Many buildings and structures are supplied by power from a source other than a utility service. If the supply—such as a transformer or generator—is customer-owned, it is not a service and, therefore, is either a feeder or branch circuit. Specific grounding and bonding rules in 250.32 of the National Electrical Code (NEC) have to be applied to a building or other structure supplied by a feeder(s) or branch circuit(s).
The purpose of grounding and bonding systems at branch-circuit or feeder- supplied separate buildings or structures is similar to service-supplied buildings or structures. A connection to ground is required to establish a reference to ground at that location. This places all normally conductive noncurrent-carrying metal parts and other conductive materials at or as close to earth potential as possible.
Grounding electrode required
Section 250.32(A) indicates a grounding electrode system is generally required at a separate building or structure supplied by feeders or branch circuits. This condition is relaxed if the building or structure is not supplied by electrical power. There is also an exception that relaxes the grounding electrode requirement if a structure or building is supplied by a single branch circuit that includes an equipment grounding conductor (EGC) for grounding non-current-carrying parts of equipment. Section 250.32(A) provides the general requirement for a grounding electrode system in accordance with Part III of Article 250. This means that the same grounding electrode requirements in Section 250.50 must be applied to separate buildings or structures supplied by feeders or branch circuits. All electrodes present at the served building or structure must be used as the grounding electrode. If none are present to form a grounding electrode system, one must be installed. The exception to the grounding electrode requirement in Section 250.32 applies to a building or structure that is supplied by a single branch circuit, and that includes an EGC for grounding electrical equipment associated with it.
Grounding electrode conductor
Section 250.32(E) provides requirements for the grounding electrode conductor (GEC) installation at separate buildings or structures supplied by feeders or branch circuits. The GEC is generally sized according to Table 250.66, which bases sizing on the largest ungrounded circuit conductor supplying the building or structure. See the sizing alternatives in 250.66(A), (B) and (C) for sole electrode connections. The GEC must be installed in accordance with the applicable requirements in Section 250.64.
Feeders and branch circuits
Feeders are generally required to include an EGC, as indicated in Section 215.6. Feeders or branch circuits installed to supply a separate building or structure must include an EGC and can be a wire type, or it can be any wiring method in Section 250.118 that qualifies as an EGC. If it is a wire type, it must be sized in accordance with Section 250.122. If a feeder to a separate building or structure is installed using rigid metal conduit (RMC), the conduit can serve as the required EGC. Note that any metal raceways provided in Section 250.118 that qualify as EGCs can be used as EGCs. The common metallic raceways installed for feeders supplying separate buildings or structures are RMC and intermediate metal conduit. Corrosion or deterioration and their effect on the life expectancy of wiring methods in contact with the earth can be an issue in some areas. Corrosion and deterioration protection rules for wiring methods are provided in Section 300.6. Good design practices provide a wire-type EGC to ensure that the safety system is not compromised in the short or long term. Note that the NEC falls short of addressing specific life expectancy of grounding electrodes or other materials in contact with the earth; however, corrosion protection is addressed in Section 300.6.
Voltage-drop design considerations
In addition, voltage-drop considerations must be applied to designs for feeders and branch circuits supplying separate buildings or structures. The EGC has to provide an effective ground-fault current path. Section 250.32(B)(1) restricts a grounded conductor from being connected to a grounding electrode or the EGC for the separate building or structure.
There is still an exception that allows the grounded (usually the neutral) conductor of feeders or branch circuits to be used for grounding at separate buildings or structures. This exception applies only to existing premises wiring systems and is not applicable to new installations. The conditions of the exception are as follows:
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.