Section 250.30/Grounding Separately Derived Systems
NEC 250.30 covers grounding and bonding for separately derived systems such as transformers and generators.
A separately derived system (such as a transformer with no direct electrical connection to supply conductors) must have a system bonding jumper connecting the grounded conductor to the equipment grounding conductor and enclosure, a grounding electrode conductor connected to a grounding electrode, and a supply-side bonding jumper. The system bonding jumper must be installed at the source of the separately derived system or at the first system disconnecting means.
Why this section exists
Transformers and generators create new electrical systems that need their own grounding reference point. Without proper grounding, the secondary system floats at an unpredictable voltage relative to ground, ground faults may not clear, and metal enclosures can become energized. The separately derived system grounding ensures a low-impedance fault return path that allows overcurrent devices to operate during ground faults on the secondary side.
What plan reviewers look for
Plan reviewers check every transformer and generator on the one-line diagram for the required grounding connections. They verify the system bonding jumper location, grounding electrode conductor sizing per Table 250.66, and the grounding electrode connection. For generators used as optional standby (not separately derived with a 4-pole transfer switch), the grounding requirements differ.
Common violations
Related NEC requirements
Section 250.24 covers grounding at the main service. Section 250.50covers grounding electrode types. Section 700.12 covers emergency system power source grounding. Table 250.66 provides GEC sizing.