Separating neutral and ground in a subpanel on a 4-wire feeder

Short answer: pull the replacement shop feeder as four conductors and split the shop cabinet into two bars. Two hots and a neutral in 3 AWG copper, an equipment grounding conductor in 6 AWG copper, with the grounds on their own bar bonded to the enclosure and the neutrals on an isolated bar. Those sizes fit a 100 A breaker feeding the shop at 75 °C terminals, read straight from the NEC ampacity table for copper and NEC Table 250.122. The old single bus was correct only while the feeder had no ground wire. With the fourth wire in the conduit the two jobs split for good, and the ground rod at the pole stays on as backup. Here is the sizing, the cabinet work in order, and the checks that prove the split before you close the door.

Why the shop end splits the bars

The pole disconnect is the one place neutral and ground may join. Downstream of it they do different jobs: the neutral carries load current home on the white wire, and the equipment grounding conductor waits for a fault and gives it a ride back to the breaker. NEC 250.4(A)(5) puts it plainly: the earth is not part of the effective ground-fault current path, so the return trip runs over metal. NEC 250.6 gives the reason the bars separate past that point: neutral current is kept off metal parts, so it cannot ride the cabinet, the conduit or the workbench you lean on.

The arithmetic shows why the fourth wire earns its slot. A 240 V fault returning over 150 ft of the copper ground wire sees about 0.07 Ω of wire. Fault current then runs into the thousands of amps, and the magnetic trip clears in a cycle or less: 16.7 ms at 60 Hz. The same fault fighting through soil faces tens of ohms and trips nothing. Your existing 3-wire feeder left the shop grounds no ride home except the neutral, which is why the old cabinet shares one bar. The service end keeps its bond; the split happens on the load side, as why bonding belongs only at the service disconnect and keeping subpanel neutral and ground apart both lay out. Industrial panels make the same call: how a PLC panel neutral bus is configured, separate versus bonded, comes down to this same physics.

What you need

• Four feeder conductors in conduit: three 3 AWG copper and one 6 AWG copper, the last with green insulation, because NEC 250.119 requires green or green-striped insulation on EGCs 6 AWG and smaller. THWN-2 is the usual insulation in conduit; the 75 °C column still governs, since that is what feeder lugs are marked.
• A ground bar kit for the cabinet: terminals for every ground wire plus a few spares, with its bonding screw or jumper to the cabinet metal.
• The isolated neutral bar. Most cabinets ship with the bonding screw or strap removable; in this panel it stays out.
• A torque wrench reading in lb-in, an ohmmeter, and a clamp meter for the leakage check after startup.
• Bonding hardware for a coated cabinet: external-tooth lock washers, or paint stripped back to bare metal. NEC 250.12 requires nonconductive coatings such as paint removed from contact surfaces, unless the fitting is designed to cut through them.
• When branch circuits and a second ground bar come later, the grounding and bonding guide covers bond and conductor sizing.

Sizing the shop feeder: 100 A at 75 °C over 150 ft

Inputs, with their sources: a 100 A two-pole breaker protects the shop feeder, the example case for a detached workshop fed under a larger main — read the actual handle rating before you order wire. Terminals are rated 75 °C, which is what feeder-grade breaker lugs carry. The run is 150 ft pole to shop, an assumption to swap for your trench length. The service is 120/240 V split phase.

Ampacity rule: the NEC ampacity table for copper at 75 °C, take the smallest conductor that covers the breaker. The column reads 6 AWG 65 A, 4 AWG 85 A, 3 AWG 100 A, 2 AWG 115 A. Check: 85 A fails against 100 A, so 4 AWG is out; 3 AWG at 100 A covers it. The neutral takes the same 3 AWG, because on a split-phase feeder the 120 V loads on one leg alone can put the full current on the neutral.

Ground rule: NEC Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device ahead of it, and the breaker rating calls for 6 AWG copper. Its 65 A ampacity is beside the point; the ground wire is sized to trip the breaker, and it does, as the fault numbers above show.

Drop check: 3 AWG copper is about 0.194 Ω per 1000 ft at 20 °C. At a 75 °C conductor, resistance rises 55 K × 0.39 % per K ≈ 21 %, to 0.235 Ω per 1000 ft. Drop at full breaker current over 150 ft: 2 × 100 A × 0.235 Ω/1000 ft × 0.150 = 7.05 V, which is 2.9 % of 240 V, inside the 3 % feeder target most designers hold. Backwards for the limit: 3 % of 240 V is 7.2 V, and the drop runs 0.047 V per foot, so 7.2 ÷ 0.047 ≈ 153 ft.

Result: three 3 AWG conductors and one green ground wire, all copper. Decision: at this breaker size, 3 AWG holds the 3 % target to about 150 ft; 2 AWG (115 A) reaches about 190 ft and 1 AWG (130 A) about 240 ft. Aluminum earns its keep on runs this long; size it from the same two tables and land it on lugs marked for aluminum.

Splitting the bars in the shop cabinet, step by step

Work the cabinet in this order:

1. Open the pole disconnect, lock it, and verify absence of voltage at the shop lugs with a meter you have just proven on a known live source.
2. Pull the bonding screw or strap that ties the neutral bar to the cabinet. This cabinet is not service equipment, so the bond stays out for good; removing a neutral-ground bond in an outdoor subpanel walks the same job on a panel replacement.
3. Mount the ground bar in a factory-punched hole with the kit's screw and an external-tooth washer. On a powder-coated cabinet, strip the coating under the washer unless the kit's fitting is built to bite through, which NEC 250.12 allows. The bond puts the cabinet metal in the fault path, which NEC 250.96(A) wants bonded so it can safely carry that current.
4. Move every bare and green conductor from the old shared bar to the new ground bar.
5. Land the white neutrals on the now-isolated neutral bar, one conductor per terminal and nothing else on that bar.
6. Land the two hots on the feeder breaker and the feeder neutral on the power lug of the neutral bar.
7. Torque every lug to its label marking. Feeder-size lugs commonly call for 20 to 45 lb-in [2.3 to 5.1 Nm]; the marking on your panel is the value that counts.
8. Re-check the torque after the first week under load, then run the checks below before you call the job finished.

The ground rod at the pole

The rod already driven at the pole keeps its job, and its job is backup. NEC 250.54 classes it as an auxiliary electrode: you may connect it to the grounding system, and the feeder EGC still runs in the conduit. Fault current takes the low-resistance wire every time, and the rod holds the pole hardware near earth potential for lightning and surges. When the rod's conductor lands in a box, bonding a grounding electrode conductor to a service enclosure shows the termination. The junction box that splits the house and shop feeds gets one rule of its own: splice hots to hots, neutrals to neutrals and EGCs to EGCs, so the next person in the box finds three clean sets.

Outdoor boxes on the run

The pole disconnect and the splice boxes live in rain, sleet and snow, so Type 3R is the working minimum there. A shop cabinet on an exterior wall takes the same treatment; inside a dry shop, Type 1 is enough. Where corrosion joins the weather, 304 stainless covers most sites and 316L covers salt air and chemical spray; the chloride stress-cracking threshold sits near 60 °C (140 °F) for both grades, so a dark box baking in coastal sun is the case for 316L.

The wall-mount stainless line is manufacturer-rated NEMA 4X-type / IP66, not UL listed. The boxes carry a hinged gasketed door, a removable mounting panel and wall-mount feet, in outside sizes from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D).

A panelboard cabinet carries its own type marking from its own listing; our boxes suit the splice and junction enclosures around the run, with every cutout machined where your conduit lands and delivery in 10 business days from your approval of the drawing. Passing the EGC through a wall and sealing the entry is its own small job; running a ground wire out of an outdoor enclosure and sealing it covers the gland work, and the outdoor electrical enclosures guide settles the type by site. Compare grades side by side in 304 and 316L stainless enclosures, and spec the cutouts in custom enclosures.

Checks before you close the door

Run these with the feeder still dead, meter set to ohms:

• Neutral bar to ground bar: open circuit. A healthy meter reads kilohms to over-range; a reading near zero means the bond screw is still in or a conductor crossed bars.
• Ground bar to the cabinet wall: 0.1 Ω or less, the field rule of thumb for a clean bond.
• Feeder EGC end to end, shop bar to the pole neutral-ground point: about 0.07 Ω over the 150 ft run; under 0.5 Ω leaves room for clamp contact resistance.

Then energize and finish the proof:

• Clamp meter around the EGC alone: milliamps is normal leakage. Half an amp or more means neutral current has found the ground path, so re-check the bar split before loading the shop.
• An IR reading on the lugs after an hour of load: a lug running hotter than the bus by more than a few degrees wants a re-torque.

Mistakes that undo the separation

The failures cluster in five places, most likely first:

• The bonding screw stays in place. Every check reads zero, and the new ground wire quietly works as a second neutral, carrying load current on the cabinet and everything bonded to it.
• The conductor sets swap bars: grounds land on the isolated bar and neutrals on the bonded one. The open-circuit check catches this before the panel ever energizes.
• The EGC comes up a size short. 8 AWG is the 60 A entry in NEC Table 250.122, two sizes under what this breaker calls for.
• The rod gets counted as the return and the feeder goes in as three wires. The wire is the path; the rod is the backup.
• Coating stays under a bond screw on a powder-coated cabinet, and the bond reads ohms instead of milliohms. Strip back to bare metal.

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