Safety guide for grounding a generator

Separately Derived vs. Non-Separately Derived: Decide First

Before you drive a single rod, determine whether the generator is a separately derived system (SDS) or not. This controls everything that follows: bonding, grounding electrode connection, and neutral handling. NEC 250.20(D) and its Informational Note define the line. If the transfer switch opens the neutral, the generator is separately derived. If the neutral remains solidly connected through the transfer equipment to the service neutral, it is not.

Get this wrong and you either create parallel neutral paths on the grounded conductor (objectionable current per 250.6) or you leave a fault current with no low-impedance return. Both will bite you, one on the meter, the other on a body.

Field tip: If you can't tell from the transfer switch label, pull the cover and trace the neutral. Switched neutral pole? SDS. Solid neutral bar through? Not SDS. Don't guess from the nameplate.

Portable Generators on a Job Site

Cord-and-plug portables feeding tools directly follow 250.34(A). The frame serves as the grounding electrode for equipment plugged into receptacles mounted on the generator, provided the equipment-grounding conductors of those receptacles are bonded to the frame. No driven rod required for this configuration. This is the OSHA-acceptable setup for most construction use.

The moment you feed a structure, a panel, or a transfer switch, the rules change. Now 250.30 or 250.34(B) applies depending on whether the neutral is switched. GFCI protection for 15, 20, and 30 amp 125V and 125/250V receptacles on portables is required by 590.6 and by the generator manufacturer under UL 2201.

• Tools plugged directly into generator receptacles: frame bond only, no rod.
• Generator feeding a building through a transfer switch: treat per 250.30 if SDS.
• All 15/20/30A 125V outlets: GFCI protected, tested before each shift.

Standby and Optional Standby Systems: 250.30 Walkthrough

For a separately derived standby generator, 250.30(A) requires a system bonding jumper, a grounding electrode conductor, and connection to a grounding electrode. Install the system bonding jumper at one location only: either at the source (generator) or at the first disconnecting means, not both. Sizing per 250.102(C) based on the largest ungrounded conductor.

The grounding electrode conductor runs from the same point where the bonding jumper is installed to the nearest effectively grounded structural metal or a grounding electrode per 250.52. Size per 250.66. If you bond at the generator, your GEC lands at the generator. If you bond at the first disconnect, it lands there. Pick one and commit.

1. Confirm SDS status by inspecting transfer switch neutral.
2. Choose bonding point: generator or first disconnect.
3. Install system bonding jumper at that single location, sized per 250.102(C).
4. Run GEC from that point to nearest qualifying electrode, sized per 250.66.
5. Verify equipment grounding conductor runs with the feeder per 250.118.

When the Neutral Stays Connected (Non-SDS)

If the transfer switch does not open the neutral, the generator is not separately derived. Do not install a bonding jumper at the generator. Do not drive a dedicated grounding electrode for the generator as its system ground. The service grounding electrode system already serves this purpose through the common neutral.

The equipment-grounding conductor from the generator frame still runs back with the feeder to the transfer equipment and bonds to the existing EGC system. This is the setup that trips up most installers on residential whole-house standbys. Check the ATS listing: many residential units come with solid neutrals specifically so the installer does not need to re-establish a grounding electrode system.

Field tip: On a 22kW residential standby with an automatic transfer switch, read the installation manual before you touch the bonding screw. Some units ship bonded from the factory and must be unbonded when used with a solid-neutral ATS. Others ship unbonded.

Grounding Electrodes and GEC Sizing

When a grounding electrode is required, 250.52 lists what qualifies: metal underground water pipe, effectively grounded structural metal, concrete-encased electrode, ground ring, rod, pipe, or plate electrodes. A single driven rod must test 25 ohms or less per 250.53(A)(2), or be supplemented by a second electrode at least 6 feet away.

Size the grounding electrode conductor using Table 250.66 based on the largest ungrounded conductor in the feeder from the generator. A GEC to a rod, pipe, or plate electrode is not required to be larger than 6 AWG copper per 250.66(A). Protect the GEC from physical damage where required by 250.64(B).

• Single rod: test or supplement. No exceptions.
• GEC to rod: 6 AWG copper maximum required.
• GEC to concrete-encased electrode: 4 AWG copper maximum required.
• Irreversible compression connectors or exothermic welding for buried connections, 250.70.

Verify Before You Energize

Commissioning is not optional. Open the main, close the ATS to generator, and measure neutral-to-ground voltage at a load-side receptacle with the generator running and loaded. Anything above a volt or two with modest load means you have a bonding problem, typically a double bond or a missing bond.

Run a ground-impedance test on the feeder EGC to confirm the fault path. Document the electrode resistance reading. If the AHJ asks how you verified the install, you want numbers on paper, not assurances.