At a grounding class, someone asked how to determine if a standby generator is required to be grounded as a separately derived system. The key to determining this is to verify the type of transfer equipment installed.
Permanently installed generators are separate power sources and are often installed as separately derived systems, which are defined in Article 100. The decision to connect a generator as a separately derived system is determined by the type of transfer equipment (transfer switch) specified for the design. It is important to understand the relationship between the type of transfer switch and how the grounding connections should be made for the generator. If the transfer equipment (switch) for a generator provides a switching action in the grounded (often a neutral) conductor, then the generator must be grounded as a separately derived system in accordance with all applicable requirements in 250.30(A).
The reason is that, in the normal power mode, the grounded (neutral) conductor for the load is connected to the grounding electrode conductor at the service. In standby mode, the grounded (neutral) conductor for the load is switched over to the generator source, which is grounded as a separately derived system. The result is that, in either position of the transfer switch, the generator electrical system output is grounded.
If there is no switching action in the grounded conductor through the transfer equipment, then the generator system remains grounded with the transfer switch in either the normal or standby mode because of the grounding electrode conductor connected at the service equipment. The grounding electrode for generator separately derived systems is required to be the same grounding electrode system for the building or structure served.
If generators are installed outdoors, the design often specifies an auxiliary grounding electrode. If the generator is a separately derived system and is located outdoors, a grounding electrode connection is required at the source location to comply with 250.30(C). The equipment grounding and bonding connections, in this case, must meet the requirements in 250.35(B) for permanently installed generators.
Informational Note No. 1 to Section 250.30 provides essential information that assists users in determining system grounding requirements for generators. The transfer equipment applied in the design is a key factor for determining if a generator must be grounded as a separately derived system. An effective ground-fault current path is required between the generator and the equipment supplied by the system. The effective ground-fault current path is required to be either a supply-side bonding jumper in accordance with 250.102(C) or a load-side equipment bonding jumper in accordance with 250.102(D).
The NEC does not require auxiliary electrodes; however, if installed, 250.54 provides specific requirements that must be met. The auxiliary electrode provides a direct connection to the earth at the generator location. This grounding connection is in addition to the grounding and bonding required by either 250.30(A) or 250.35(B), as covered above. The auxiliary grounding electrode must be connected to the equipment grounding conductor in addition to the generator’s frame. The earth is never permitted as an effective ground-fault current path. When auxiliary grounding electrodes are installed for equipment, they are not required to meet the 25-ohm requirements specified in 250.53(A)(2) Exception.
Section 250.35 covers important requirements for providing an effective ground-fault current path, which 250.4 addresses generally. Generators grounded as separately derived systems meet this requirement when installed according to the applicable rules in 250.30(A). If the generator is not installed as a separately derived system, an effective ground-fault current path must be provided in accordance with 250.35(B). This simply means a supply-side bonding jumper is required to be installed between the generator and the equipment grounding terminal bar or bus of the first disconnecting means enclosure supplied by the system. The supply-side bonding jumper must be installed in accordance with 250.102(C).
The minimum size of the supply-side bonding jumper must be based on either Table 250.102(C) or by using the 12.5% rule based on the total circular mil area of the largest ungrounded derived phase conductors connected to the generator. If the generator has an overcurrent device, the supply feeders from the output side of the overcurrent protective device on the breaker must include an equipment grounding conductor sized in accordance with 250.122 based on the rating of the overcurrent device at the generator’s output.