NEC 250.30 / 250.32 — Separately Derived Systems and Bonding at the Generator

A "separately derived system" (SDS) is a power source whose output has NO direct electrical connection to any other source's grounded conductor (neutral). Most residential standby generators with a SWITCHED-NEUTRAL automatic transfer switch are separately derived. Generators with a SOLID-NEUTRAL ATS are NOT separately derived — they share the utility's neutral.

Why this matters: if the system is separately derived, NEC 250.30 requires its own grounding-electrode connection at the generator. If it's NOT separately derived, you do NOT add a separate ground rod at the generator — bonding it back at the main service is sufficient. Adding a ground rod at a non-separately-derived generator creates a parallel grounding path and is a common AHJ rejection.

How to tell which type of ATS you have:

• Switched-neutral ATS — the ATS opens BOTH the hot AND the neutral when it transfers from utility to generator. The generator becomes a separately derived system. NEC 250.30 applies → install a ground rod at the generator AND bond the neutral to ground at the generator's first overcurrent device.
• Solid-neutral ATS — the ATS opens only the HOTS; the neutral remains continuous between utility and generator. The system is NOT separately derived. The neutral-to-ground bond stays at the main service only. NO ground rod at the generator.

Most residential whole-house ATS units (Generac RXSW, Kohler RXT) are SOLID-NEUTRAL by default. This means most NJ residential generator installs do NOT need a separate ground rod at the gen pad. Verify by reading the ATS manual — the neutral configuration is always specified.

For commercial standby gens with their own dedicated branch system, switched-neutral ATS is more common and the SDS rules apply: install at least one ground rod within 6 ft of the generator, size the GEC per Table 250.66 to the generator's largest service-entrance conductor.

NEC 250.32 (separate buildings/structures): if a feeder runs to a detached garage with a sub-panel, the sub-panel needs its own grounding electrode (typically a ground rod at the garage). The neutral and ground at the detached sub-panel stay isolated — same four-wire-feeder rule as any other sub-panel.

For generator + ATS SLDs, the free Malfettone SLD Builder at /tools/single-line-diagram has a "Whole-house standby generator + ATS" template assuming a solid-neutral 200A whole-house ATS — the most common NJ residential configuration.