Floating vs bonded neutral
A bonded-neutral generator connects its neutral to the frame (its own ground); a floating-neutral generator leaves them separate, expecting the building’s panel to provide the bond. The wrong pairing with a transfer switch can trip GFCIs or defeat ground-fault protection.
When a generator feeds a house through a transfer switch that does not switch the neutral, the panel’s existing neutral-ground bond serves the system — a floating-neutral generator is the correct match, and a bonded one creates a second bond and nuisance GFCI trips.
Used standalone with extension cords, a bonded-neutral design is what OSHA expects on work sites. Check the spec sheet or the plate near the outlets; some models have a switchable bond. This is exactly the kind of detail worth confirming with the electrician who installs your inlet.
| Bonded neutral | Neutral tied to the frame inside the generator — measures near 0 Ω between neutral and ground contacts |
|---|---|
| Floating neutral | Neutral isolated from the frame — measures open |
| Code hinge | NEC 250.30 applies only if the generator is a separately derived system, which depends on whether the transfer switch switches the neutral |
| 3-pole switch (solid neutral) | Generator is not separately derived — pair it with a floating-neutral generator |
| 4-pole switch (switched neutral) | Generator is separately derived — it needs its own neutral-ground bond and grounding electrode connection |
| NEC 250.34 | A portable generator’s frame needs no grounding electrode when it supplies only cord-and-plug equipment through receptacles mounted on the generator |
What the bond is, and why its location is the whole question
In any AC system there is exactly one place where the grounded conductor — the neutral — is connected to the equipment grounding system. In a house that place is the main service panel. Its job is to give fault current a low-impedance path back to the source so an overcurrent device opens fast, and to hold every piece of grounded metal at the same potential. Exactly one bond is the requirement. Zero bonds and fault current has nowhere to go, so a faulted enclosure simply sits energised. Two bonds and normal neutral current divides between the neutral conductor and the grounding conductors, putting working current on metal that is supposed to be dead.
A bonded-neutral generator makes that connection inside the machine. A floating-neutral generator does not, and expects to be connected to a system that already has one. Neither is safer in the abstract. Each is correct for exactly one wiring arrangement, and fitting the wrong one produces a genuine hazard rather than an inconvenience.
Finding out which you have takes two minutes. The manual states it; failing that, with the generator off and nothing plugged in, measure resistance between the neutral and ground contacts of one of its 120 V receptacles. Near zero means bonded, open means floating. Some models ship with a bonding screw or jumper that the manual documents as removable, and a few have a switch. Establish this before the electrician wires the inlet, not afterwards.
Matching the generator to the transfer equipment
The question that decides everything is whether your transfer equipment switches the neutral. Most residential transfer switches, and every interlock arrangement, do not: they switch only the two hot legs, and the neutral runs straight through, permanently tied to the panel’s neutral bus and therefore to the panel’s bond. In that arrangement the generator is not a separately derived system, NEC 250.30 does not apply, and the generator needs a floating neutral. Bring a bonded generator to that install and you have created a second bond — neutral current now returns partly through the equipment grounding conductor and the ground rod, GFCIs trip for no visible reason, and grounded metal in the house carries current it should not.
A four-pole transfer switch switches the neutral along with the hots. The generator’s output is then isolated from the utility-derived system entirely, making it a separately derived system, which under NEC 250.30 must have its own neutral-to-ground bond and a connection to a grounding electrode. Here a bonded generator, or a bond added at the switch, is the correct configuration. Four-pole switches turn up more often on standby installations and wherever there is ground-fault protection at the service that a second bond would confuse.
Getting it wrong in the other direction — a floating generator behind a four-pole switch, with no bond anywhere — is the more dangerous mistake, because nothing appears to be wrong. A hot conductor faulting to a metal enclosure produces almost no current, so no breaker trips and no GFCI operates. The enclosure sits at line potential waiting for a person to complete the circuit. Nuisance GFCI trips at least announce themselves; this failure mode is silent until it is not.
Standalone use, GFCI receptacles, and bonding plugs
Used the way most portables actually get used — extension cords from the generator’s own receptacles to appliances and tools — a bonded neutral is what you want, and it is what OSHA assumes on a job site. Both OSHA and NEC 250.34 accept that a portable generator supplying only equipment through receptacles mounted on it does not need a driven ground rod, provided the non-current-carrying metal parts are bonded to the frame and the neutral is bonded to the frame as well. The generator is its own reference; a rod driven into dry soil adds resistance, not safety, and is not required.
This is also what makes the generator’s own GFCI receptacles work as designed. A GFCI detects a mismatch between the current leaving on the hot and returning on the neutral. On a bonded machine, a hot-to-frame fault has a return path through the frame back to the bonded neutral, which produces exactly that mismatch and trips the device. On a floating machine there is no such path, so a fault to the frame can bring the entire chassis to line potential without tripping anything at all.
Bonding plugs are the field fix: a plug wired with neutral tied to ground, inserted into a spare receptacle, which bonds a floating-neutral generator for standalone use. They are standard practice in RV and equipment-rental circles. The plug must come out before the generator is connected to a house through a non-switched-neutral transfer switch or interlock, because leaving it in reinstates the double bond. Any safety control that depends on a person remembering to move a part between two modes is a weak one — if the generator’s main job is backing up the house, buy the neutral configuration that install needs and leave it configured.
Frequently asked questions
Should my generator have a floating or bonded neutral?
It depends on how it will be connected. For house backup through the usual three-pole transfer switch or a panel interlock — where the neutral is not switched — you need a floating-neutral generator, because the panel already provides the system bond. For standalone use with extension cords, or behind a four-pole switch that does switch the neutral, you need a bonded one. Decide from the transfer equipment, not from a general preference.
Why does my GFCI trip when I connect my generator to the house?
Almost always a double neutral-ground bond. The house panel bonds neutral to ground, and if the transfer switch does not switch the neutral, a bonded-neutral generator adds a second bond. Neutral current then splits between the neutral conductor and the grounding path, and the GFCI reads that as leakage. The fix is either a floating-neutral generator, removal of the generator’s bonding screw where the manufacturer documents that as permitted, or a transfer switch that switches the neutral.
Do I need to drive a ground rod for my portable generator?
For ordinary standalone use, no. NEC 250.34 permits a portable generator supplying only cord-and-plug equipment through receptacles mounted on the generator to use its own frame as the reference, with no grounding electrode, as long as the frame is bonded to the receptacle grounding terminals. When the generator feeds a building through an inlet, the building’s grounding electrode system serves that function. Where a rod is required — some separately derived installations — that is an electrician’s determination, not a default precaution.
How do I tell if my generator is bonded?
With the engine off and nothing plugged in, put an ohmmeter between the neutral and ground contacts of one of its 120 V receptacles. A reading near zero ohms means the neutral is bonded to the frame; an open reading means it floats. The manual and the plate near the outlet panel usually state it too.
Can I unbond a bonded-neutral generator?
Only where the manufacturer documents a removable bonding screw or jumper and explains how. Cutting a bond that is not designed to be removed modifies a listed product and voids its listing. It also disables the protection the onboard GFCI receptacles provide during standalone use, so a generator converted that way should never go back to cord-and-plug duty without a bonding plug fitted.