At some properties, a single electric utility service supplies multiple buildings or structures. The service could directly supply one of the buildings, and feeders or branch circuits supply other buildings from that service equipment. Another arrangement might be that the utility service is freestanding, such as on a pole or pad-mounted, and feeders or branch circuits supply all buildings or structures. 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 that 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, metal building frame electrodes, concrete-encased electrodes, and so forth, as provided in 250.52(A). 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 (EGC).
Building, structure or equipment
For consistent application of the NEC, it is important to differentiate between a building, a structure and equipment. The definitions provide some clarity in that all buildings are structures, while not all structures are buildings. As an 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. As examples, 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, which is the structure. However, this does not render the combination of the equipment and footing a structure, thereby predicating 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 is produced or manufactured typically in a facility and delivered to the site for installation.
It is important to establish a clear differentiation between equipment and structures to effectively determine when a grounding electrode is required to be installed by 250.32(A) and when it is an option in accordance with 250.54. Note that it might be a good design practice to install an auxiliary grounding electrode for equipment that is freestanding and remote from buildings, but it is optional.
In accordance with 250.32(B)(1), a feeder(s) or branch circuit(s) supplying a separate building or structure generally must include an EGC. The EGC can be a wire, or it can be any wiring method 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 for 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 common electrically continuous metallic paths between the feeder source and the destination at the building or structure served. And finally, there is no ground-fault protection of equipment installed on the supply side of the feeder. This exception applies only to existing premises wiring systems. For existing premises wiring systems that use a grounded conductor in this manner, the grounded conductor must not be smaller than the sizes required in Section 250.122 or 220.61 as applicable. Exception No. 2 also permits the grounded conductor to be used in the manner specified in 250.32(A)(1) Exception No. 2.
Bonding piping systems
Section 250.104 provides 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 bonding jumper must not be less than the sizes provided in 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. Understanding what constitutes equipment, and what qualifies as a building or structure, will help you determine the requirements for grounding electrodes or the option of installing an auxiliary.