Why you should grounding a generator

Grounded vs. Bonded: Know Which One You're Dealing With

Generator grounding confuses more electricians than almost any other topic, and the reason is simple: the NEC treats portable and permanently installed generators very differently. Before you drive a rod or land a jumper, you need to know whether the generator is a separately derived system (SDS) or not. That single determination drives every grounding decision you'll make on the job.

A separately derived system has no direct electrical connection to the supply conductors of another system, including a solidly grounded neutral. If your transfer switch breaks the neutral, you've got an SDS. If it doesn't, you don't. NEC 250.30 governs SDS grounding; NEC 250.34 covers portable and vehicle-mounted units.

Get this wrong and you'll either create parallel neutral paths (current on the grounding conductor, nuisance GFCI trips, hot equipment cases) or leave a fault with no low-impedance return path. Both are dangerous. Neither passes inspection.

Portable Generators on a Jobsite

Per NEC 250.34, a portable generator does not require connection to a grounding electrode (a ground rod) if two conditions are met: the generator only supplies equipment mounted on the generator or cord-and-plug-connected equipment through receptacles on the generator, and the non-current-carrying metal parts of that equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame.

Translation: if you're running a saw or a light stand plugged directly into the generator, the frame is your ground. Driving a rod adds nothing to the fault-clearing path and can actually mislead people into thinking they've added protection when they haven't.

Field tip: most OEMs bond neutral to frame at the factory on portable units. Check with a meter before you assume. A quick continuity test between the neutral pin and the frame tells you everything.

When the Portable Feeds a Structure

The moment that portable generator feeds a structure through an inlet and transfer switch, the rules change. Now you're looking at whether the transfer equipment switches the neutral. If it does, you have an SDS and the generator needs its own grounding electrode system per NEC 250.30(A). If the neutral is not switched, the service grounding electrode system handles it, and the generator's neutral must not be bonded to the frame.

This is where a lot of installs go sideways. An electrician ties a portable into a manual transfer switch that doesn't break the neutral, forgets to remove the factory neutral-frame bond, and ends up with neutral current flowing on the EGC back to the main panel. GFCI receptacles on the generator will trip the instant you load them.

• Neutral switched at the transfer: bond neutral to frame at generator, drive/connect a grounding electrode.
• Neutral not switched (solid neutral): remove the neutral-frame bond, do not drive a separate rod for the generator.
• Always verify per manufacturer instructions; some inverter generators have floating neutrals by default.

Permanently Installed Standby Generators

Standby gensets at residential and commercial sites almost always function as SDSs because automatic transfer switches typically break the neutral. NEC 250.30(A) requires the system bonding jumper at the generator (or at the first disconnect), a grounding electrode conductor sized per 250.66, and connection to a grounding electrode.

Key points that inspectors flag repeatedly:

1. System bonding jumper is installed in exactly one location, either at the source or at the first disconnecting means, not both (250.30(A)(1)).
2. Grounding electrode conductor is sized from Table 250.66 based on the largest ungrounded conductor (250.30(A)(5)).
3. Supply-side bonding jumper runs with the feeder between the generator and the transfer switch, sized per 250.102(C) (250.30(A)(2)).
4. Grounding electrode is the nearest effective one: building steel, a concrete-encased electrode, or a driven rod meeting 250.52 requirements.

Vehicle-Mounted and Trailer Generators

Line truck generators, RV gensets, and trailer-mounted rental units fall under NEC 250.34(B). The vehicle frame serves as the grounding means provided the frame is bonded to the generator and the generator only supplies equipment mounted on the vehicle or cord-and-plug equipment through receptacles mounted on the vehicle or generator.

Trailer rigs used to feed temporary power on a job need the same SDS evaluation as any portable. Read the transfer equipment's listing, check whether the neutral is switched, and configure bonding accordingly. Rental units often ship with the neutral-frame bond intact, which is correct for standalone use but wrong for most transfer applications.

Field tip: tape a laminated card inside the generator's control cabinet documenting how it's configured (bonded vs. floating neutral) and the date. Saves the next tech an hour of troubleshooting.

Testing Before You Energize

Paper configuration is worthless without verification. Before you hand the job off, confirm the grounding system actually works.

• Measure neutral-to-ground voltage at the generator under load. Should read near zero volts with a proper single-point bond; a reading of several volts points to a parallel path or a missing bond.
• Verify continuity from the EGC at the furthest receptacle back to the generator frame and grounding electrode.
• For SDS installs, measure ground rod resistance per 250.53(A)(2). If a single rod exceeds 25 ohms, install a second rod at least 6 feet away.
• Test every GFCI receptacle on the generator before and after connection to the structure.

Document your readings. If something trips or smokes six months from now, those notes prove the system was correctly grounded the day you left it. That paper trail protects your license.