Short answer: on the load side of a 4000 A breaker, every bonding jumper at the wall is 500 kcmil copper, the size NEC Table 250.122 sets for an equipment grounding conductor fed by that overcurrent device. Land each jumper on a listed insulated grounding bushing whose lug is marked for 500 kcmil copper. On the supply side of a service or a separately derived system, the code lets you size each jumper to its own raceway's conductors, or loop one jumper through the whole set.
What size bonding jumper does a service entrance need?
Run one full-size jumper at each bushing on the load side of the breaker: the table reads the device ahead of the conductor, so nothing shrinks with the raceway.
Table 250.122 reads the overcurrent device ahead of the conductor, so a 4000 A generator breaker governs every raceway it feeds. The worked case is six parallel raceways leaving that breaker. Each raceway carries about 667 A of load, 4000 ÷ 6, yet its jumper is still the full 500 kcmil copper. Six bushings take six jumpers: 6 × 500 kcmil = 3000 kcmil of bonding copper at the wall. The fundamentals sit in the grounding and bonding guide.
The large rows of NEC Table 250.122, which sizes the equipment grounding conductor from the overcurrent device rating ahead of it:
| Breaker or fuse rating (A) | Copper jumper (kcmil) | Aluminum jumper (kcmil) |
|---|---|---|
| 1200 | 3/0 | 250 |
| 1600 | 4/0 | 300 |
| 2000 | 250 | 400 |
| 2500 | 300 | 500 |
| 3000 | 400 | 600 |
| 4000 | 500 | 700 |
The aluminum column is for aluminum jumpers, whatever the phase conductors are made of. The two sizing rules that meet at a feeder into a subpanel are worked through in the subpanel jumper sizing article.
Which raceways are sized to their own conductors instead?
On the supply side of a service or a separately derived system, the code lets you size each raceway's bonding jumper from the conductors in that one raceway, which lands far below the 500 kcmil the load-side table gives at the top row.
The branch turns on where the bushing sits. Supply side of the service, or supply side of a separately derived system: read each jumper from that raceway's largest ungrounded conductor in the code's bonding-jumper table. A generator that moves its neutral in the transfer switch is a separately derived system; one that carries the neutral straight through usually is not. Load side of the breaker, the per-raceway option stops: every jumper reads that breaker's row and comes out 500 kcmil copper. The same two-way split governs EGC sizing from a service disconnect to a downstream panel.
Can one jumper loop through several bushings?
Yes on the supply side: one jumper threads through each bushing in turn and lands on every lug, and you size that loop from the combined conductors of all the raceways it bonds, not from one raceway's share.
The arithmetic: six raceways of 750 kcmil copper per phase put 6 × 750 = 4500 kcmil of ungrounded conductor behind one jumper. Past 1100 kcmil total, the code's bonding-jumper table sets the jumper at 12.5% of that area: 4500 × 0.125 = 562.5 kcmil, rounded up to a 600 kcmil copper loop. One conductor, one lug per bushing, one pull through the row. Load side, keep the jumpers separate: the parallel-conductor rule puts an equipment grounding conductor in each raceway, each at the full 500 kcmil. Sizing a service wireway jumper to the largest conductor walks that method step by step.
What bushing and lug actually take 500 kcmil?
Use a listed insulated grounding bushing whose lug is marked for 500 kcmil copper: the common hardware is a threadless set-screw body with a copper lug and a 105 °C rated plastic throat liner, sold by trade size to match the raceway.
The insulated throat does two jobs. Its rounded face shields the conductor insulation where the raceway meets the wall, and the liner's rating suits the hot air around a generator set. The listing mark names the metals: copper or aluminum grounding conductors, so a copper jumper takes the copper-marked lug. Read the lug range off the bushing sheet before the box goes on the truck. When the wall entry fitting itself is in question, bonding bushing or bonding hub at an enclosure compares the two.
What if the bushing's factory lug is too small?
Swap the factory lug for one marked to 500 kcmil copper: unbolt the small lug from the bushing screw, fit the larger barrel on the same set screw, and torque the new lug to the value its maker prints.
Panel builders do this swap routinely on big parallel runs. The bushing body carries the bond to the hub; the lug only lands the conductor, so a larger barrel on the same screw changes nothing about the throat. Keep the replacement a listed lug of the right metal, face the throat squarely into the enclosure, and clamp the set screw. De-energize the board and verify absence of voltage before opening a lug; a generator that is off can still be backfed from the utility through the transfer gear.
Where does the jumper land on the enclosure?
Land it on the bushing's lug right at the wall, because NEC 250.96(A) requires metal parts serving as grounding conductors — raceways, enclosures included — to be bonded together so they safely carry the fault current likely to be imposed on them.
The contact surfaces carry that current, so NEC 250.12 has paint, lacquer and enamel stripped from threads and contact surfaces at grounding and bonding connections unless the fitting is designed to bite through them. A bare 304 or 316L stainless wall lands on bare metal as delivered. A powder-coated carbon steel box needs the coating cut back under the bushing face and the locknut teeth, or fittings listed to cut through on installation. Enclosure grounding screw threads and lug fit covers the thread and lug details at the landing point.
What do you put on the enclosure order?
Order the wall with cutouts matched to your hub schedule and leave 2 in [50 mm] of edge-to-edge spacing between entries as a wrench-clearance rule of thumb: a 4 in raceway entry leaves room for the bushing, locknut and a lug for 500 kcmil.
Put the entries on the drawing: cutouts on the bottom wall, sized to each hub's spigot, spaced so bushings and locknuts clear each other and a wrench. The cutout designer lays holes on any face, checks the spacing and exports the layout with the quote request. Order the wall in 304 stainless for most outdoor generator yards, and 316L where salt air, brine or chlorides reach the box. Custom stainless enclosures take your width, height, depth and every cutout from one piece, delivered in 10 business days from your approval of the drawing, rated NEMA 4X / IP66 by the manufacturer; not UL listed.
Do these bushings belong on the box at all?
Fit bonding bushings where a wall entry has a concentric or slotted ring that breaks, where the job spec calls for them, or where you want the insulated throat or a lug to land a jumper; full knockouts with standard threaded hubs often need none.
Where the knockout ring is concentric or slotted, when a bonding bushing is required at a concentric knockout works the decision through. A listed bonding locknut is the lighter alternative on a full knockout, but it gives up the insulated throat and the jumper lug this job runs on. Where the job spec lists bonding bushings at every entry, the spec wins and the wall takes the jumpers.
The decision, case by case:
- Load side of the breaker: one 500 kcmil copper jumper per raceway, each on an insulated bushing with a 500 kcmil copper lug.
- Supply side of a service or separately derived system: per-raceway jumpers sized to each raceway's conductors, or one loop at 12.5% of the combined conductor area.
- Aluminum jumpers: 700 kcmil at the top row, landed on AL/CU-marked lugs.
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 |
| 48 × 36 × 16 in | 304 stainless | 48 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 12 in | 304 stainless | 60 x 36 x 12 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How many bonding bushings does a 4000 A parallel run need?
One at every raceway wall entry, and one full-size jumper at each: six parallel raceways on the load side of that breaker means six insulated bushings and six 500 kcmil copper jumpers, 3000 kcmil of bonding copper in total. Table 250.122 reads the breaker rating, so no jumper drops to a raceway's share of the load.
What torque lands a 500 kcmil jumper on a bushing lug?
Use the torque the lug maker prints for that barrel; a single-barrel mechanical lug for 500 kcmil copper commonly calls for about 375 lb-in [42 N·m]. Set screws and locknuts take far less, so a calibrated torque wrench and the printed values beat a guess on every connection at the wall.
Does the jumper size change for aluminum phase conductors?
The jumper follows its own metal, not the phase conductors: NEC Table 250.122 calls for 700 kcmil aluminum where the copper row at the same breaker size gives 500 kcmil. Land aluminum on a lug marked AL/CU and treat the contact the way the lug's instructions set out.
