Rules for Separately Derived Systems: To ground or not to ground and other questions

By Michael Johnston | Sep 11, 2023
Transfer equipment that provides a switching action in the grounded conductor. Michael Johnston
The NEC provides an extensive set of rules specifically related to grounding and bonding for separately derived systems.

Electrical wiring and power distribution systems for commercial, industrial, institutional and some residential occupancies typically include the installation and use of separately derived systems. A separately derived system is generally a separate power source, such as a battery, generator, photovoltaic (PV) system, transformer, wind turbine or other source that produces electrical power. The NEC provides an extensive set of rules specifically related to grounding and bonding for separately derived systems.

Connection to another source?

One of the keys to determining if a system (source) is separately derived is related to whether the system has a “direct electrical connection” to another source. Some examples of separately derived systems include generators, batteries, converter windings, transformers, solar PV systems and wind turbine generators. Common separately derived systems include transformers and generators. Some transformers do not qualify because one winding in the transformer is common to the input and the output side. These are autotransformers, and they are addressed by requirements in Article 450. Examples include core and coil ballasts in fluorescent luminaires and “buck-boost” transformers used for raising or lowering voltage levels for specific applications or individual pieces of utilization equipment.

Grounding a separately derived system means that the system will be connected to ground (earth) through a grounding electrode, in addition to the enclosure containing the system, if applicable. When a system is connected to the earth, one conductor supplied by the system is intentionally grounded. This creates a grounded conductor of this system to which all NEC rules for grounded conductors must apply. 

Section 250.30 indicates that the grounding requirements of 250.30(A) and (B) apply to separately derived systems. Sections 250.20, 250.21 and 250.26 are also directly related to the requirement for grounding a separately derived system.

Type of transfer equipment

Generators can be separately derived systems. How the grounding and bonding connections are made at a generator is usually determined by the type of transfer equipment. There is an important informational note following 250.30 that describes the transfer switch and how the grounding connections are made for the generator.

First, if a transfer switch for a generator includes a switching action in the grounded conductor, then the generator must be grounded as a separately derived system in accordance with all applicable requirements in 250.30(A). This is necessary because in the normal mode the grounded conductor is connected to the service grounding electrode, whereas in standby mode the grounded conductor is switched over to the generator, which must be grounded as a separately derived system. The result is that in either transfer switch position, the system is grounded. If there is no switching action in the grounded conductor by the transfer switch, then the generator system remains grounded with the transfer switch in either position if there is a grounded conductor. A generator system can be grounded (neutral point connected to earth through a grounding electrode conductor), but if no line-to-neutral loads are supplied, there might not be a grounded conductor. 

Although the generator in this case is not a separately derived system, the grounding and bonding connections must meet the requirements in 250.35 for permanently installed generators. 

Performance concepts of 250.35 focus on providing an effective ground-fault current path with the supply conductors from the generator to the first disconnecting means or equipment supplied. Generators grounded as separately derived systems meet this requirement when installed according to the rules in 250.30(A). If the system does not have an overcurrent protective device (OCPD) at the generator, a supply-side bonding jumper is required between the generator and transfer equipment. The sizing requirements for this bonding jumper are related to the location of the first system OCPD. If the bonding jumper is on the supply side of the first system OCPD, it is sized as a supply-side bonding jumper in accordance with 250.102(C) and Table 250.102(C)(1). This means that the size is based on Table 250.102(C)(1) or 12.5%, based on the circular mil area of the ungrounded derived phase conductors supplied from the generator.

Generators are often installed outside buildings or structures with the feeder routed to the transfer equipment, typically inside the building or structure. The conductor installed on the load side of a generator OCPD is an equipment grounding conductor (EGC) because it is a load-side installation. This EGC is required to be sized using Table 250.122, based on the rating of the OCPD supplied. The importance of the grounding and bonding connections and sizing rules is to provide an effective path for the ground-fault current back to the generator (source).

Header image: Transfer equipment that provides a switching action in the grounded conductor. Photo by Michael Johnston.

About The Author

A man, Mike Johnston, in front of a gray background.

Michael Johnston

NECA Executive Director of Codes and Standards

JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].






featured Video


New from Lutron: Lumaris tape light

Want an easier way to do tunable white tape light?


Related Articles