Bond the neutral to ground exactly once, at the meter-main outside the fence. Everything from there into your control enclosure — the 30 ft [9 m] PVC run, the fused disconnect, the automatic transfer switch and the 120/240 V panel — is a subpanel in every sense the code cares about. That means isolated neutral, grounds on a ground bar, and a copper equipment grounding conductor sized to the main riding the full run. Land that conductor on the 4/0 station ground grid. Driven rods at the enclosure stay auxiliary. Here is where each bond goes, why the copper rides in the PVC anyway, and what the enclosure needs to hold it all.
One bond, at the meter-main
The service point is the combination meter socket and main breaker, 30 ft outside the fence. The NEC puts a system's single neutral-to-ground bond at that first disconnect. The grounded conductor and the grounding system meet in the meter-main and nowhere else. From there the layout follows the subpanel rule that has been on the books for a century: run a separate ground and neutral, land the neutral on an isolated terminal, and keep grounds on their own bar. NEC 250.6 states the aim directly: keep neutral current off metal parts, neutrals on the isolated neutral terminal, grounds on the ground bus. The station service transformer is a second system, and it gets its own bond at its own first disconnect. Two systems, two bonds, one each.
The second bond inside the enclosure is the mistake that burns panels. Add one and the grid, the copper EGC and the neutral run in parallel. The 4/0 grid then carries load current around the clock, and a lost neutral turns that load current into a fault on everything bonded. That is why a bond left behind in a subpanel scorches equipment, as the page on why a neutral-ground bond in a subpanel burns equipment explains. The same reasoning for ordinary yards sits in removing a neutral ground bond in an outdoor subpanel and in keeping subpanel neutral and ground bonding apart. Do not give the grid a second job.
Settle the transfer switch before you lay out the bars. A 3-pole ATS transfers the neutral with the sources, so only one system's neutral is in circuit at a time. A 2-pole switch leaves both neutrals tied through it. That works when both bonds sit at their own disconnects, and the common neutral carries unbalanced current back to whichever source feeds the load.
The copper still goes in the PVC
PVC conduit is an insulator and contributes nothing to fault clearing. That leaves two jobs for copper: an equipment grounding conductor, sized to the main, from the meter-main ground bar to the enclosure ground bar, full length, and bonds between every metal part along the way. NEC 250.4(A)(5) draws the line for you: the effective ground-fault current path is metal, and the earth is not considered part of it. A metallic raceway can serve as that path on its own. With PVC the wire is the only path, which is the contrast in rigid conduit as a grounding conductor with NEMA 4X boxes.
NEC 250.96(A) covers the metalwork: raceways, cables, equipment and enclosures that may carry fault current get bonded together so they can conduct it safely. The entries are where builds slip. Stub the PVC into bottom cutouts and seal them as in connecting schedule 40 PVC conduit to an outdoor enclosure. Run and seal the ground wire leaving an outdoor enclosure the way that guide sets out, and land the conductor with a listed entry connector and bond. On a powder-coated carbon steel box, NEC 250.12 asks for paint and enamel cleared from the contact surfaces. A stainless box has no coating to clear, which is one more reason it wins in a yard.
Carry the 4/0 grid to the rack; rods stay auxiliary
The yard question is whether to extend the 4/0 station ground grid to the structure rack or isolate the enclosure and ground it with rods. Carry the grid. NEC 250.54 lets a rod at the enclosure work as an auxiliary electrode: you may connect it to the EGC, and it supplements the copper rather than replacing it. The physics is lopsided. Soil around a driven rod reads tens of ohms; 30 ft of the copper EGC adds milliohms. A fault returned through dirt never reaches the trip level of the main breaker, and one returned through copper clears in cycles. Bond the frame, the ground bar and the grid riser into one piece of metal, as NEC 250.96(A) asks, so everything inside the fence moves together.
The ground-potential worry behind the debate runs the other way. A transmission fault lifts the yard grid against remote earth. The copper EGC ties your enclosure and the meter-main into that lift, so the whole system rises and falls together. Leave the copper out and the fault still finds the dirt, while your metal sits at grid potential and the incoming service neutral sits near remote earth — the flashover you were trying to avoid. Grid design and utility protection coordination are where that lift gets managed. One safety rule before you land anything on the grid: lock out the service and test for absence of voltage, because the grid can carry utility fault current while you work.
Sizing the bond for a 220 A feed at 120/240 V
The inputs, and where each comes from: a 220 A, 120/240 V single-phase main breaker and a 30 ft [9 m] underground PVC run, both from the yard case above. NEC Table 250.122 sizes the EGC from the rating of the overcurrent device ahead of it. Take 25 Ω as the long-accepted working resistance of a single driven rod, and standard copper wire tables at 75 °C for conductor resistance.
The rule in words: find the first row of Table 250.122 whose ampere rating covers the breaker, and the EGC is at least that row's copper size. The arithmetic: the 200 A row asks for 6 AWG copper, which does not cover a 220 A trip. The 300 A row governs and calls for 4 AWG copper. Over the run, the conductor adds 0.25 Ω per 1000 ft × 30 ft = 7.5 mΩ. The soil path by comparison: at 120 V to ground, 120 V ÷ 25 Ω = 4.8 A, about 2% of the main's rating — a current a thermal breaker holds forever without tripping. The copper segment sits roughly 3,300 times lower in resistance than the rod path, and the full fault loop adds the source and service conductors on top of it.
The result: 4 AWG copper minimum, full 30 ft, isolated from the neutral, landed on bars at both ends. The decision: pull the row that covers the breaker — the 300 A row here — and step up or down one row as the rating moves. The EGC runs heavier than ampacity would pick because it carries fault current for cycles, not load current for hours. As a load conductor at 75 °C, that same 4 AWG copper is good for 85 A under the NEC ampacity tables.
| Main breaker rating | Minimum copper EGC (NEC Table 250.122) |
|---|---|
| 100 A | 8 AWG |
| 200 A | 6 AWG |
| 300 A | 4 AWG |
| 400 A | 3 AWG |
What the enclosure itself needs
Working space comes first, and NEC 110.26(A) sets it: clear space in front of equipment serviced while energized, at least 3 ft [900 mm] deep for 0-150 V to ground in Condition 1 — your 120/240 V panel qualifies. Width: at least 30 in, or the equipment width. Leave room for the door to open at least 90 degrees, and keep at least 6.5 ft of headroom or the height of the equipment. The fused disconnect's handle follows NEC 240.24(A): the center of its grip, in its highest position, stays at or under 2.0 m [6 ft 7 in] above the floor or platform.
Inside, NEC 312.6 sets the minimum wire bending space at terminals for cabinets and cutout boxes. Table 312.6(A) covers conductors entering the same wall as the terminal, and Table 312.6(B) covers conductors crossing to the opposite wall. A 4/0 riser and parallel feeder entries eat bending space fast, so lay out the bars before you choose the depth. NEC 312.8 also limits how much of an enclosure for switches or overcurrent devices may be filled with conductors, splices and taps. Bond the ground bar to the cabinet with the supplied screw, the detail in bonding the ground bus to a subpanel enclosure, and jumper the door as in control panel door bonding.
The fundamentals sit in our enclosure grounding and bonding guide and in how to size an electrical enclosure. Mounting height and clearances come together in the enclosure mounting guide. If the station service transformer shares the fence line, its grounding is the companion case in grounding a transformer to the enclosure instead of the grid.
The box to order
In a substation yard you are specifying against rain, hose-down washing, dust and sun. Order a manufacturer-rated NEMA 4X / IP66 stainless box; our ratings are manufacturer ratings and the boxes are not UL listed. NEC 110.28 keeps the scales separate in US work, and Table 110.28 lists the enclosure type for each location, so a US specification needs the type number, with the IP code riding along for IEC-side specs. The NEMA 4 vs 4X vs IP66 guide lays out the difference. Pick 304 stainless for most yards. Step to 316L where salt, brine or chemical drift reaches the fence.
Size the box around the layout. A 60 x 36 x 16 in wall-mount enclosure gives you the full 16 in depth — room for a 120/240 V loadcenter, the fused disconnect and the ATS with bending space to spare. The same 60 x 36 x 16 in size comes in 316L for harsh sites (60 x 36 x 16 in 316L). With the ATS in its own cabinet, the 48 x 36 x 16 in box carries the rest at the same depth. We cut every entry from one piece on the custom enclosure line — bottom PVC stub-ups, a gland plate sized for the EGC and the 4/0 riser. Every wall-mount size is listed under wall-mount enclosures. Each box is delivered in 10 business days anywhere in the world; for custom work the count starts at your approval of the drawing.
| Box | Outside (H x W x D) | Rating |
|---|---|---|
| 60 x 36 x 16 in | 60 x 36 x 16 in | Manufacturer-rated NEMA 4X / IP66 |
| 48 x 36 x 16 in | 48 x 36 x 16 in | Manufacturer-rated NEMA 4X / IP66 |
So the three cases end like this. Bonded yard as described: one bond at the meter-main, the copper EGC sized from Table 250.122 in the PVC, 4/0 riser from the grid, rods as auxiliary only. A future 300 A or 400 A feed: the table above steps the copper to the next row, and the box grows with the bending space. A spec that asks the enclosure to float on rods alone: send it back with the arithmetic — 4.8 A at 120 V to ground against the main's trip clears nothing.
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 |
|---|---|---|---|
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 16 in | 316L stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 48 × 36 × 16 in | 304 stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does the copper EGC in the PVC run have to be green?
The green marking rule in NEC 250.119 covers insulated equipment grounding conductors of 6 AWG and smaller, so a larger feeder EGC falls outside it. Green insulation is still the practical pick: it reads at a glance inside the box and keeps a future hand from landing it on the neutral bar. Identify both ends at the bars either way.
What changes if the transfer switch switches the neutral?
A 3-pole automatic transfer switch moves the neutral with the sources, so only one system's neutral is in circuit at a time and each system keeps its single bond at its own first disconnect. A 2-pole switch with a solid neutral ties the two neutrals together; both bonds stay at their disconnects and the common neutral carries the unbalanced current back to whichever source feeds the load. Count bonds: one per system, two systems here, and none added at the enclosure.
Can the enclosure sit on ground rods alone if the grid stops short?
Rods at the enclosure are auxiliary electrodes under NEC 250.54: you may bond them to the EGC, and they supplement the copper rather than replacing it. The numbers show why: soil reads about 25 ohms, so at 120 V to ground the earth path draws 4.8 A, roughly 2% of the main breaker's rating and far below any trip point. The 4/0 grid riser and the copper EGC do the clearing.
What lands the 4/0 grid conductor on a stainless ground bar?
Use a ground bar with lugs marked for the conductor metals you are joining: the 4/0 riser under a lug rated for aluminum or dual-rated for both metals, the copper EGC under its own lug. NEC 250.12 asks for paint and enamel cleared from contact surfaces; bare stainless needs none of that. Tighten each lug to the torque printed on it and recheck after the first season of temperature swings.
