Short answer: make the service-rated ATS the system's first and only neutral-to-ground connection. Its main bonding jumper stays bolted to the cabinet. The grounding electrode conductor lands on its neutral bus, and everything downstream runs on its own equipment ground. For a 200 A main, NEC Table 250.122 sizes that ground at 6 AWG copper. The load center then works like any subpanel: whites on an isolated bar, grounds on a bar bonded to the can, and the generator's neutral jumper comes out. The two ground rods stay for lightning and utility surges, because NEC 250.4(A)(5) states plainly that the earth is not an effective ground-fault current path. Every step below carries a size or a test number you can check at the bench.
Where the bond and the GEC land
Read the label first. An ATS marked as suitable for service equipment is the new first disconnect, and from its line lugs back to the meter everything is service conductors. The factory main bonding jumper — the strap or screw tying the neutral bus to the cabinet — stays right where it is. Torque the 4/0 aluminum service lugs to the value printed inside the door; a 200 A service gives a loose lug no second chance.
The grounding electrode conductor moves to the ATS neutral bus as one unspliced run from the electrodes. The NEC sizes it from the largest ungrounded service conductor: 4/0 aluminum calls for 2 AWG copper, and a run touching only ground rods is allowed the smaller 6 AWG copper. The arrangement already on this kind of service also works: rods, then a pass-through lug, then a bonding bushing in the meter. The meter can is bonded, and the short nipple ties it to the ATS. Bonding a grounding electrode conductor to a service enclosure covers the lug and pass-through details.
Bond the nipple while you are in there. A 2 in metallic nipple between meter and ATS takes grounding bushings at each end, or a bonding locknut where the raceway lands; the grounding bushings on the old install stay and do this job. Keep the space around the cabinet too. NEC 110.26(A) asks for 3 ft [900 mm] of clear depth in front of 0-150 V-to-ground equipment in Condition 1, and 30 in of width, with the door swinging 90 degrees. NEC 240.24(A) holds the highest breaker handle's grip center to 2.0 m [6 ft 7 in] above the floor. The grounding and bonding guide covers the hardware side of all of this.
Why the load center buses must split
Once the bond lives in the ATS, a second bond downstream turns the grounding system into a spare neutral. NEC 250.6 aims to keep neutral current off metal parts. A combined bus at the load center does the opposite: part of every load amp returns through the grounds and any bonded nipple instead of the white wire. The white and the green run side by side for the whole 25 ft, so the panel is where you break the pair apart.
The fix is mechanical and takes an afternoon. Pull the factory bond strap or green screw that ties the neutral bar to the can, so the neutral bar floats. Fit the ground bar kit — a factory accessory that bolts into the can's tapped holes with its own bond screw — and move every bare and green conductor onto it. Removing the neutral bond in an outdoor subpanel shows the strap and the screw up close, and bonding the ground bus to the subpanel can covers the bond screw and its contact face.
| Bus at the load center | Tied to the can? | What lands there | What it carries |
|---|---|---|---|
| Neutral bar | No — isolated | Every white wire | The full load return, all day |
| Ground bar | Yes — bond screw or strap | Bare and green grounds | Fault current; near 0 A on normal loads |
Branch ground sizes survive the move unchanged. NEC Table 250.122 sizes each from its breaker: 12 AWG copper behind a 20 A breaker, 14 AWG copper behind 15 A. Why neutral and ground bond only at the service disconnect lays out the full reasoning.
A 200 A worked example
Put numbers on the job so the parts order writes itself. The setup is a 200 A meter socket with a service-rated ATS 3 ft away on the existing 2 in nipple. The load center sits 25 ft of feeder away, two ground rods are in the earth, and a standby generator runs behind a solid-neutral ATS. Service conductors are 4/0 aluminum.
| Part of the job | Size | Where the number comes from |
|---|---|---|
| Main bonding jumper in the ATS | Factory strap sized for the service; a field-built bond is 2 AWG copper | The NEC sizes it from the largest ungrounded service conductor, here 4/0 aluminum |
| GEC, ATS neutral bus to the rods | 6 AWG copper (2 AWG copper once a water pipe or other electrode joins) | The NEC lets a run touching only rods be this size |
| Feeder EGC, ATS to load center | 6 AWG copper, green insulated | NEC Table 250.122, from the 200 A main |
| Feeder neutral | Insulated, sized for the load, separate for the full 25 ft | Carries the whole load return and touches the can nowhere |
| Branch grounds on the 20 A circuits | 12 AWG copper | NEC Table 250.122, from each breaker |
Carry one number out of this table: the resistance of the 25 ft feeder EGC. This size of copper measures about 0.395 Ω per 1000 ft at 20 °C, so 0.395 × 25 ÷ 1000 = 0.0099 Ω — call it 0.01 Ω end to end. That is why the clamp test later expects zero amps on this wire. The fault path is metal, about 0.01 Ω of it, and the rods contribute nothing worth measuring.
What you need
- Ground bar kit matched to the load center, with its bond screw.
- Green insulated copper EGC for the 25 ft feeder in the worked-example size; NEC 250.119 calls for insulated EGCs of 6 AWG and smaller to be green or green with one or more yellow stripes.
- GEC stock for the rod run in the size from the bonding section, or 2 AWG copper if a water pipe or other electrode joins it.
- Bonding bushings or a bonding locknut for each end of the 2 in nipple, unless the grounding bushings carried over.
- Insulated feeder white sized for the load, kept clear of every ground for the full run.
- Torque screwdriver, clamp meter, ohmmeter, and the load center maker's sheet for the bar kit.
Run the job in this order
- Lock the service out and prove it dead: open the disconnect, apply your lock, and test for absence of voltage at the ATS line lugs before the first terminal turns.
- Leave the ATS bond alone. The main bonding jumper stays bolted neutral bus to cabinet, and the 4/0 aluminum lugs go back down at the torque printed inside the door.
- Move the GEC to the ATS neutral bus: one unspliced run from the two rods in the rod-run size, under the neutral lug beside the service white.
- Bond the nipple. Grounding bushings at each end of the 2 in offset, jumpers tight, contact metal clean.
- Pull the new feeder: two hots, one insulated white, one green EGC sized from the 200 A main by NEC Table 250.122. EGC sizing from the service disconnect to a subpanel walks the same table for bigger feeders.
- Split the load center. Pull the bond strap or green screw, land every white on the floating neutral bar, and read the bar clear of the can on an ohmmeter.
- Fit the ground bar kit and move every bare and green conductor onto it. Scrape paint and enamel off the contact face first; NEC 250.12 calls for nonconductive coatings to come off bonding surfaces.
- Float the generator. Pull the neutral-to-case jumper, land its white on the neutral bar like any other load, and bond its frame with the green EGC.
The generator end of it
The jumper question turns on one detail: does the ATS carry the neutral through solid or break it? A solid-neutral ATS — hots switched, neutral carried straight through — keeps the whole system grounded at the ATS alone. The generator then runs as a load with a grounded frame, and its jumper comes out. A neutral-switching ATS is the other case: it makes the generator a separately derived system while it runs, so the jumper stays in the generator and the bond and a GEC land there. That is the same logic that grounds a transformer secondary. How transformer secondary grounding compares with bonding walks that case, and the generator's installation sheet names which layout your switch is.
Checks when done
Three tests, in order, prove the bond is right and single:
- Ohmmeter, system dead: ATS neutral bus to load center ground bar reads the wire plus terminations — about 0.01 Ω for the 25 ft feeder EGC, straight from the worked example. Neutral bar to can at the load center reads open.
- Clamp meter, system live: run a 10 A load, clamp the feeder EGC, and read near 0 A. Amps on the green mean a bond still hides downstream, usually a strap that never came out.
- Voltage, system live: neutral bar to can at the load center reads about 0 V, because they meet 0.01 Ω away at the ATS bond — while the dead-system ohmmeter shows them separate.
Close with the eyeball items: 3 ft [900 mm] clear in front of the ATS, the door at a full 90 degrees, and the highest handle grip at or under 2.0 m [6 ft 7 in].
Mistakes that cost you the bond
- Keeping the generator's jumper behind a solid-neutral ATS. Return current then splits onto the EGC and the generator frame, and the clamp test fails at the first load.
- Leaving the green screw in the load center. The same failure one panel down: grounds carry load amps and every bonded nipple on the run warms with them.
- Sizing the feeder EGC by copying the white or by feel. Size it from the 200 A main; NEC Table 250.122 fixes it, and the worked example carries the answer.
- Treating the ground rods as fault protection. NEC 250.4(A)(5) counts the earth out of the effective ground-fault current path; the metal EGC is what clears the fault.
- Anchoring the ATS before laying out the 4/0 tails. NEC 312.6 sets the bending space at those terminals, and 4/0 aluminum eats real room per lug, so mock the tails up first and fasten the cabinet after.
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.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can the load center keep neutrals and grounds on one bus?
No. Once the main bonding jumper sits in the ATS, the load center is a subpanel, and a second bond sends load current back through the equipment grounding conductor. For the worked example that EGC is sized at 6 AWG copper from the 200 A main; with a 10 A load running, part of those amps can ride the green instead of the white, heating connections along the whole 25 ft. Pull the strap or green screw and land grounds on a bar bonded to the can.
Where does the grounding electrode conductor land — meter or ATS?
Either end of the bonded service raceway works; the ATS neutral bus is the clean spot after the swap. Keep the run unspliced from the rods to the lug. Size it from the largest ungrounded service conductor, 2 AWG copper for 4/0 aluminum, and a rods-only run may take the smaller size shown in the worked example below.
Does the standby generator keep its neutral-to-case jumper?
Not behind a solid-neutral ATS. The generator is not a separately derived system, so its frame rides the equipment grounding conductor and its jumper comes out. If the ATS breaks and reconnects the neutral, the generator becomes a separately derived system: the jumper stays and the bond lands at the generator, with its own GEC. The transfer switch label or the generator manual tells you which of the two you have.
Do the two ground rods still do anything?
Yes, but not fault clearing. They bleed lightning and utility surges to earth, which is why they stay bonded to the neutral at the first disconnect. NEC 250.4(A)(5) is explicit that the earth is not part of the effective ground-fault current path, so the 25 ft feeder EGC — about 0.01 ohm end to end — is what clears a fault, not the rods. A rod added at the ATS is an auxiliary electrode under NEC 250.54: it may join the set, and the EGC stays the fault path.
