Bonded Versus Floating Neutral on Portable Generators

Why this matters

A homeowner buys a portable, an electrician installs an inlet and an interlock, and the first time the furnace and a few lights come on the generator's GFCI trips. Or it never trips, because the 240 V outlet has no GFCI, and the house runs for days with neutral current riding the equipment grounding conductor. Both outcomes come from one fact on the generator: whether its neutral is bonded to its frame. This card explains the two configurations, which one belongs on a house connection, and why the GFCI behaves the way it does. The narrative case is 'The Portable Hookup That Tripped the GFCI Every Time'.

Before any of it: a portable runs outdoors only, at least 20 feet from the house with its exhaust pointed away from doors, windows and vents, never in a garage, crawlspace or porch alcove. See 'Exhaust Clearance and CO Safety Standard'.

The two configurations

Bonded neutral. Inside the generator, the neutral of the winding is jumpered to the frame, and the frame connects to the ground pin of every receptacle. The generator creates its own neutral-to-ground reference. This is the configuration for standalone use, cords run straight from the generator to tools or appliances: a hot-to-frame fault in a tool returns on the cord's grounding conductor to the bond and back to the winding, enough current flows to trip the generator's breaker or GFCI, and the fault clears. Many contractor-grade portables ship bonded for that reason.

Floating neutral. The neutral is not connected to the frame. The frame still connects to the ground pins, but the generator has no neutral-to-ground reference of its own. Plugged into a house through a solid-neutral transfer, it takes the reference from the house's single bond at the service. Many homeowner portables marketed for house connection ship floating.

Which one you have is on the generator's label or in its manual. Where neither says, test it: generator off, cooled, unplugged from everything, set a meter to continuity and read between the neutral and ground pins of a receptacle. Near zero ohms is bonded; open is floating. Do this only with the engine stopped and the unit disconnected, so the receptacle cannot be live.

Why an interlock means solid neutral

An interlock kit and power inlet ('How to Install an Interlock Kit and Power Inlet Box') switches only the two hot legs: the main breaker and the generator back-feed breaker cannot both be on. The neutral is never switched. The inlet's neutral lands on the panel's neutral bar, and that bar is bonded to ground at the service. So with the generator connected, its neutral is permanently tied to the utility neutral and to the house's one bond. That makes the generator, in NEC terms, not a separately derived system, and the house's existing bond is the only one that should exist.

A bonded portable connected this way has two bonds: one at the service and one in its own frame. Two bonds with a neutral and a grounding conductor running between them in the same cord is a loop, and neutral current splits between both paths.

The parallel path, traced

This is a bonded portable on an inlet and interlock, panel bonded at the service.

PORTABLE (bonded)            HOUSE SERVICE PANEL
 L1 -------- cord L1 ------- inlet -> interlock bkr
 L2 -------- cord L2 ------- inlet -> interlock bkr
 N ---+----- cord N -------- inlet -> N bar ---+
      |                                        |
     bond                            main bonding
      |                                 jumper |
 FRAME+----- cord G -------- inlet -> G bar ---+
                                         |
                                  GEC to rod

Return 1: load -> N bar -> cord N -> winding
Return 2: load -> N bar -> bonding jumper -> G bar
          -> cord G -> frame -> bond -> winding

Return 1 is the path the circuit is designed for. Return 2 exists only because there are two bonds. Current divides between them in inverse proportion to their resistance. In a cord where the neutral and grounding conductors are the same gauge and length, the bonding jumper and bars add almost nothing, so the two paths are close to equal and the neutral current splits roughly in half.

Worked: why the GFCI trips at the first unbalanced load

The portable is rated about 7,500 W running, with a 120/240 V 30 A locking receptacle protected by a GFCI. The cord is 25 feet of four-conductor cord, all four conductors the same gauge. The interlock is on, the generator is running, and the homeowner turns loads on.

The load. On L1, the furnace blower and the refrigerator draw 12 A at 120 V. On L2, lights and the freezer draw 7 A at 120 V. The neutral carries the difference between the two legs: 12 minus 7 is 5 A.

The split. With the neutral and grounding conductors matched, 5 A divides roughly evenly: about 2.5 A on the cord neutral, about 2.5 A on the cord grounding conductor.

What the GFCI sees. A GFCI compares current leaving on the hots with current returning on the neutral through its own sensing coil. Out on L1 and L2, back on the neutral, the difference should be zero. Here about 2.5 A comes home on the grounding conductor and bypasses the coil, so the GFCI reads a 2.5 A imbalance. A personnel GFCI trips at a few milliamps, around 5 mA. 2.5 A is 2,500 mA, about 500 times the trip level. The device trips the instant the legs unbalance, which is why it trips every time and never seems to depend on which load came on.

A floating portable with a GFCI can trip too, for a different reason. Many GFCI designs also detect a neutral that is connected to ground downstream of the device. Connected to the house, a floating portable's neutral meets the service bond a few feet past its GFCI, and the device reads it as a grounded-neutral fault. That trip can happen with almost no load, which is the tell that separates it from the parallel-path trip above: a bonded unit trips when the legs unbalance, a floating unit with this kind of GFCI can trip on connection.

When there is no GFCI on the 240 V outlet

Older portables, and some current ones depending on their size and the edition the maker built to, have no GFCI on the 120/240 V receptacle. NEC 445.20 (2017 edition onward), in substance, requires GFCI protection on certain receptacles of portable generators rated 15 kW or smaller, with conditions that vary by edition; check the edition your jurisdiction has adopted and the unit's own marking.

Without a GFCI, the bonded portable on an interlock still has the parallel path; nothing trips to tell anyone. In the worked case, about 2.5 A of normal neutral current rides the grounding conductor for the whole outage. NEC 250.6, as adopted, in substance prohibits arrangements that put objectionable current on grounding conductors. The practical risks: metal parts tied to the grounding system carry a small voltage whenever the conductor's resistance rises, and if the cord's neutral is damaged or a pin loosens, the grounding conductor becomes the only neutral and carries all of it.

What the right setup is, and who changes it

On an interlock or solid-neutral transfer switch, the portable's neutral should be floating. One bond, at the service. That is the configuration the house wiring was designed around.

Changing a bonded portable to floating is done only per the generator maker's instructions. Some makers publish a procedure or sell a kit. Many say the bond must not be removed, and opening the unit to move a jumper can void the listing and the warranty. If the maker gives no procedure, the answer is not a field modification. Two alternatives an electrician can offer:

• A transfer switch with a switched neutral. The neutral opens with the hots, the portable becomes a separately derived source, and its bond is then correct. NEC 250.30, as adopted, governs that case. How the switch label tells you which kind you have is in 'Reading a Transfer Switch Nameplate'.
• A different generator, one that ships floating or has a maker-approved conversion.

A floating portable used standalone loses its own fault return: a hot-to-frame fault in a tool may not draw enough current to trip anything, and the GFCI is then the protection that matters. A unit switched between house use and job-site use needs its configuration set for each use per its manual, and the owner needs to know which state it is in.

The house side of all of this, inlet, interlock, panel and service, goes to a licensed electrician wherever the locality requires one. The generator tech's part is identifying the configuration and telling the customer and the electrician which way it has to be.

What never fixes it

• Plugging into a non-GFCI outlet to stop the trip. The parallel path is still there; you have only removed the device reporting it.
• Cutting or lifting the grounding conductor in the cord. That trades the parallel path for a worse one. The generator frame is then tied to the house grounding system only through its own bond and the cord neutral, so it rides at neutral voltage under load, and if that neutral opens or a pin loosens, the frame is not grounded at all and a fault can leave it energized for whoever touches it.
• A cheater plug, or a back-feed cord without an inlet and interlock. It energizes the utility side through the house's main and can injure a line crew; no neutral setting makes that acceptable.
• Removing the house's service bond so the generator's bond is the only one. The house is then unbonded on utility power, which is the condition the service bond exists to prevent.

How to confirm it is right

With the generator correctly floating on a solid-neutral connection, the checks are simple and done by the electrician at hookup:

• Generator off and disconnected, neutral to ground pin reads open at the generator receptacle.
• Connected through the inlet, generator still off, neutral to ground at the inlet reads near zero, because the house bond is now in the loop. That is the one bond.
• Running under an unbalanced load, a clamp on the grounding conductor of the cord reads essentially nothing, and a clamp on the neutral reads the leg difference. If the grounding conductor reads a meaningful share of the neutral current, there is still a second bond somewhere.

Clamping conductors at a running inlet or panel is energized work, done only by someone qualified for that exposure in the PPE the shop's NFPA 70E program sets.

References

• NFPA 70, National Electrical Code, as adopted by your jurisdiction: 250.6 (objectionable current), 250.30 (separately derived systems), 445.20 (GFCI on portable generator receptacles, 2017 edition onward); check the current edition.
• Generator maker's manual for the neutral configuration and any approved change to it.
• NFPA 70E, current edition, for energized measurement.
• See related: How to Install an Interlock Kit and Power Inlet Box; Reading a Transfer Switch Nameplate; The Portable Hookup That Tripped the GFCI Every Time; Exhaust Clearance and CO Safety Standard.