Electrical
Generator grounding and bonding: the separately derived system question
Decide whether the set is a separately derived system, put the one neutral-ground bond in the right place, give an SDS its own electrode, and verify one bond and only one.
Direct answer
Generator grounding bonds the frame to earth. Generator neutral bonding, the neutral-to-ground connection, belongs at the generator only when the set is a separately derived system. A 4-pole transfer switch that switches the neutral makes it one, so bond there; a solid-neutral 3-pole switch does not, so the bond stays at the service. The adopted NEC and AHJ control.
Key takeaways
- A generator is separately derived only when its neutral has no direct connection to the utility neutral, governed by NEC Article 100 and 250.30.
- A 4-pole switched-neutral transfer switch makes the generator separately derived, so bond the neutral at the generator and give it a grounding electrode.
- A 3-pole solid-neutral transfer switch leaves the generator not separately derived, so remove the factory jumper and keep the single bond at the service.
- Keep exactly one neutral-ground bond in the running configuration, not zero and not two; two bonds create a parallel neutral path causing GFCI and ground-fault nuisance trips.
- The generator frame is always grounded through an equipment grounding conductor run with the feeder, regardless of the separately derived neutral-bond decision.
Generator grounding and bonding, and why it gets fought about
Two different things hide under the word grounding on a generator job, and mixing them up is where most of the trouble starts. Grounding is connecting the frame and the non-current-carrying metal to earth so a fault has a low-impedance path back to the source. Bonding, in the sense people argue about, is the neutral-to-ground connection, the deliberate tie between the grounded conductor and the equipment grounding system. The frame ground is never in question. It is the neutral bond that has to land in exactly one place.
Here is the part that catches good electricians. Whether you bond the generator's neutral to ground at the generator depends on whether the set is a separately derived system, and that in turn depends on whether the transfer switch switches the neutral. Get it wrong in one direction and you have an ungrounded system with no fault-clearing path. Get it wrong in the other and you have two neutral-ground bonds, a parallel neutral path, and nuisance trips that will have you chasing ghosts for a week.
This is the most misunderstood topic in generator work, and it is misunderstood because the right answer flips on a single piece of hardware most people never look inside. The transfer switch decides it. Everything else, the electrode, the jumper, the conductor sizes, follows from that one call. This guide walks the decision, then the wiring that follows. For the set, the pad, and the transfer switch itself, see the standby generator and ATS installation guide; for the service grounding electrode system the building already has, see the grounding electrode and bonding guide.
What is a separately derived system?
A separately derived system is a source of power that has no direct electrical connection, including through the grounded conductor, to any other supply. That last clause is the whole game. The NEC defines it in Article 100, and the rules for grounding and bonding it live in 250.30. A transformer secondary is the classic example. A generator can be one too, but only sometimes.
The deciding question is the neutral. If the generator's grounded conductor, the neutral, has no solid electrical connection back to the utility's grounded conductor, the generator is separately derived. If the neutral stays tied to the utility neutral through the transfer switch even when the generator is running, there is a direct connection through the grounded conductor, and the generator is not separately derived. The phase conductors get switched in both cases. The neutral is what tells you which kind of system you have.
Why does the code care? Because a separately derived system is treated as its own source. It needs its own neutral-to-ground bond and its own grounding electrode connection, the same way the service does, so that a fault on the generator side has a defined path back to the generator to trip its overcurrent device. A non-separately-derived generator does not get that bond, because it borrows the service's bond through the shared neutral. One source, one bond. That principle does not bend.
What decides whether the generator is separately derived?
The transfer switch, specifically whether it switches the neutral. This is the single fact that determines everything downstream, so it is worth confirming with your own eyes and not the submittal.
A 3-pole transfer switch on a typical three-phase, four-wire system switches the three phase conductors and leaves the neutral solidly connected straight through. The utility neutral and the generator neutral are the same continuous conductor, bonded only back at the service. Because the grounded conductor is shared, the generator is not separately derived. A 4-pole transfer switch adds a switched neutral pole, so it breaks the neutral along with the phases. When the switch transfers to the generator, the generator neutral is fully isolated from the utility neutral. No shared grounded conductor means the generator is separately derived.
Count the poles and trace the neutral. On a single-phase, three-wire residential set the language is two-pole versus three-pole, but the logic is identical: the extra switched pole is the neutral, and the presence of that switched neutral is what makes the source separately derived. The pole count is the decision, and it points to two completely different bonding rules at the generator. Confirm what the switch actually does before you decide where the bond goes, because a switch labeled for one configuration in the catalog can be ordered or strapped for the other.
| Transfer switch | Neutral | Separately derived? | Neutral bond at the generator |
|---|---|---|---|
| 3-pole (solid neutral) | Not switched, continuous through | No | No bond at the generator |
| 4-pole (switched neutral) | Switched with the phases | Yes | Bond the neutral at the generator |
The 3-pole, solid-neutral case
With a 3-pole transfer switch the neutral runs straight through, so the generator is not separately derived and you do not bond the neutral to ground at the generator. The single system neutral-ground bond stays where it already is, at the service main bonding jumper. The generator simply feeds a system that is already bonded back at the service.
The practical consequence is a step people forget: most permanently installed gensets ship with a factory bonding jumper that ties the neutral to the frame inside the alternator housing or the output box. On a 3-pole, solid-neutral install, that jumper has to come out. Leave it in and you have created a second neutral-ground bond in parallel with the service bond, which is the exact failure the next two sections cover. Find the jumper, remove it or open the link, and document that you did.
The neutral and the frame still relate to each other, just not by a bond at the generator. The frame is grounded through the equipment grounding conductor that runs with the feeder back to the service, and the neutral is grounded through the service bond. Fault current on the generator side returns over that equipment grounding conductor to the service, where the bond sends it back to the source. The bond location is the service. The generator borrows it.
The 4-pole, switched-neutral case
With a 4-pole transfer switch the neutral is switched, the generator is separately derived, and now you do bond the neutral to ground at the generator. This is the mirror image of the 3-pole rule, and getting them backward is the classic mistake. The 4-pole set needs its own system bonding jumper, neutral to equipment ground, established at the generator under the rules in 250.30.
On most permanently installed sets the factory bonding jumper is already there to do this, so on a 4-pole install you generally keep it, confirm it, and treat it as the system bonding jumper for the separately derived system. The bond can be made at the generator or at the first disconnecting means or overcurrent device for the derived system, but it goes in one of those places and only one. Pick the location, make the bond there, and make sure it is the only neutral-ground connection on the generator side.
Because the 4-pole set is its own source, it also needs a grounding electrode connection, covered a few sections down. The shorthand worth carrying: 4-pole switches the neutral, the generator is separately derived, bond at the generator and give it an electrode. 3-pole leaves the neutral solid, the generator is not separately derived, no bond and no separate electrode, the service handles it. The adopted code edition and any local amendments control the details, so verify against the edition the AHJ enforces.
Why the bond location matters: the parallel neutral path
Put a neutral-ground bond at both the service and a 3-pole generator and you have bonded the neutral to ground in two places on one continuous neutral. That creates a parallel path, and it is not a harmless redundancy. It is a wiring fault that happens to keep the lights on, which is the worst kind, because it works fine until it does not.
Normal neutral current wants to return to the source on the neutral conductor. With two bonds tying that neutral to the grounding system, the return current now sees two paths home: the neutral, and the grounding system, the equipment grounding conductors, the conduit, the building steel, the electrode. Current divides between them by impedance, so part of the neutral current rides on metal that is supposed to be carrying zero current under normal conditions. That is objectionable current, and the NEC spends real effort in Article 250 trying to keep it from existing.
The hazards are concrete. Grounded metal that should be at earth potential now has current and a voltage on it, which is a shock path. Ground-fault sensing reads the neutral current as if it were a fault and trips. And the parallel path can carry enough current to heat connections that were never sized for it. One neutral-ground bond and only one is not a style preference. It is what keeps current off the metal people touch.
Why does my generator trip the GFCI when I connect it?
A bonded-neutral generator connected through a neutral-switching transfer switch is the most common cause, and the mechanism is the parallel neutral path from the last section. Once the generator's neutral is bonded to its frame and the service neutral is bonded at the service, with the neutral solidly connected between them, return current splits across the neutral and the grounding path. A GFCI or a ground-fault sensor measures the imbalance between the conductors it is protecting. The portion of neutral current that left on the grounding path looks exactly like a ground fault, so the device trips, often the instant the generator picks up load.
This is why a portable generator with a frame-bonded neutral, perfectly legal and OSHA-correct on its own at a jobsite, will nuisance-trip a GFCI when you wire it into a home or building that already has its own neutral-ground bond. Two bonds, one neutral, current on the ground, the GFCI does its job on a fault that is really a wiring configuration.
There are two clean fixes and one dirty one. Clean: match the hardware to the bonding. Use a transfer switch that switches the neutral so the generator is properly separately derived with its frame bond as the only bond, or use a solid-neutral arrangement with a floating-neutral generator so the only bond is at the service. The dirty workaround is leaving a mismatch and defeating the protection, which is never the answer. The trip is the symptom. The double bond is the disease.
The factory bonding jumper at the generator
Most permanently installed generators leave the factory with a bonding jumper installed between the neutral and the frame, because the manufacturer cannot know which transfer switch you are pairing it with and the bonded state is the safe default for standalone use. Whether that jumper stays or goes is the single physical action that puts your install on the right side of the SDS question.
On a 3-pole, solid-neutral, non-separately-derived install, the jumper comes out. It would otherwise be the second bond. On a 4-pole, switched-neutral, separately-derived install, the jumper stays and serves as the system bonding jumper. Same piece of copper, opposite decision, driven entirely by what the transfer switch does to the neutral. Locate it before you energize: it is usually a visible strap, a removable link, or a labeled lug inside the generator's connection box, and the installation manual calls out where it is and how to remove it.
Do not assume. Open the box and look. Generators get swapped, reused, and reconfigured, and the jumper state on the unit in front of you may not match the paperwork or the last job it came off. Confirm it physically, set it to match the transfer switch, and write down which way you left it so the next person and the inspector can see the decision was made on purpose.
The grounding electrode for a separately derived generator
A separately derived generator, the 4-pole switched-neutral case, needs its own grounding electrode connection, the same as any other separately derived system under 250.30. The neutral point of the generator, where the system bonding jumper is made, gets connected to a grounding electrode through a grounding electrode conductor. The code gives a hierarchy for which electrode to use, generally favoring an effectively grounded building electrode near the generator, such as building steel or a metal water pipe within the first few feet, with a made electrode like a ground rod where those are not available.
The grounding electrode conductor is sized from the derived phase conductors, commonly by the same table used for service grounding electrode conductors, which many editions reference as Table 250.66. Confirm the sizing rule and the table number against the adopted edition, because the section references move between cycles. The point is that the bond and the electrode connection are a matched pair: the system bonding jumper establishes the neutral-ground reference, and the grounding electrode conductor ties that reference to earth.
A 3-pole, non-separately-derived generator does not get its own separate grounding electrode for the neutral, because it is not its own source. Its frame is grounded through the equipment grounding conductor back to the service, and the service electrode system serves the whole thing. Driving a lone ground rod at a 3-pole generator and bonding the neutral to it is a common, well-meant error that creates the second bond all over again. The frame bond is required; a neutral bond to a local rod is not, and on a 3-pole set it is wrong.
Equipment grounding: the frame is always grounded
Whatever the answer on the neutral bond, the generator frame and the non-current-carrying metal are always connected to the equipment grounding system. This never depends on the SDS question. The equipment grounding conductor runs with the feeder between the generator and the transfer switch and ties the generator frame, the enclosure, and the equipment ground bus together so a fault to the frame has a low-impedance path back to the source.
On a 4-pole separately derived set, that equipment grounding path carries fault current back to the generator's own neutral-ground bond, where it returns to the windings and trips the generator overcurrent device. On a 3-pole non-separately-derived set, the same equipment grounding path carries the fault back to the service bond. Either way the frame is bonded and either way a frame fault has somewhere to go. The neutral bond decision changes where the fault current ends up, not whether the frame is grounded.
Run a proper equipment grounding conductor with the conductors, sized to the overcurrent device protecting the feeder. Do not rely on the conduit alone as the only ground path on a generator feeder, and do not confuse the frame ground with the neutral bond. They are separate connections doing separate jobs, and the trouble starts when someone treats one as a substitute for the other.
Do you bond the neutral on a portable generator?
It depends on how the portable set is used, and the two common cases pull in opposite directions. Used standalone on a jobsite, feeding tools through its own receptacles, a portable generator should have a bonded neutral, neutral tied to the frame, and that is what OSHA expects. A label reading neutral bonded to frame means it is built that way; neutral floating means it is not. The bonded-frame design is what makes the generator's own GFCI receptacles work and gives a frame fault a return path.
The code gives portable, cord-and-plug-connected generators a break on the grounding electrode. Under the provision many editions place at 250.34, a portable generator does not need a connection to a grounding electrode when it only supplies equipment mounted on the generator or cord-and-plug equipment through receptacles on the generator, provided the receptacle ground terminals, the equipment ground, and the generator frame are all bonded together. The frame serves as the ground reference for that limited system. Confirm the section against the adopted edition.
The case that bites people is wiring a portable into a building through a transfer switch. Now the building already has a neutral-ground bond, so a frame-bonded portable can create the double bond and the nuisance trips. The handling depends on whether the transfer switch switches the neutral: a neutral-switching connection lets the portable be its own separately derived source with its frame bond as the only bond, while a solid-neutral connection wants the portable's neutral bond removed so the service bond is the only one. Removing a factory neutral bond can take the unit out of OSHA-compliant standalone use, so that becomes a unit dedicated to the connected application. Match the bonding to the connection and verify there is exactly one neutral-ground bond in the running configuration.
Multiple and paralleled generators
Paralleled generators add a wrinkle, because tying alternators together neutral-to-neutral with a bond on each set creates circulating current between the machines on the grounding path, the same parallel-path problem multiplied across sources. The accepted approach is single-point grounding: one neutral-ground bond for the paralleled system, at the paralleling bus or a designated point, rather than a separate bond at each generator.
Where the system bond lands then drives the transfer switch choice, and it can go either way. A single-point-grounded parallel system with the bond fixed at the grounding point often pairs with 3-pole, solid-neutral switches, because the neutral reference is already established and switching it is unnecessary. A parallel system built as a separately derived source bonded at the paralleling switchgear often uses 4-pole switches to isolate that neutral from the utility. Either is workable; what is not workable is a bond at every machine.
Paralleling grounding is a design decision, not a field call, and it should arrive on the drawings with the bond point and the switch poles specified. If it does not, that is a question for the engineer of record before anything gets energized, not something to sort out at the genset with a wrench. The single-point principle is the thing to protect.
Ground-fault protection and the generator
Ground-fault protection makes the neutral-bond decision unforgiving, because GFP works by measuring current that should sum to zero. Any neutral current that escapes onto the grounding path because of a second bond shows up as a fault to the sensor, and on a system with GFP that means a trip or, on emergency sources, at least an indication. This is why GFP coordination is one of the strongest reasons to specify a 4-pole switch.
A 4-pole, switched-neutral transfer switch isolates the generator neutral from the service neutral so each source has a single, clean neutral-ground bond that its own ground-fault sensing can see correctly. With a solid neutral shared between two bonded sources, the ground-fault sensor on one side can pick up neutral current that belongs to the other side and trip on it. Where ground-fault protection is present on the service and the generator has to coordinate with it, the switched neutral is what keeps the sensing honest.
Some editions require ground-fault indication at the emergency source, and large services at higher voltages and ampere ratings carry ground-fault protection of equipment requirements that the generator system has to live alongside. The specific thresholds, the article that mandates indication on emergency systems, and the GFP-of-equipment trigger points are edition-dependent, so verify them against the adopted NEC and the project documents. The design principle holds regardless: if there is ground-fault sensing in the path, the neutrals usually need to be switched.
What is the difference between a 3-pole and 4-pole transfer switch, and when do you need each?
The difference is one pole, the neutral, and that pole changes the whole grounding scheme. A 3-pole switch switches the phases and leaves the neutral solid, so the generator is not separately derived, the neutral bond stays at the service, and there is no separate generator electrode. A 4-pole switch switches the neutral too, so the generator is separately derived, you bond at the generator, and you give it an electrode. The 3-pole costs less and has fewer parts; the 4-pole costs more and buys you an isolated neutral.
Reach for the 4-pole, switched-neutral switch when the system has ground-fault protection that has to coordinate, when there are multiple sources or paralleled generators that need clean neutral isolation, or when the project specification or the engineer calls for a separately derived generator. Reach for the 3-pole, solid-neutral switch when there is no ground-fault sensing to confuse, a single source, and the design intends the bond to stay at the service. Most straightforward standby installs without GFP run fine on a 3-pole; the moment ground-fault protection or paralleling enters, the 4-pole earns its cost.
Decide this before the gear is ordered, because the switch pole count and the generator bonding have to agree, and changing one after the fact means rewiring the other. The selection drives the jumper, the electrode, and the conductor count. Pick it on purpose, off the one-line and the protection scheme, not off whatever switch showed up on the truck.
| Consideration | 3-pole (solid neutral) | 4-pole (switched neutral) |
|---|---|---|
| System type | Not separately derived | Separately derived |
| Neutral bond | At the service only | At the generator (SDS) |
| Generator grounding electrode | Not separate; service serves it | Required for the SDS |
| Factory neutral-frame jumper | Remove it | Keep it as the system bonding jumper |
| Ground-fault protection coordination | Risk of false sensing if double-bonded | Clean neutral isolation |
| Relative cost | Lower | Higher |
Testing and verification: one bond, not zero, not two
The verification that matters most is confirming there is one neutral-ground bond in the running configuration and only one. Not zero, which leaves the system ungrounded with no fault-clearing path. Not two, which builds the parallel neutral path. One. Everything else in commissioning is downstream of getting that single fact right.
Check it with the system de-energized and locked out. With the main bonding jumper and any generator system bonding jumper accessible, you can confirm continuity from neutral to ground at the intended bond point and confirm the absence of that connection everywhere it should not exist. On a 3-pole install, prove the generator neutral is isolated from the generator frame and ground, that the factory jumper is out. On a 4-pole install, prove the system bonding jumper is present at the generator and that no second bond exists downstream. A low-resistance ohmmeter reads the bond you want; an insulation resistance tester, a megger, helps confirm the neutral is clear of ground where it should be, with the conductors disconnected so you are not reading the connected system.
The blunt test in the field: with the generator running and carrying load, neutral current belongs on the neutral. If a clamp meter reads meaningful current on the equipment grounding conductor, the grounding electrode conductor, or the conduit, you have a second bond somewhere and current is on the metal. Find it and clear it before the system goes to the owner. That stray current is the parallel path announcing itself, and it will be a nuisance trip or a shock complaint later if you leave it.
Commissioning the grounding before turnover
Commissioning is where the bonding decision gets proven under real conditions instead of on paper. Before the set goes to the owner, walk the grounding the same way every time: confirm the factory jumper state matches the transfer switch, confirm the system bonding jumper and grounding electrode connection on a separately derived set, and confirm there is no second bond anywhere on the system.
Then run it. Transfer to the generator under load and watch for the symptoms of a double bond: ground-fault indication, GFCI or GFP trips, or current on the grounding conductors. A clean system transfers, carries load, and shows neutral current on the neutral and none on the ground. Transfer back and confirm the service side behaves. The transfer event under load is the test that catches what a static continuity check can miss, because it puts real return current on the system and lets a parallel path show itself.
Document the configuration as you leave it, the same way the standby generator and ATS installation guide lays out for the rest of the commissioning. Whoever services this set in five years inherits the bonding decision you made, and an undocumented neutral bond is a trap waiting for the next person who opens the box.
Data-center and critical-power generator grounding
Critical-power installations push almost every reason to switch the neutral at once. They run ground-fault protection on the service that has to coordinate, they often parallel multiple generators, and they carry heavy non-linear and unbalanced loads that put real current on the neutral. Each of those, on its own, argues for a 4-pole, switched-neutral arrangement; together they make it the default on most critical-power one-lines.
Non-linear loads matter here in a way they do not on a simple standby set. Harmonic and unbalanced neutral current, if it can split onto the grounding path through a second bond, both trips ground-fault sensing and dumps current onto metal that connects a lot of sensitive equipment together. Isolating the neutral with switched-neutral transfer switches and holding a single, deliberate neutral-ground bond per source keeps that current where it belongs. When 4-pole switches are used across paralleled sources, the neutral-ground bond and its conductors have to be rated for the combined fault current the design can deliver.
On these jobs the grounding scheme is engineered, specified on the drawings, and not a field improvisation. The installer's job is to build exactly what the one-line and the grounding details show, then verify the single-bond-per-source reality with the same testing as any other set. The stakes are higher, so the verification is not optional.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
What to document
The grounding configuration on a generator is invisible once the covers are on, so the record is the only thing that tells the next person whether the bond is where it should be. Capture the transfer switch type and neutral handling, whether the system is separately derived, where the single neutral-ground bond lives, the state of the factory jumper as you left it, the grounding electrode and conductor for a separately derived set, and the equipment grounding conductor size. Note the verification, what you measured and that there was one bond and only one.
The table below is the minimum that should travel with the closeout. It is short on purpose, because the decisions are few and each one is load-bearing. If a service tech can read this and know whether to expect a jumper in the box and a bond at the generator or at the service, the documentation has done its job.
| What to record | Why it matters |
|---|---|
| Transfer switch type and neutral handling | Decides separately derived or not, the whole scheme |
| Separately derived: yes or no | States the bonding rule in one line |
| Neutral-ground bond location | The single bond; where to expect it and where not |
| Factory jumper state (in or out) | Tells the next tech what is inside the box |
| Grounding electrode and GEC (SDS) | Confirms the SDS earth connection and its size |
| Equipment grounding conductor size | The frame fault path, required in every case |
| Verification result | Proof of one bond, not zero, not two |
Common mistakes
- Leaving the factory jumper in on a 3-pole solid-neutral set, creating a second bond, a parallel neutral path, GFCI or GFP nuisance trips, and current on the grounding metal.
- Omitting the neutral bond on a 4-pole separately derived set, leaving the generator side ungrounded with no fault-clearing path.
- Not providing a grounding electrode for a separately derived generator, so the derived neutral has no connection to earth.
- Driving a lone ground rod at a 3-pole generator and bonding the neutral to it, which rebuilds the second bond.
- Specifying the wrong pole count for the protection scheme, a 3-pole where ground-fault coordination or paralleling needs a switched neutral.
- Wiring a frame-bonded portable into a building through the wrong transfer switch and chasing the resulting trips instead of fixing the bond.
- Trusting the submittal or the last job for the jumper state instead of opening the box and confirming it.
Standards and references
The NEC, NFPA 70, is where this lives. Article 100 defines a separately derived system, and Article 250, specifically the separately derived system provisions commonly cited at 250.30, sets the rules for the system bonding jumper, the grounding electrode, and the grounding electrode conductor when the generator qualifies as separately derived. Portable, cord-and-plug-connected generators get the grounding-electrode relief many editions place at 250.34, and the proportional and general grounding-conductor sizing comes from the rest of Article 250.
Article 445 covers generators themselves, including overcurrent protection, conductor ampacity from the terminals, and disconnecting means. Emergency and legally required standby systems carry their own requirements in Articles 700 and 701, including ground-fault indication on emergency sources and selective coordination, and large services carry ground-fault protection of equipment requirements that the generator system has to coordinate with. The manufacturer's installation instructions are part of the listing and tell you where the factory bonding jumper is and how to remove it; on a listed assembly those instructions are enforceable.
Section numbers and the exact thresholds move between code cycles, so treat the references here as the topics to look up, not as gospel chapter and verse. Confirm every section against the edition the jurisdiction has actually adopted, honor local amendments, and let the AHJ settle anything ambiguous before you energize. The principles that do not move: a generator is separately derived only if its neutral has no direct connection to the supply, the 3-pole solid-neutral set is not bonded at the generator while the 4-pole switched-neutral set is, and there is one neutral-ground bond in the system and only one.
Units and terms
The vocabulary around generator grounding is where a lot of the confusion comes from, because several terms get used loosely for connections that are not interchangeable. The terms below are the ones to keep straight before the bonding decision makes sense.
- Separately derived system (SDS)
- A power source with no direct electrical connection, including through the neutral, to another supply; gets its own neutral-ground bond and electrode
- System bonding jumper (SBJ)
- The neutral-to-ground connection made at a separately derived source such as a 4-pole generator, the counterpart to the service main bonding jumper
- Main bonding jumper (MBJ)
- The single neutral-to-ground bond at the service, where a non-separately-derived generator's neutral stays bonded
- Grounding electrode conductor (GEC)
- The conductor connecting the bonded neutral point of a source to its grounding electrode, sized from the derived conductors
- Equipment grounding conductor (EGC)
- The conductor that bonds the generator frame and enclosures to the grounding system; required regardless of the SDS question
- 3-pole vs 4-pole switch
- 3-pole switches the phases and leaves the neutral solid (not SDS); 4-pole switches the neutral too (SDS). The single-phase equivalent is 2-pole versus 3-pole
- Parallel neutral path
- Neutral current riding on the grounding system because the neutral is bonded to ground in more than one place; the source of nuisance trips and shock hazard
- Floating neutral
- A generator with no neutral-to-frame bond; both output conductors are treated as live, used so the only bond can be elsewhere
FAQ
What is a separately derived system?
A separately derived system is a power source with no direct electrical connection, including through the grounded conductor, to another supply. The NEC defines it in Article 100 and grounds it under 250.30. A generator is separately derived only when its neutral is isolated from the utility neutral, which happens with a neutral-switching transfer switch.
Do you bond the neutral to ground at a generator?
Only when the generator is a separately derived system, meaning a 4-pole transfer switch switches the neutral. Then you bond at the generator with a system bonding jumper and add a grounding electrode. With a 3-pole solid-neutral switch the generator is not separately derived, so you remove the bond and the service bond serves it.
What is the difference between a 3-pole and 4-pole transfer switch?
A 3-pole switch switches the phases and leaves the neutral solidly connected, so the generator is not separately derived and the neutral bond stays at the service. A 4-pole switch switches the neutral too, making the generator separately derived, so you bond the neutral at the generator and provide a grounding electrode.
Why does my generator trip the GFCI when I connect it?
Usually because a bonded-neutral generator is connected through a solid neutral to a building that already has a neutral-ground bond. Two bonds on one neutral let return current split onto the grounding path, and the GFCI reads that as a fault and trips. Match the transfer switch to the bonding so there is one neutral-ground bond, not two.
What happens if you bond the neutral at both the service and the generator?
You create a parallel neutral path. Return current divides between the neutral and the grounding system, putting current and voltage on grounded metal. That causes ground-fault and GFCI nuisance trips, a shock hazard on metal that should be at earth potential, and heating of connections never sized for it. The rule is one neutral-ground bond and only one.
Does a portable generator need its neutral bonded?
Used standalone on a jobsite, yes, a frame-bonded neutral is OSHA-correct and makes its receptacles and GFCI work. Wired into a building through a transfer switch, it depends on the switch: a switched-neutral connection keeps the frame bond as the only bond, while a solid-neutral connection wants the bond removed so the service bond is the only one.
Does a separately derived generator need its own grounding electrode?
Yes. A separately derived generator, the 4-pole switched-neutral case, needs a grounding electrode connection under 250.30, with a grounding electrode conductor from the bonded neutral point to a qualifying electrode, sized from the derived conductors. A 3-pole non-separately-derived generator does not get a separate electrode; the service electrode system serves it.
When should I specify a 4-pole transfer switch over a 3-pole?
Specify a 4-pole, switched-neutral switch when the system has ground-fault protection that must coordinate, when generators are paralleled or there are multiple sources needing clean neutral isolation, or when the design calls for a separately derived generator. A 3-pole solid-neutral switch suits a single source with no ground-fault sensing and the bond kept at the service.
Is the generator frame grounded even when the neutral is not bonded there?
Yes. The frame and non-current-carrying metal are always tied to the equipment grounding system through an equipment grounding conductor run with the feeder, regardless of the separately derived question. That path carries a frame fault back to whichever neutral-ground bond serves the system. The neutral bond decision changes where fault current returns, not whether the frame is grounded.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.