Neutral ground bonding in a service rated transfer switch

Short answer: when a service-rated automatic transfer switch and its 200 A main become the first point of disconnect, the neutral-ground bond and both grounding electrode conductors move into the ATS enclosure. The old meter/main is rewired as a subpanel with neutrals and grounds separated. The layout worked through below is the common retrofit: a 200 A meter/main feeding a service-rated ATS through four 2/0 copper conductors in 2 in EMT, with a 24 kW generator behind it. In that layout the conductors are already the right size. The bond sits in the wrong enclosure, and the factory bond in the meter/main cannot stay. Here is what moves, what the inspector tests for, and the two ways to finish the job.

One bond, at the new first disconnect

Follow the power path and the change is obvious. The utility conductors land on the ATS line lugs, pass through the 200 A main, and return to the old panel's main lugs. The ATS main therefore sits ahead of every breaker in the building, which makes the ATS enclosure the service disconnect. The grounding follows it. The neutral bus bonds to the ground bus and enclosure right there, and both electrode conductors terminate in the same enclosure. In this layout that means the #4 copper to the cold water pipe and the #4 to the concrete-encased electrode land on the ATS ground bus. The termination details are in bonding a grounding electrode conductor to a service enclosure, and the grounding and bonding guide carries the fundamentals behind them.

Everything after the ATS is a subpanel. Neutrals land on an isolated bar, grounds on a bar bonded to the enclosure, and the two bars meet only inside the ATS. That single-bond principle is the subject of why the neutral-ground bond lives only at the service disconnect, and the downstream wiring is laid out in subpanel neutral and ground bonding: keep them apart.

What the inspector's two items mean

The correction list reads like generalities, but each item is a physical check:

• Bond at the ATS. A jumper connects the neutral bus to the ground bus inside the ATS enclosure, and the enclosure itself is bonded.
• GEC relocated. Both #4 electrode conductors terminate on the ATS ground bus instead of in the meter/main.
• Downstream isolated. Every panel after the ATS shows its neutral bar floating from the enclosure, with grounds on their own bonded bar.

Prove the last item with an ohmmeter. Open the service and the generator, prove the buses dead, then lift the neutral at the ATS and measure between neutral and ground at a downstream receptacle. A correct layout reads open, kilohms or more. A reading of a few hundred ohms or less means a bond is still hiding downstream. In this layout that bond is the one factory-installed in the old meter/main.

Why the meter/main bond cannot stay

In this panel the neutral bus is bolted to the enclosure at the factory across four lugs, and the utility neutral lands on that same bus. The bond is built into the assembly as the factory labeled it: a meter/main is labeled for use as service equipment, where carrying the bond is its job. Once the ATS main sits ahead of it, the panel is a subpanel, and a subpanel holds no bond of its own.

Leave the factory bond in place and neutral current returns on two paths at once. Part flows through the neutral, part through the grounds, the EMT and the panel steel. That is how load current ends up on parts meant to carry current only during a fault, and it is why a bond left behind in a subpanel scorches connections; the mechanism is spelled out in why a neutral-ground bond in a subpanel burns equipment. Field removal is the other wall, and the bond you can and cannot remove in a subpanel separates a bond screw from a bus bolted to the can. This bus is the bolted kind, so the retrofit hits a limit no wire-size check will catch.

The #2 copper neutral on a 100 A house feed

The conductors between the enclosures already pass, and the check is worth writing down before anyone buys wire. The inputs come straight from this layout: a 100 A two-pole breaker feeding the house panel, a 60 A two-pole breaker feeding a 5 ton condenser that runs line-to-line at 240 V, and a #2 AWG copper neutral between the two enclosures.

The rule: a neutral carries only the unbalanced load, so its ampacity must cover the largest line-to-neutral leg. #2 copper carries 115 A at 75 °C in the NEC ampacity table for copper conductors at 75 °C. With the condenser running line-to-line, its neutral current is 0 A, so the worst case is the house leg pulling its full 100 A on one side: 100 A − 0 A = 100 A on the neutral. Then 115 A − 100 A = 15 A spare, 15% above the load, and the same #2 clears its protection with 115 A of ampacity against a 100 A trip.

Decision: leave the copper alone. The threshold moves only when the house side grows past 115 A of one-leg load, at which point the neutral upsizes with the hots. Everything else in this retrofit is bond location, not wire size.

Bonding the steel raceway between the enclosures

The 2 in EMT run with a 2 ft flexible steel section carries fault current only where it is bonded. NEC 250.96(A) requires metal parts serving as equipment grounding conductors, raceways included, to be bonded together so they can conduct any fault current likely to hit them. The flexible section is the weak link, and the grounding bushing on its end is the fix: the #4 green stranded conductor runs from the bushing lug to the ground bus in the ATS.

Size that conductor by the overcurrent device ahead of it, per NEC Table 250.122: a 200 A main calls for 6 AWG copper, so the #4 installed is two sizes larger and passes. At both lugs, NEC 250.12 requires paint, lacquer and enamel off the threads and contact surfaces, because a bonding lug clamped through paint is a high-resistance joint. And under NEC 250.4(A)(5) the earth is no part of the fault-current path: the bushing bond carries a fault around that flex, not the soil.

When the generator is the source

A 24 kW genset at 240 V supplies 24,000 W ÷ 240 V = 100 A, which matches the 100 A output on its nameplate. Whether it needs its own bond depends on the switch, and the answer changes the wiring. With a 3-pole ATS the neutral passes through unswitched, the generator is not a separately derived system, and the single bond stays at the service disconnect; the generator's frame bond stays as its manual ships it. With a 4-pole ATS the neutral is broken at transfer, the generator becomes a separately derived system while it runs, and the bond and electrodes follow the generator per its documentation. Decide the pole count before the wire pull, because it decides where a second bond lives, or whether there is one.

Two ways to finish the job

Case 1 gives the better system: the whole house sits behind one 200 A main, and one throw kills everything. It works only if the old panel becomes a true subpanel. The factory-bonded four-space bus cannot be converted in the field, so the meter/main comes out and an isolated-neutral meter-combo goes in, fed from the ATS load side.

Case 2 keeps every factory assembly as labeled. The meter/main stays the service disconnect, its two two-pole breakers are two throws well inside the six-disconnect grouping (the background is in six motions of the hand: when a sub panel needs a main breaker), and the ATS wires as a subpanel in both positions, with its ground bar bonded to the enclosure the way bond the ground bus to the panel enclosure with the supplied screw describes. Size the EGC run from the ATS to the panel as in EGC from service disconnect to subpanel.

Decision: pick Case 1 when the whole house must transfer and the old panel is due for replacement anyway. Pick Case 2 when the existing panel stays, because it ends the argument with the least metal moved.

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.

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