Generator Neutral Bonding: Bonded vs Floating

This is the technical detail most often gotten wrong in generator installations, and it's invisible until it causes a problem.

The rule

The neutral-to-ground bond must exist exactly once in the system.

That single bond is what gives fault current a low-impedance path back to the source, so an overcurrent device trips. Too few bonds and faults don't clear. Too many and normal return current flows where it shouldn't.

See grounding vs bonding for the underlying principle.

Two generator configurations

Bonded neutral. The generator's neutral is connected internally to its frame and grounding system. It provides its own bond.

Floating neutral. No internal connection between neutral and frame. The generator relies on a bond somewhere else in the system.

Most portable generators ship bonded, because the common use case is standalone operation feeding extension cords — where the generator is the only source and needs to provide its own bond.

Which one you need

It depends entirely on what the transfer equipment does with the neutral.

Transfer equipment does NOT switch the neutral

This covers most interlock kits and many transfer switches. The neutral stays continuously connected through to the main panel, where the service bond exists.

That service bond is still in the circuit. So the generator must be floating — otherwise you have two bonds.

This is the most common residential case, and it's why a bonded portable generator often needs converting before being connected to a house.

Transfer equipment DOES switch the neutral

A 3-pole transfer switch (for a 240 V system) switches both energized conductors and the neutral. When transferred, the house's service bond is disconnected from the generator's circuit.

Now nothing else provides a bond, so the generator must be bonded. It's operating as a separately derived system.

Standalone operation

Generator feeding extension cords only, nothing connected to the house. The generator is the only source and must be bonded to provide the reference its own GFCI receptacles need.

Summary

What goes wrong

Two bonds. Normal return current divides between the neutral conductor and the grounding path. Everyday load current then flows on equipment grounding conductors, conduit, metal enclosures, and water pipes — continuously, not just during a fault. That's precisely what the grounding system is supposed to be free of so it can do its job.

It also commonly causes GFCI devices to trip, because current returning by an unexpected path looks like leakage. Nuisance GFCI tripping on a generator is very often this.

No bond. Fault current has no low-impedance path back to the source, so a breaker may not trip on a ground fault. The fault persists, and metal parts can stay energized.

Both failures are invisible during normal operation, which is why this gets missed.

How to tell what you have

Check the manufacturer's documentation. The manual states whether the generator is bonded or floating, and usually documents the conversion procedure if one exists.

Look for a bonding strap or screw at the generator's neutral terminal, as described in the manual.

Ask the installer. Anyone installing a transfer switch or interlock should have determined this. It's a reasonable question and a good test of whether the work was thought through.

Don't test it by trial and error. Nuisance GFCI tripping is a symptom, not a diagnostic method.

Converting

Many generators can be converted, and manufacturers document how — usually removing or adding a bonding strap or screw.

Follow the manufacturer's procedure. Don't improvise, and don't cut wires. If the manual doesn't describe a conversion, the generator may not support one, and the transfer equipment should be selected to match instead.

Some manufacturers sell bonding plugs that adapt a floating generator for standalone use — a plug inserted into a receptacle that establishes the bond temporarily. Useful if you use the same generator both ways.

Why is this electrician work?

Beyond the panel-side hazards, this decision requires knowing what the transfer equipment does with the neutral — which is not something you can determine by looking at the outside of a switch.

It's also exactly the kind of thing an inspector checks, and exactly the kind of defect that passes unnoticed for years until a fault occurs. See generator permits and inspection.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NEC 250.34 — portable and vehicle-mounted generators
• NEC 250.30 — separately derived systems, which decides bonded vs floating

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Transfer switch explained
• Generator electrical requirements
• Grounding vs bonding
• Size it properly: generator sizing calculator

More in our generator and backup power guides.