivered to the site for installation. It is important to differentiate between equipment and structures to effectively determine when 250.32(A) requires a grounding electrode and when it is an option in accordance with 250.54.

In accordance with 250.32(B)(1), a feeder or branch circuit supplying a separate building or structure generally must include an equipment grounding conductor (EGC). The EGC can be a wire or any wiring method in provided in Section 250.118 that qualifies as an EGC. If the EGC is a wire type, it must be sized in accordance with 250.122. Exception No. 1 allows the grounded (usually the neutral) conductor of feeders or branch circuits to be used for grounding at separate buildings or structures under specific and prescriptive conditions. First, an EGC is not included with the circuit supplying the separate building or structure. Second, there are no continuous metallic paths between the feeder source and the destination at the building or structure served.

Finally, there is no ground-fault protection of equipment installed on the supply side of the feeder. This exception only applies to existing premises wiring systems. For existing systems that use a grounded conductor in this manner, the conductor must not be smaller than the sizes required in 250.122 or 220.61 as applicable. Exception No. 2 permits the grounded conductor to be used in the manner specified in 250.32(A)(1) Exception No. 2.

Section 250.104 provides new requirements for bonding of metal water piping systems and other metal piping installed in or attached to a building or structure supplied by feeders or branch circuits. The size of the bonding jumper for metal water piping and interconnected building steel must not be less than the sizes provided in Table 250.102(C)(1) rather than Table 250.66.

Specific grounding and bonding rules apply to separate buildings or structures supplied by a feeder(s) or branch circuit(s). A grounding electrode is generally required at such buildings or structures, unless the building or structure is supplied by a single branch circuit and meets the requirements in the exception to 250.32(A). Understanding what constitutes equipment as compared to what qualifies as a building or structure helps Code users determine the requirements for grounding electrodes, or the option of installing an auxiliary grounding electrode.

About the Author

Michael Johnston

Executive Director of Standards, NECA
Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Technical Correlating Committee. He served as a principal representative on NEC CMP-5 representing IAEI for the 2002, 2005, and 2008 cycles and is currently...

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