Neutral ground bonding: why only at the service disconnect

Bond the neutral to ground once, inside the enclosure that holds the service disconnecting means — in this case an automatic transfer switch (ATS) carrying the 200 A main breaker. The old main panel beside it, joined through a 2 in rigid close nipple, is downstream equipment from that moment. Its neutral bar floats; its grounds land on the case. The shared steel is the fault path and the neutral bar is the load path, and NEC 250.6 exists to keep steady load current off metal parts. Leave the bond in the old panel and the system grows a second neutral return. In the 60 A worked example below, 17 A of neutral current rides the EGCs and raceway, heating joints and tripping GFCIs.

Where does neutral ground bonding belong?

At one point only: inside the enclosure holding the service disconnecting means, here the ATS with its main breaker; the NEC allows the grounded conductor to touch the grounding system once per system, and metering is the listed exception you may still meet downstream.

A breaker alone is half a service. The ATS with the main breaker inside is both disconnecting means and overcurrent protection, so it becomes service equipment, and the old panel drops to the load side the moment it takes over. That is a change in the wiring system, not in the cabinets.

The single bond does two jobs. It pins the neutral near earth potential, and it hands fault current a low-impedance metal loop back to the source so the 200 A breaker clears within cycles. NEC 250.4(A)(5) calls that loop the effective ground-fault current path and states that the earth is not part of it. The enclosure grounding and bonding guide walks that loop piece by piece.

Does a metal nipple make both enclosures one piece of service equipment?

The metal is one, and that is the point of the nipple: it bonds the two enclosures for fault current, while the rule follows the service disconnect rather than the spacing, so a panel 2 in away is downstream all the same.

Threaded rigid steel across a 2 in gap is an excellent fault path: it carries thousands of amperes for the few cycles a short circuit lasts, which is the job NEC 250.96(A) gives bonded metal parts, raceways and enclosures. It makes a poor neutral, because neutral current runs for hours rather than cycles. That steady current is what NEC 250.6 keeps off metal parts, so neutrals land on an isolated terminal and grounds on the ground bus.

The walls, the nipple and the ground bars form one metal body; the neutral bar is a separate conductor inside it. Joining the cabinets never joined the systems. The close nipple is not the main leak either: the bond in the old panel parallels its neutral bar with the EGC and raceway of every feeder leaving it. Subpanel neutral and ground bonding covers that conversion step by step.

How much neutral current takes the metal path?

Impedance splits the return current, and in the worked example below a 60 A load sends 17 A — more than a quarter — through the EGC and raceway instead of the neutral conductor, while equal-size conductors split close to half and half.

With the bond left in place, return current reaches the ATS over two parallel routes: the neutral conductor and the metal. Take a 150 ft feeder from the old panel, run in rigid steel conduit with a 2 AWG copper neutral and a 6 AWG copper EGC, carrying a 60 A load. Standard copper resistance tables give about 0.19 Ω per 1,000 ft for 2 AWG and about 0.49 Ω per 1,000 ft for 6 AWG at 75 °C. Over the 150 ft run:

• Neutral conductor: 0.19 Ω/1,000 ft × 150/1,000 = 0.029 Ω, about 29 mΩ
• EGC: 0.49 Ω/1,000 ft × 150/1,000 = 0.074 Ω, about 74 mΩ

Current divides by impedance, so the metal share is 29 / (29 + 74) ≈ 28 %. That is 28 % of 60 A ≈ 17 A on the EGCs and raceway, leaving 43 A on the neutral conductor. Reactance adds a similar few milliohms to both paths, so the ratio stands. The raceway in parallel with the EGC pulls the metal path below 74 mΩ, so treat 17 A as the floor.

A 4 AWG EGC (about 0.31 Ω per 1,000 ft, 47 mΩ over the run) lifts the metal share to 38 %, or 23 A. Equal-size neutral and EGC land near half and half, 30 A, and building steel bonded into the same grid pushes it further up. Where a raceway earns its keep as a grounding conductor, and where it just collects stray current, is covered in metal raceway as an equipment grounding conductor.

What goes wrong first when neutral current rides the metal?

GFCI-protected circuits trip first, because a Class A GFCI trips at 4-6 mA of imbalance and a bonded panel leaks amps through the ground paths; after that come hot joints, lifted metal, and the open-neutral case that puts the whole load current through equipment someone can touch.

1. Nuisance trips. A Class A GFCI trips at 4-6 mA of imbalance between line and neutral current. On a 20 A branch with the metal share at 28 %, about 6 A of neutral bypasses the sensor, a thousand times the trip level.
2. Hot joints. 17 A through one corroded coupling at 0.05 Ω dissipates 17 × 17 × 0.05 ≈ 14 W around the clock. Joints carbonize and insulation cooks; a conduit run warm in a cool room is the tell.
3. Lifted metal. The far enclosure sits 17 A × 0.074 Ω ≈ 1.3 V above the service earth. A volt of lift is harmless; the 14 W joint beside that reading is the fire risk.
4. The open neutral. Break the neutral and all 60 A hunts through metal: 60 A × 0.074 Ω ≈ 4.4 V end to end, with every imperfect joint taking its cut, and contact points on grounded parts rising toward tens of volts.

A clamp meter finds the first three, because a grounding system under load should carry almost nothing:

Does distance or PVC change the rule?

Distance changes nothing, because the code writes the one-bond rule per electrical system with no distance term, so the old panel 2 in away and a panel on the floor below follow the same separation; PVC changes the leak path but keeps the principle.

The rule attaches to the system, not to the cabinet spacing. The old panel is load side because the ATS feeds it, whether the join is 2 in of steel, 20 ft of PVC, or a floor slab between them. A plastic nipple does change the leak path: the raceway drops out, and stray current takes the EGCs, the water pipe and any bonded building steel instead. Cable assemblies with a bare or green equipment grounding conductor leak the same way. The metal join between cabinets was never the test; the connection point of the grounded conductor is.

What moves to the ATS when it becomes the service equipment?

Three things land in the ATS enclosure: the main bonding jumper, sized at 6 AWG copper minimum from the 200 A main; the grounding electrode conductor connection, moved over from the old panel; and listed hubs on both ends of the 2 in rigid nipple.

• The main bonding jumper. It lands on the neutral bar and bonds it to the enclosure. NEC Table 250.122 sizes it from the overcurrent device ahead of it: a 200 A main takes a 6 AWG copper jumper.
• The grounding electrode conductor. Its connection moves out of the old panel and into the ATS enclosure, as bonding a grounding electrode conductor to a service enclosure shows. A rod driven beside the ATS is an auxiliary electrode under NEC 250.54: connect it by all means, and keep the conductor to the grounding electrode system as well.
• Listed hubs on the nipple. Threaded rigid steel lands in hubs at both ends, and NEC 250.12 wants paint, lacquer and enamel cleared from threads and contact surfaces so the joint carries fault current. Bolt-on and built-in hubs for service raceway bonding compares the choices.
• The generator neutral. A 3-wire load keeps its neutral solid through the switch and stays unbonded downstream, which is what keeps the one-bond rule intact when the generator runs.

Lock out and test for absence of voltage before opening either enclosure: the line lugs of a service disconnect stay live even with the main breaker off.

What do you change in the old panel?

Take the bonding strap or screw out of the old panel, land every neutral on an isolated bar, land every ground on a bar bonded to the case, and size each feeder's EGC from its breaker at 8 AWG copper for a 100 A feeder.

Find the strap or screw that bonds the neutral bar to the case and take it out; bonding the ground bus to a subpanel enclosure shows where that screw usually sits. Insulated EGCs stay green or green-striped at 6 AWG and smaller under NEC 250.119. Feeder EGCs follow NEC Table 250.122 from each breaker: 8 AWG copper on a 100 A feeder. For an outdoor box the removal is a sequence of its own, and removing a neutral ground bond in an outdoor subpanel gives it step by step.

If the conversion is the moment to replace the box, order the subpanel enclosure with an isolated neutral terminal assembly and factory cutouts sized round for the 2 in hubs. Our 304 and 316L boxes are cut to your layout from one piece and delivered in 10 business days (custom work counts from your approval of the drawing), rated NEMA 4X / IP66 by the manufacturer; not UL listed. The cutout, gland and window guide covers hub openings, and the custom enclosure builder takes your layout.

The decision, enclosure by enclosure:

Bond once, at the ATS. Convert the old panel to an isolated neutral bar and a bonded ground bar. Clamp-check the grounding conductors under load and expect near zero. That is the whole job, 2 in or 200 ft from the service.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions