Code-compliant approach to grounding a generator

Separately Derived or Not: Make the Call First

Every generator grounding decision starts with one question: is this a separately derived system (SDS) or not? The answer drives bonding, grounding electrode connections, and how you handle the neutral. Get this wrong and you either create parallel neutral paths or lose your ground-fault return entirely.

Per NEC 250.30, a generator is an SDS when the transfer switch switches the neutral along with the hot conductors. If the neutral is solidly connected through to the service (non-switched neutral in the transfer switch), it is not separately derived. Portable generators under NEC 250.34 have their own rules and rarely get bonded to a premises grounding electrode.

Field tip: pop the transfer switch cover and trace the neutral bar before you quote the job. A 3-pole switch means SDS. A 2-pole (in a 120/240 single-phase) with a solid neutral means non-SDS. That five-minute check changes the entire bonding plan.

Bonding the Neutral: One Point Only

The neutral-to-ground bond exists at exactly one location in any system. For an SDS generator, that bond lives at the generator per NEC 250.30(A)(1), and the service main bond must not carry generator neutral current. For a non-SDS generator, the bond stays at the service, and the generator's internal bond must be removed or lifted.

Most packaged standby gensets ship with a factory neutral-ground jumper. If you are installing it as non-SDS, that jumper comes out. Document it, photograph it, and note it on the as-builts. Inspectors look for this specifically.

• SDS with 3-pole ATS: bond at the generator, size per NEC 250.102(C) and Table 250.66.
• Non-SDS with 2-pole ATS (single-phase) or solid-neutral 3-pole: remove factory bond at generator.
• Never leave bonds at both ends. Objectionable current on the equipment grounding conductor is a 250.6 violation and a shock hazard.

Grounding Electrode Conductor Sizing and Routing

For an SDS, NEC 250.30(A)(4) requires a grounding electrode conductor (GEC) run from the generator neutral point to the nearest effectively grounded structural metal, metal water pipe within 5 feet of entry, or a made electrode if those are not available. Size the GEC per NEC Table 250.66 based on the largest ungrounded conductor from the generator.

For non-SDS, you do not run a separate GEC to the generator. The equipment grounding conductor (EGC) pulled with the feeders back to the service handles fault current return. That EGC is sized per NEC Table 250.122 based on the overcurrent device protecting the feeder.

Keep the GEC continuous where practical. If you must splice, use an irreversible compression connector or exothermic weld per NEC 250.64(C). Standard split bolts under tape are not acceptable for GEC splices.

Portable and Vehicle-Mounted Generators

NEC 250.34 is the shortcut most installers miss. A portable generator supplying only cord-and-plug-connected equipment plugged directly into the generator does not require a connection to a grounding electrode, provided the frame is bonded to the generator's EGC terminal and the non-current-carrying metal parts of the equipment are bonded to the frame.

The moment that portable feeds a structure through an inlet or transfer switch, different rules apply. Now you are back in 250.30 or 250.20 territory depending on how the neutral is handled. OSHA 1926.404(f)(3) mirrors this for construction sites but adds assured equipment grounding conductor program requirements if you are not using GFCI.

On jobsite temp power, a trailer-mounted generator feeding a spider box through a 50A twist-lock is almost always non-SDS. Check the genset's internal bonding; if the neutral is bonded to the frame and your spider box also bonds neutral to ground, you have two bonds and nuisance GFCI trips are coming.

Ground Rods, Made Electrodes, and the 25-Ohm Rule

If the premises grounding electrode system is not accessible (remote pad-mount genset, standalone pump house), you are driving rods. NEC 250.53(A)(2) requires a single rod to be supplemented unless you can document 25 ohms or less to ground. In practice, drive two rods minimum, spaced at least 6 feet apart per NEC 250.53(A)(3), and skip the fall-of-potential test.

Bond the generator frame, the GEC, and any metallic raceways together at a single point. Use listed ground clamps rated for direct burial (NEC 250.70) and protect the GEC from physical damage per NEC 250.64(B).

1. Drive two 8-foot copper-clad rods, 6 feet apart minimum.
2. Run a continuous #6 copper GEC (minimum for rod electrodes per NEC 250.66(A)).
3. Use an acorn clamp or listed compression connector below grade.
4. Bond to generator frame lug, not to a random bolt on the enclosure.

GFCI, Ground Fault Protection, and Inspector Checks

Generators over 150V to ground and rated more than 1000A require ground-fault protection of equipment per NEC 230.95 when installed as a service, and NEC 700.31/701.26 restrict GFPE on emergency and legally required standby systems to alarm-only. Know which system you are wiring. A nuisance GFPE trip on a life-safety feeder is a code violation, not just a callback.

Before you call for inspection, walk the install with this list. Inspectors in most AHJs check these exact points:

• Neutral bonded at one location only, verified with a clamp meter on the EGC (should read near zero under load).
• GEC sized correctly and continuous, or spliced with listed irreversible means.
• Transfer switch pole configuration matches the SDS determination.
• Equipment grounding conductor present and bonded at both ends of the feeder.
• Working clearances per NEC 110.26 around the generator and ATS.

Document the bonding decision in writing on the panel schedule or a label inside the ATS. The next electrician on the job will thank you, and so will the inspector three years from now when the AHJ changes.