Is the grounded conductor the same as the ground wire?

No, and this pair of names causes more confusion than anything else in the subject. The grounded conductor is the NEUTRAL. It is called that because it is connected to ground at exactly one point, the service. The ground wire is the equipment grounding conductor, usually written EGC. One carries current all day. The other carries none until there is a fault. A third similar name, the grounding electrode conductor, is the single heavy wire running from the service equipment out to the ground rods and other electrodes.

Why are neutral and ground separated in a subpanel?

The common assumption is that a second connection is redundant because the two meet back at the main anyway. The opposite is true: bonding them in two places creates two parallel paths for normal return current. Some of the current that should come back on the neutral instead rides the equipment grounding conductor, the metal conduit, and anything else conductive between the two enclosures, including a person touching both. NEC 250.6(A) requires that condition to be prevented, and NEC 250.142(B) bars using the grounded conductor to ground equipment downstream of the service disconnecting means. That rule carries narrow exceptions, the two you will meet being existing range and dryer frames under NEC 250.140, and meter enclosures immediately adjacent to the service disconnect. Neither of them reopens the subpanel.

Does the ground bar in a subpanel get bonded to the enclosure?

Yes. This is the part that gets misread. The equipment grounding bar is bonded to the metal enclosure in an ordinary subpanel exactly as in the main panel. Two things sit outside that: a nonmetallic enclosure has no metal to bond, and a deliberately isolated-ground bar is insulated from the enclosure by design with its conductors carried back to the service. What is isolated in a subpanel is the NEUTRAL bar, which sits on insulating standoffs with no connection to the can. Separation means the neutral is separated from the enclosure, not that the ground floats.

Where does the neutral-to-ground bond actually go?

At the service disconnecting means, which is the equipment the service conductors terminate in and that is listed and marked suitable for use as service equipment. NEC 250.24(B) requires an unspliced main bonding jumper there, for each service disconnect, connecting the grounded conductor, the equipment grounding conductors and the enclosure. NEC 250.24(A)(5) then prohibits further grounding connections to the grounded conductor on the load side of it. Do not use "the first thing after the meter that shuts the building off" as the test: since the 2020 cycle NEC 230.85 requires an outdoor emergency disconnect at one- and two-family dwellings, and two of its three permitted markings say NOT SERVICE EQUIPMENT.

Will the ground rod trip the breaker on a ground fault?

No. NEC 250.4(A)(5) states plainly that the earth shall not be considered as an effective ground-fault current path. Earth resistance through a pair of electrodes is typically tens of ohms, which on a 120V fault permits a few amps. That is far below what a 20A breaker needs to trip and far above what is safe to touch. Fault current clears through metal, back through the main bonding jumper, or it does not clear.

Can I run three wires to a detached garage subpanel?

Not on new work. The allowance that let a 3-wire feeder serve a separate structure with the neutral doubling as the equipment ground was deleted in the 2008 NEC. New feeders run four wires: two ungrounded, one grounded, one equipment grounding conductor, with the neutral isolated at the remote panel per NEC 250.32(B)(1). Existing 3-wire installations were legal when made and are not retroactively illegal, so you will still meet them.

Is a backup loads panel a subpanel?

Almost always, yes. A protected or backup loads panel fed from a gateway or automatic transfer switch sits downstream of the service disconnecting means, so its neutral is isolated and its ground bar is bonded to the enclosure. The one arrangement that changes the answer is where the backup equipment itself is the first disconnecting means after the meter. That moves the service disconnect and the bond onto the backup equipment. The loads panel is still a subpanel either way.

How can I tell whether a subpanel is improperly bonded without opening it?

You largely cannot, and that is the defining problem with this defect. It does not trip anything, it does not run hot, and the system works normally. There is a non-invasive indicator an electrician can use: a clamp meter around the feeder equipment grounding conductor, or around the whole feeder assembly, should read essentially zero under load. A reading that tracks the building load is a strong sign that return current has found a second path. Interpreting that is qualified work, and confirming it still means opening the panel.

Does a subpanel in a plastic enclosure still need the bars separated?

Yes. The rule is about keeping the neutral and the equipment grounding conductor from being connected downstream of the service, not about the enclosure material. A non-metallic panel simply has no metal box to bond, so there is no enclosure bonding to get wrong. The two terminal bars still stay separate, the equipment grounding conductors still land on the ground bar, and that bar still ties to the equipment grounding system.

Do I have to remove the bonding screw in a subpanel?

Yes, if the panel is downstream of the service disconnecting means and the screw or strap is bonding the neutral bar to the enclosure. Panels ship with that bonding means included precisely because the same enclosure may be used as service equipment, where it is required. Used as a subpanel, it has to come out, and the equipment grounding conductors move to a separate ground bar listed for that panel. This is work for an electrician inside energized equipment, and on some panels there is more than one bonding path to remove.

What is the difference between a subpanel and a main panel?

The main panel, more precisely the service equipment, is where the utility supply terminates and where the single neutral-to-ground bond and the grounding electrode connection live. A subpanel is any panel fed from that equipment, or from another panel downstream of it. It has its own busbars and breakers, its overcurrent protection is the device feeding it, its neutral bar is isolated from the enclosure, and it needs no grounding electrode of its own unless it is in a separate building.

What size does the equipment grounding conductor to a subpanel need to be?

It is sized from NEC Table 250.122 against the rating of the overcurrent device protecting the feeder, not against the size of the phase conductors. The trap is NEC 250.122(B): where the ungrounded conductors are increased in size for any reason, including voltage drop on a long run to a detached structure, the equipment grounding conductor has to be increased proportionally. That one gets red-lined constantly, because the feeder was upsized for distance and the ground was left at the table value.

Does a subpanel that feeds only 240V loads need a neutral?

Not necessarily. A neutral carries return current for 120V loads, so a panel supplying nothing but straight 240V loads such as a welder or a well pump may not need one. The equipment grounding conductor is not optional in either case. Before leaving the neutral out, check that nothing in the panel needs 120V, including anything added later, and check whether any of the equipment has a 120V control circuit. Adding a neutral during the original install is far cheaper than pulling one afterward.

Grounding & bonding

Subpanels: why neutral and ground are separated

In the service panel of a typical house the neutrals and the equipment grounds land together, on bars that are tied to each other and to the metal enclosure. In every subpanel downstream they land on two bars that must not touch. The same two conductors, joined in one enclosure and deliberately kept apart in the next.

The usual reasoning is that neutral and ground meet back at the main anyway, so one more connection downstream is harmless. It is not a redundant connection. It is a second path, it changes where normal load current flows, and it does that silently, on a system that keeps working and passes a casual look.

A technical reference for solar designers, project managers, and electricians. Written around the single-phase 120/240V dwelling service, which is where the question usually comes up. Three-phase and 208Y/120 systems follow the same bonding rule with more conductors. Section numbers are NEC 2023. Last updated August 2026.

The short answer

Neutral and ground are separated in a subpanel so that normal return current has exactly one path home, and fault current has exactly one path home, and those two paths are different conductors. Bond them together twice and every load in the building starts sending part of its return current down the equipment grounding conductor, the metal conduit, and anything else conductive that bridges the two enclosures.

That single connection is made at the service disconnecting means, the service equipment where the utility supply terminates. NEC 250.24(B) requires an unspliced main bonding jumper there. NEC 250.24(A)(5) then prohibits further grounding connections to the neutral on the load side of it, and NEC 250.142(B) bars using the neutral to ground equipment downstream. One bond per system, at the service, and not again in any panel it feeds. Two of them is what the trade calls a double bond.

The earth is not part of either path. NEC 250.4(A)(5) states that the earth shall not be considered as an effective ground-fault current path, which is why the bond, and not the ground rod, is what lets a breaker trip on a ground fault.

The detail that gets misread: separating them does not mean the ground is left disconnected. The equipment grounding bar is bonded to the metal enclosure in a subpanel exactly as it is in the main panel. It is the neutral bar that is isolated, sitting on insulating standoffs with no path to the enclosure. NEC 408.40 is the section behind this: the equipment grounding terminal bar is bonded to the cabinet and panelboard frame, and equipment grounding conductors may not land on the bar provided for grounded conductors except where Article 250 permits or requires that interconnection, which downstream of the service it does not. Where a subpanel has been installed with that neutral bar still factory-bonded, the correction is for an electrician to remove the bonding screw or strap. It is a small piece of hardware and it is not a small job to get at, because the work is inside energized equipment.

A bar is a terminal strip inside the enclosure where the branch wires land, not the busbar the breakers clip onto. At the service, neutrals and equipment grounding conductors share one bar and a main bonding jumper ties it to the metal enclosure. In every subpanel there are two bars: the neutral bar sits on insulating standoffs, and the ground bar is fastened to the enclosure.

Who does what

The designer decides which device is the service disconnecting means and draws the bond and the grounding electrode conductor accordingly.
The electrician does every physical step described here. Opening a panel, lifting a bond, and confirming what is inside an existing enclosure are qualified-person work.
The project manager reads the drawing, requests the photographs, and knows which of the two above to call.

On this page

1.The short answer
2.What a subpanel is
3.The four wires that feed a subpanel
4.What the bond is actually for
5.What happens, case by case
6.Finding the one legal bond point
7.Where solar and battery storage change the answer
8.Detached structures
9.How to tell a subpanel is bonded correctly
10.Frequently asked questions
11.The short version

What a subpanel is, and when you need one

A subpanel, also written sub panel, is any panel fed from equipment further upstream rather than directly from the service, which is the utility supply where it lands at the building. Usually the feed comes from a breaker in the panel above it. Sometimes it comes from a set of lugs instead, which changes other things but not the bonding answer. Electrically a subpanel is an extension of what feeds it: the same two hot conductors, the same voltage, the same grounding system, just distributed further into the building. It has its own busbars, the metal blades the breakers clip onto, its own branch breakers, and its own pair of terminal bars where the neutrals and grounds land.

Its overcurrent protection lives upstream. Whatever device protects the conductors feeding it is what limits the current it can draw, which is why a subpanel is described by two ratings that people mix up: the busbar rating stamped in the enclosure, and the rating of the breaker feeding it.

Four reasons one gets installed:

The main panel is out of spaces. When a panel is full, a subpanel is usually cheaper than replacing it. Full is a label question rather than a headcount: many load centers accept tandem breakers in designated slots, and the panel label says which.
Circuits are clustered somewhere far from the panel. A shop, a kitchen remodel, or a finished basement can need eight circuits at one end of the building. One feeder, meaning the set of conductors running from one panel to the next, beats eight separate cables run back to the main panel.
A separate building or structure needs power. A detached garage, a barn, a well house. This case has extra rules and gets its own section below.
The design calls for a specific group of loads to be separated. This is the one solar designers meet most often: a backup loads panel holding the circuits that stay energized when the grid drops.

Most subpanels are main lug only, written MLO, meaning they have lugs where a main breaker would go and no breaker installed in them. Nothing is missing. NEC 408.36 requires a panelboard to be protected by an overcurrent device rated no higher than the panelboard, and the breaker feeding it from upstream is that device. Some subpanels do carry a main breaker, usually because the panel is far from the one feeding it or because a local shut-off is wanted, and that is a choice rather than a rule. A subpanel in a separate building is the case where a local disconnect stops being optional, which the detached structures section covers.

The four wires that feed a subpanel

On a single-phase 120/240V system the feeder to a subpanel is normally four conductors, and each one has exactly one job. The code names and the trade names are not the same words, and the code names are what a plan reviewer will use.

Trade name	Code name	Carries
Two hots	Ungrounded conductors	Load current out. 240V between them, 120V from either one to neutral.
Neutral	Grounded conductor	Return current from 120V loads, continuously, in normal operation.
Ground	Equipment grounding conductor	Essentially nothing until there is a fault. Then fault current, shared with every metallic path in parallel with it.

The neutral is a working conductor that carries current every second the building is occupied. The equipment grounding conductor is a standby path that carries current only during a fault, and only for the fraction of a second it takes to trip the device. They look similar on a drawing and they do completely different jobs.

Because the neutral is a working conductor, it is not at the same potential along its whole length. Under load there is a voltage drop across it, so the neutral bar in the subpanel sits a volt or two above the neutral bar at the service. That is normal and harmless while those two points are joined by one conductor.

Bond the neutral to the enclosure at the subpanel as well, and those same two points are now joined by two conductors: the feeder neutral, and the equipment grounding conductor together with every piece of metal running alongside it. A voltage difference across two parallel paths puts current in both, in inverse proportion to their impedance. That is the whole mechanism, and every failure below is a consequence of it.

The bond completes a metal path to the source, not to earth

The main bonding jumper is the connection between the grounded conductor, the equipment grounding conductors, and the enclosure at the service. Physically it is often a green screw through the neutral bar into the can, or a strap supplied with the panel. NEC 250.24(B) requires it to be unspliced and requires it at the service disconnect. NEC 250.28 covers what it is made of and how it is sized.

Its job is not to send fault current into the earth. It is to complete a metallic loop back to the source so that a ground fault becomes a short circuit rather than a live enclosure. Fault current leaves the source, reaches the fault, returns on the equipment grounding conductor, crosses the main bonding jumper onto the grounded conductor, and goes back to the transformer winding it came from. The path is low impedance, the current is high, and the device trips fast.

The grounding electrode system, the rods and the concrete-encased electrode and the metal water pipe, is a separate job. It references the system to earth and gives lightning and utility-side surges somewhere to go. It is not a fault return path and the code says so directly: NEC 250.4(A)(5), the earth shall not be considered as an effective ground-fault current path. Removing the main bonding jumper and expecting the ground rod to cover for it leaves fault current with no path that can trip a breaker.

Two sizing points travel with it, because both get missed. The main bonding jumper is sized from NEC 250.28(D), which routes to NEC 250.66 and Table 250.102(C)(1) against the service-entrance conductors. And the grounded conductor run to the service disconnect is sized as a supply-side bonding jumper under NEC 250.24(C), not to the calculated neutral load, which is what makes a long meter-to-gateway run need more neutral than the load alone would suggest.

One bond, and only one, per system. Two bonds put the grounded conductor and the equipment grounding conductor in parallel between the bonding points, which is the condition NEC 250.6(A) requires to be prevented and which the next section walks through case by case.

What happens when the bonding is right, and when it is not

Six situations, each one traced by following the current. The first two are a correct system doing its job. Three are the ways the arrangement fails. The last is a utility fault that arrives looking like a bonding problem and is not one.

If everything is correct and the house is just running

The ground wire carries nothing.

Current leaves the service, runs down the subpanel busbar, through the load, back on the neutral to the subpanel neutral bar, along the feeder neutral to the service, and out to the transformer. The equipment grounding conductor is not part of that loop. Clamp it and you will not read a clean zero, because electronic power supplies, LED drivers and filters leak a small current to ground by design and milliamps are normal. What you should not see is a reading that rises and falls with the building load. That is the baseline every case below is measured against.

If a hot conductor touches a metal enclosure

The breaker trips, in a fraction of a second.

Current flows onto the enclosure, back along the equipment grounding conductor to the service, across the main bonding jumper onto the neutral, and out to the transformer. That path is metal end to end, so its impedance is very low and the current is very high, which is what opens the breaker. Note where the current does not go: not into the earth. The bond is what closes the loop, and without it there is no loop.

If there is no main bonding jumper anywhere, but the ground rods are in

The breaker holds and the enclosure stays live.

The ground rods do not stand in for the bond. NEC 250.4(A)(5) states that the earth shall not be considered as an effective ground-fault current path, and the arithmetic is not close. A pair of driven electrodes plus the utility electrode gives a loop on the order of 30 to 50 ohms, so a 120V fault drives roughly 2 to 4 amps through the soil. A residential 20A breaker opens instantaneously at 8 to 13 times its rating, meaning 160 to 260 amps, and at 3 amps it never opens at all. The fault stays live, and the faulted enclosure sits at the electrode share of that divider, tens of volts above remote earth, until someone touches it and offers a better path.

If the neutral is bonded at the service and again in the subpanel

Part of the return current in the building leaves the neutral, permanently.

This is the one that gets waved through, on the reasoning that the two are joined back at the main anyway. They are, and that is the problem: the second connection closes a loop rather than duplicating a connection. Return current now has two paths home and divides between them in inverse proportion to their impedance. The share is not small, because a feeder equipment grounding conductor is often the same size as the neutral and a metal raceway in parallel with it lowers the impedance further. It rides the equipment grounding conductor, the raceway, the metallic water and gas piping, the building steel, and any other metal bridging the two enclosures, including a person contacting both. NEC 250.6(A) requires this to be prevented and NEC 250.142(B) bars the arrangement that causes it. Nothing trips, nothing smells hot, and the drawing looks fine. The system is not safe, it is quiet, and it stays quiet until someone becomes part of the path.

If the subpanel is improperly bonded and then the feeder neutral opens

All of the return current transfers to the ground wire.

The improper bond has already given the current a second route, so losing the neutral simply hands it the whole load. Those conductors were never meant to carry load current continuously, the loads keep running so nothing reports the fault, and it stays invisible until an inspection or an incident finds it. Worth saying plainly, because it is the natural next assumption: an open neutral on a properly separated subpanel does not politely shut the loads off either. The 120V loads on the two legs end up in series across 240V and the lightly loaded leg climbs toward 240. An open neutral is destructive either way. The improper bond changes where the current goes, not whether the voltage divides.

If everything is bonded correctly and the service neutral opens

The two legs go unequal and equipment starts failing.

This one is usually not a bonding error at all, and it gets misdiagnosed as a panel problem. With the service neutral open, the two 120V load groups are left in series across the full 240V between the ungrounded conductors, and the neutral point floats to wherever the ratio of their impedances puts it. The lightly loaded leg climbs toward 240V and the heavily loaded leg collapses, and both move every time a load switches. Whatever else is bonded to the service carries some return and softens the excursion, usually the metallic water service and through it the neighboring premises rather than the ground rods, which at tens of ohms cannot hold the neutral. Bonded metal can rise above earth potential, and the symptoms are the classic ones: warm water pipes, a tingle at a hose bib or a pool. The open is most often on the utility side, but corroded meter socket jaws and customer-owned service-entrance conductors produce the identical signature. The fix is the conductor, not the bonding. Do not remove the water pipe bond to chase the stray current, and never cut or lift a service neutral or a water bond without a temporary bonding jumper in place first.

A ground-fault circuit interrupter works on a different mechanism. Its sensing transformer takes the vector sum of the circuit conductors passing through it, which is zero while everything that leaves comes back on the same circuit, and it opens when the imbalance reaches the Class A threshold of 4 to 6 milliamps, because that missing current went home by some other route.

It is not a substitute for correct bonding, though the relationship is more useful than a flat "it does not help". A Class A GFCI also contains a grounded-neutral detector and will trip on a neutral-to-ground connection on its load side, which is why unexplained GFCI and AFCI tripping is one of the most common reasons anyone opens a panel and finds an improper bond in the first place. Treat repeated nuisance tripping as a reason to look.

Position decides what a device can see. A GFCI on a branch circuit downstream of the improper bond will not detect it, because that circuit's own conductors still balance at the breaker. A ground-fault sensing device upstream of it will: a GFCI breaker feeding the subpanel, a GFCI-protected feeder to an outbuilding or pool equipment, or service ground-fault protection under NEC 230.95 all watch part of the return current leave on the equipment grounding conductor, and they trip. A GFCI breaker feeding a subpanel that will not hold is one of the standard signatures of case 04.

In case 03, where nothing is bonded anywhere, a GFCI protects at the moment of contact rather than clearing the fault. That is real protection and a different claim from tripping the breaker.

Finding the one legal bond point

The bond belongs at the service disconnecting means, which is a role rather than a type of box. Everything downstream of it is a subpanel with an isolated neutral, regardless of what the enclosure is called.

The obvious test is the wrong test. "The first thing after the meter that shuts the building off" is the rule of thumb most people carry, and current code has broken it twice. NEC 230.85 requires an emergency disconnect outside one- and two-family dwellings, and two of its three permitted markings read "not service equipment". NEC 230.82(3) allows a meter disconnect ahead of the service disconnecting means, which is on the supply side and is explicitly not service equipment either. Both can shut the whole building off. Neither takes the bond.

The test that holds is the listing and the marking. Service equipment is equipment that is listed and marked Suitable for Use as Service Equipment, often written SUSE, and it is where the service conductors terminate. Read the label rather than the position in the line-up.

The common arrangements:

Meter, then a main-breaker panel. The panel is the service disconnect. Bond and grounding electrode conductor land there, neutral and ground share a bar, and every other panel in the building is separated.
Meter-main combination, then an indoor panel. The meter-main is the service disconnect. The bond and the electrode conductor are outside at the meter, and the indoor panel that everyone calls the main panel is separated. It is the arrangement most often found bonded twice.
Meter, separate disconnect, then a panel. Same as above. The disconnect holds the role.

Whether the downstream panel is fed by a breaker or by feedthrough lugs does not change the bonding answer. Both are on the load side of the service disconnecting means, so both have isolated neutrals. It changes other things, including which interconnection methods are open to you, which is covered in spotting a feedthrough panel.

The grounding electrode conductor does not have to land in the same box

The bond has one legal home. The grounding electrode conductor is more flexible. NEC 250.24(A)(1) permits the connection at any accessible point from the load end of the service drop or lateral up to and including the terminal where the grounded service conductor lands at the service disconnect. In practice that often means the meter enclosure or a wireway ahead of the disconnect, because that is what the utility wanted.

This matters on retrofits. When new service equipment goes in ahead of an existing panel, the run from the meter to that equipment becomes service conductors, so an existing grounding electrode conductor landed in the meter can is still within the permitted zone and does not automatically have to be moved. Specifying a relocation that the code does not require is a real cost added to a job for no reason.

Where two bonds are legitimate

A second bond is correct wherever there is genuinely a second system, and there are three of those. NEC 230.71 permits two to six service disconnecting means for one service, and since the 2020 cycle each of them has to sit in its own enclosure or its own barriered compartment rather than as six handles in one can. Where that arrangement genuinely exists, each disconnect carries its own main bonding jumper, because NEC 250.24(B) requires one for each service disconnect. A supply-side connection for solar under NEC 705.11 brings its own service disconnect and its own bond with it. And a separately derived system, which for most purposes means a transformer, gets a system bonding jumper at the transformer or at the first disconnecting means per NEC 250.30(A). That panel is downstream of the service and is correctly bonded, which is the one case where "never bond downstream" is the wrong instruction. It matters on any job with a step-down transformer.

A standby generator is the same question wearing different hardware. If the transfer switch switches the neutral, the generator is a separately derived system and gets a system bonding jumper under NEC 250.30(A). If it does not, the neutral stays solidly interconnected, the generator is not separately derived, and a bonded-neutral unit puts a second bond on a system that already has one. Decide the transfer switch first, then configure the generator to match.

What is never correct is a second bond on a panel that is simply fed through another disconnect.

The supply side plays by different rules

Everything on the supply side of the service disconnect is bonded to the grounded conductor, and that is not a second bond. NEC 250.92 requires the service raceways and enclosures to be bonded, and NEC 250.142(A) permits using the grounded conductor to do it. A bonded meter can is normal, not a finding. NEC 250.142(B) Exception No. 2 also permits it for a meter enclosure on the load side under defined conditions, which is worth knowing before red-lining a production meter on a solar job.

One consequence catches people out on retrofits. Moving the service disconnect downstream turns the raceway feeding it into a service raceway, and standard locknuts and bushings are no longer sufficient there. NEC 250.92(B) calls for bonding bushings, bonding locknuts, threaded hubs, or a bonding jumper, with the supply-side bonding jumper sized per NEC 250.102(C).

Where solar and battery storage change the answer

Solar and storage work rarely adds a subpanel for the usual reasons. It adds one by moving the service disconnect, which reclassifies a panel that has been sitting there for twenty years.

A gateway ahead of the main panel

Whole-home backup usually puts a gateway or a transfer switch between the meter and the existing panel. Whether that equipment becomes the service disconnect turns on one fact: it must be listed and marked Suitable for Use as Service Equipment. Equipment without that listing cannot be service equipment no matter where it sits in the line-up, and a service disconnect is still required ahead of it. You cannot confer the role by adding a bonding screw.

Where it does have the listing and the meter was plain, the gateway becomes the service disconnect, the bond moves to it, and the old main panel becomes a subpanel whose existing bond has to be removed. It was correct the day it was installed and it is a violation the day the gateway is energized. Where the house already had a meter-main or a separate disconnect, nothing moves: that equipment was already the service disconnect, and the gateway is switching equipment in the middle of the system holding neither the bond nor the electrode conductor. What the gateway does to your service walks that comparison in more detail.

De-bonding an existing service panel is not a screw

That sentence, "its existing bond has to be removed", hides the biggest cost surprise on gateway retrofits. The panel was service equipment, which means every branch-circuit equipment grounding conductor and every neutral is landed together on the factory bars. Converting it to a subpanel means fitting a ground bar kit listed for that specific panel, and then moving thirty or forty grounds onto it in an enclosure that was full when it was built. Some of them will be cut too short. On older panels a listed kit may not exist.

The item nobody prices is 3-wire ranges and dryers. The NEC 250.140 allowance that lets those frames be grounded through the grounded conductor depends, for the uninsulated service-entrance cable case, on the branch circuit originating at the service equipment. Move the service disconnect upstream and that condition is gone, so those circuits have to be converted to 4-wire. Old range and dryer cable is not rare.

There is also equipment that cannot be converted at all. Plenty of meter-mains and all-in-one service devices are built with a neutral that is factory-bonded and not intended to be isolated, because the enclosure is listed as service equipment and nothing else. If the design turns one of those into a subpanel, the answer is to replace it. And on panels that can be converted, check for more than one bonding path: a green screw plus a strap to a second neutral bar is common, and pulling only the screw leaves the panel bonded.

The backup loads panel is a subpanel

A protected loads panel is a subpanel. Isolated neutral, ground bar bonded to the enclosure, no grounding electrode of its own if it is in the same building. So is the non-backup panel holding everything that is not on the battery, which is often the larger of the two and gets forgotten because it is the leftover.

Being energized by the battery during an outage does not make the loads panel service equipment, and it is not the place to add a bond. In the common architecture the backup equipment switches only the ungrounded conductors and passes the neutral straight through, so the island is not a separately derived system and the one existing bond at the service serves it. Some products instead switch the neutral and close an internal bonding relay while islanded. Which one you have is a question for the installation manual rather than an assumption.

The trap this sets is worth naming, because it is the article's own case 04 arriving through the front door. Backup equipment is frequently shipped with a factory neutral-to-ground bond installed, because the same enclosure is listed to be used as service equipment. Tesla's Backup Gateway is the obvious example: its installation manual has a step for removing the main neutral-ground bonding screw whenever the Gateway is not installed as service equipment. Put one downstream of an existing meter-main, leave that screw in, and the system is double bonded on day one with nothing to show for it.

Detached structures, and the rule that changed

A subpanel in a separate building or structure follows the same bonding rule and adds two requirements. First, the structure gets a disconnecting means of its own. NEC Article 225, Part II requires means to disconnect all ungrounded conductors supplying or passing through a building or structure, at a readily accessible location nearest where the conductors enter. So the detached-garage case is one of the few where the panel out there usually does carry a main breaker rather than being main lug only. Article 225 was reorganized for the 2023 cycle, so confirm the subsection numbers against the edition your jurisdiction has adopted before putting them on a drawing.

Second, per NEC 250.32(A), each building or structure supplied by a feeder or branch circuit needs a grounding electrode system of its own, connected to the equipment grounding terminal bar in that disconnecting means. The exception is a structure supplied by a single branch circuit, a multiwire branch circuit included, where that circuit carries an equipment grounding conductor. The new electrode connects to the equipment grounding system out there, not to the neutral, and it does not travel back to the house.

A subpanel in the same building needs no second electrode, because the building already has one. A subpanel in a detached garage does, even though the neutral is still isolated and the bond is still back at the service.

The rule that changed. Older editions permitted a 3-wire feeder to a separate structure, with the neutral doing double duty as the equipment grounding conductor. That allowance for new work was removed in the 2008 NEC, and new feeders run four wires with the neutral isolated at the remote panel per NEC 250.32(B)(1).

Existing runs are the part worth understanding, because the code did not simply grandfather them. NEC 250.32(B)(1) Exception No. 1 still permits the grounded conductor to serve as the fault return path for existing premises wiring, and it is conditional on three things continuing to hold: no equipment grounding conductor is run with the supply, no ground-fault protection is installed on the supply side of the feeder, and there are no continuous metallic paths bonded to the grounding system in both structures.

That third condition is the one that bites. Somebody runs a metal water line, a gas line, or a coaxial cable out to the garage years later and the exception stops applying, without anyone touching the wiring. So a 3-wire feeder you find is not automatically a correction, and it is not automatically fine either. Check for a metallic path between the buildings before deciding.

How to tell a subpanel is bonded correctly

Qualified-person work. Everything described below is behind a deadfront, where busbars and line-side terminals are energized, and line-side terminals stay energized with the main breaker switched off. If you are a designer or a project manager, the correct move is to have the licensed electrician on site look and photograph it. Nothing here is an instruction to open equipment yourself.

Two rules for whoever does the work. De-energize before lifting a bond, because removing a main bonding jumper on a panel that has an improper downstream bond puts you in series with the building's return current. And never open a grounded conductor under load: a neutral broken while carrying current can arc and put full line voltage across the gap.

The test that actually finds this:

Put a clamp meter around the feeder equipment grounding conductor with the building under load. It should read close to nothing. A reading that rises and falls with the load means return current has found a second path, which is either a bond in the subpanel or a neutral-to-ground contact somewhere downstream. Clamping the whole feeder assembly at once is the same test from the other direction: everything belonging to that circuit should sum to zero, and a reading means current is returning outside it. With the panel de-energized and the feeder neutral lifted, an ohmmeter from the neutral bar to the enclosure should read open.

What a correct subpanel looks like inside:

Two separate terminal bars, one for neutrals and one for equipment grounding conductors.
The neutral bar on insulating standoffs, with no bonding screw through it into the enclosure and no strap tying it to the enclosure or to a second bonded bar.
The ground bar fastened to the back pan with the plain machine screws that came with a kit listed for that panel. The green screw is the main bonding jumper and it belongs in the neutral bar at the service, not in a subpanel ground bar. A green screw in a subpanel is the defect, not the proof of correctness.
Metal-to-metal contact where the ground bar lands. A bar bolted over powder coat, or a lug on painted steel with no paint-cutting washer, will pass a continuity beep and fail under fault current. NEC 250.12 requires the paint to be removed or a fitting designed to make the connection through it.
One neutral per terminal, per NEC 408.41. Equipment grounding conductors are different: most ground bars are listed for more than one conductor per hole, so doubled grounds are usually fine and doubled neutrals are not.
The feeder equipment grounding conductor on the ground bar, and the feeder neutral on the neutral bar.

What to check on the drawing, where most of this is actually caught:

Trace from the meter and identify the service equipment, which is what the service conductors terminate in and what is marked suitable for use as service equipment. Do not use "first thing that shuts the building off": an emergency disconnect or a meter disconnect can hold that position without being service equipment.
Confirm the neutral-to-ground bond is shown at that equipment and nowhere it should not be.
Confirm a grounding electrode conductor is shown, landing within the zone NEC 250.24(A)(1) permits. With more than one service disconnect this becomes a tap arrangement under NEC 250.64(D) rather than a single landing point.
Confirm every downstream panel is drawn with separate neutral and ground bars. Count the conductors in each feeder, remembering two things: a panel serving only 240V loads may legitimately have no neutral, and NEC 250.118 lets a metal raceway or cable armor be the equipment grounding conductor, so three wires in an EMT run is not automatically a missing ground.
Where new service equipment is being added ahead of an existing panel, confirm the drawing shows the existing bond being removed. Adding the new bond without removing the old one is the most common version of this error on solar plans, and it reads as complete unless you look for the removal note. On a real sheet that note reads something like "remove existing main bonding jumper at panel P1; install listed equipment grounding bar and relocate all EGCs to it".
Where there is a second bond, confirm it belongs to a genuine second service disconnect, a supply-side connection, or a separately derived system, rather than to a panel that is simply fed through another disconnect.

Frequently asked questions

Is the grounded conductor the same as the ground wire?: No, and this pair of names causes more confusion than anything else in the subject. The grounded conductor is the NEUTRAL. It is called that because it is connected to ground at exactly one point, the service. The ground wire is the equipment grounding conductor, usually written EGC. One carries current all day. The other carries none until there is a fault. A third similar name, the grounding electrode conductor, is the single heavy wire running from the service equipment out to the ground rods and other electrodes.
Why are neutral and ground separated in a subpanel?: The common assumption is that a second connection is redundant because the two meet back at the main anyway. The opposite is true: bonding them in two places creates two parallel paths for normal return current. Some of the current that should come back on the neutral instead rides the equipment grounding conductor, the metal conduit, and anything else conductive between the two enclosures, including a person touching both. NEC 250.6(A) requires that condition to be prevented, and NEC 250.142(B) bars using the grounded conductor to ground equipment downstream of the service disconnecting means. That rule carries narrow exceptions, the two you will meet being existing range and dryer frames under NEC 250.140, and meter enclosures immediately adjacent to the service disconnect. Neither of them reopens the subpanel.
Does the ground bar in a subpanel get bonded to the enclosure?: Yes. This is the part that gets misread. The equipment grounding bar is bonded to the metal enclosure in an ordinary subpanel exactly as in the main panel. Two things sit outside that: a nonmetallic enclosure has no metal to bond, and a deliberately isolated-ground bar is insulated from the enclosure by design with its conductors carried back to the service. What is isolated in a subpanel is the NEUTRAL bar, which sits on insulating standoffs with no connection to the can. Separation means the neutral is separated from the enclosure, not that the ground floats.
Where does the neutral-to-ground bond actually go?: At the service disconnecting means, which is the equipment the service conductors terminate in and that is listed and marked suitable for use as service equipment. NEC 250.24(B) requires an unspliced main bonding jumper there, for each service disconnect, connecting the grounded conductor, the equipment grounding conductors and the enclosure. NEC 250.24(A)(5) then prohibits further grounding connections to the grounded conductor on the load side of it. Do not use "the first thing after the meter that shuts the building off" as the test: since the 2020 cycle NEC 230.85 requires an outdoor emergency disconnect at one- and two-family dwellings, and two of its three permitted markings say NOT SERVICE EQUIPMENT.
Will the ground rod trip the breaker on a ground fault?: No. NEC 250.4(A)(5) states plainly that the earth shall not be considered as an effective ground-fault current path. Earth resistance through a pair of electrodes is typically tens of ohms, which on a 120V fault permits a few amps. That is far below what a 20A breaker needs to trip and far above what is safe to touch. Fault current clears through metal, back through the main bonding jumper, or it does not clear.
Does a subpanel need its own grounding electrode?: Only if it is in a separate building or structure. NEC 250.32(A) requires a grounding electrode at each building or structure supplied by a feeder or branch circuit. The exception is a structure supplied by a single branch circuit, a multiwire branch circuit included, where that circuit carries an equipment grounding conductor. A subpanel in the same building as the service uses the existing grounding electrode system and needs no second one.
Can I run three wires to a detached garage subpanel?: Not on new work. The allowance that let a 3-wire feeder serve a separate structure with the neutral doubling as the equipment ground was deleted in the 2008 NEC. New feeders run four wires: two ungrounded, one grounded, one equipment grounding conductor, with the neutral isolated at the remote panel per NEC 250.32(B)(1). Existing 3-wire installations were legal when made and are not retroactively illegal, so you will still meet them.
Is a backup loads panel a subpanel?: Almost always, yes. A protected or backup loads panel fed from a gateway or automatic transfer switch sits downstream of the service disconnecting means, so its neutral is isolated and its ground bar is bonded to the enclosure. The one arrangement that changes the answer is where the backup equipment itself is the first disconnecting means after the meter. That moves the service disconnect and the bond onto the backup equipment. The loads panel is still a subpanel either way.
How can I tell whether a subpanel is improperly bonded without opening it?: You largely cannot, and that is the defining problem with this defect. It does not trip anything, it does not run hot, and the system works normally. There is a non-invasive indicator an electrician can use: a clamp meter around the feeder equipment grounding conductor, or around the whole feeder assembly, should read essentially zero under load. A reading that tracks the building load is a strong sign that return current has found a second path. Interpreting that is qualified work, and confirming it still means opening the panel.
Does a subpanel in a plastic enclosure still need the bars separated?: Yes. The rule is about keeping the neutral and the equipment grounding conductor from being connected downstream of the service, not about the enclosure material. A non-metallic panel simply has no metal box to bond, so there is no enclosure bonding to get wrong. The two terminal bars still stay separate, the equipment grounding conductors still land on the ground bar, and that bar still ties to the equipment grounding system.
Do I have to remove the bonding screw in a subpanel?: Yes, if the panel is downstream of the service disconnecting means and the screw or strap is bonding the neutral bar to the enclosure. Panels ship with that bonding means included precisely because the same enclosure may be used as service equipment, where it is required. Used as a subpanel, it has to come out, and the equipment grounding conductors move to a separate ground bar listed for that panel. This is work for an electrician inside energized equipment, and on some panels there is more than one bonding path to remove.
What is the difference between a subpanel and a main panel?: The main panel, more precisely the service equipment, is where the utility supply terminates and where the single neutral-to-ground bond and the grounding electrode connection live. A subpanel is any panel fed from that equipment, or from another panel downstream of it. It has its own busbars and breakers, its overcurrent protection is the device feeding it, its neutral bar is isolated from the enclosure, and it needs no grounding electrode of its own unless it is in a separate building.
What size does the equipment grounding conductor to a subpanel need to be?: It is sized from NEC Table 250.122 against the rating of the overcurrent device protecting the feeder, not against the size of the phase conductors. The trap is NEC 250.122(B): where the ungrounded conductors are increased in size for any reason, including voltage drop on a long run to a detached structure, the equipment grounding conductor has to be increased proportionally. That one gets red-lined constantly, because the feeder was upsized for distance and the ground was left at the table value.
Does a subpanel that feeds only 240V loads need a neutral?: Not necessarily. A neutral carries return current for 120V loads, so a panel supplying nothing but straight 240V loads such as a welder or a well pump may not need one. The equipment grounding conductor is not optional in either case. Before leaving the neutral out, check that nothing in the panel needs 120V, including anything added later, and check whether any of the equipment has a 120V control circuit. Adding a neutral during the original install is far cheaper than pulling one afterward.

The short version

Bond the neutral to the ground once, at the service disconnecting means, so that a ground fault has a metallic path back to the source and clears the breaker. The earth is not that path and never was.

Separate them everywhere downstream, so that normal return current cannot divide between the neutral and the equipment grounding conductor. In a subpanel the neutral bar is isolated from the enclosure and the ground bar is bonded to it.

On solar work, the question to ask first is which device is the service disconnecting means, because installing a gateway or a disconnect ahead of an existing panel moves that role and turns the old main panel into a subpanel that still has its bond in it.

Draw the service, and the bond lands where the wiring says it goes.

Solar Design Lab works out which device is the service disconnecting means from the way the design is actually wired, and puts the neutral-to-ground bond and the grounding electrode there. A gateway added ahead of an existing bonded main panel, the case on this page, moves both. Free to use, no card required.

Open the free editor See every check we run

More resources

Related guides & tools

The 120% rule (NEC 705.12)The 83% rule and solar feeder taps Interconnection method selector Solar planset requirements, ranked Solar wiring diagram guide Spotting a feedthrough panel What the gateway does to your service

Section numbers are NEC 2023. Adoption is uneven, so confirm every citation against the cycle your jurisdiction has adopted before putting it on a plan set, and check for local amendments. Equipment listings and manufacturer installation instructions govern how a specific product treats the neutral, particularly backup and transfer equipment. This is an educational technical reference for reading and drawing systems, not instruction for electrical work and not a substitute for project-specific engineering. Work on service equipment, panels, and conductors is for a licensed electrician.