Short answer: a grounding-type bolt-on hub is itself one of the NEC's accepted ways to bond a metal service raceway, so the raceway that lands on it takes no bonding bushing. A hub built into the wall bonds the same way, through bare metal threads. The trap is the standard Myers hub: the trade sells two kinds, and only the grounding type carries the lug and the bonding listing. Keep one more rule straight: a raceway that carries a grounding electrode conductor is bonded at both ends, and the hub can only ever be one of them. Below you get the hub cases that pass, the hole sizes for the common trade sizes, eight installation steps, and a worked 200 A riser from hole diameter to meter reading. The fault-path basics live in the grounding and bonding guide.
What the NEC asks when the raceway lands on a hub
The NEC names the methods that bond a metal service raceway to the service equipment: bonding jumpers, threaded hubs and threaded connections, and listed bonding fittings. A bolt-on hub over a round hole sits on that list, so the hub is the bond and the bushing list at that end stays empty. A hub that is part of the wall, a threaded boss or tapped opening, bonds through its bare metal threads, the same path a threaded coupling uses.
A raceway that carries a grounding electrode conductor is stricter: the NEC requires the bond at both ends, at your enclosure and at the far cabinet or electrode. The grounding-type hub can be the enclosure end; the far end keeps its own bond. On systems over 250 V to ground the listed parts matter most, and the deepest dive on that side is the walk-through of bond bushing requirements for 480 V EMT raceways.
| Where the raceway lands | What makes the bond | Bonding bushing at that end? |
|---|---|---|
| Grounding-type bolt-on hub | the hub's listed bond path and its lug | No |
| Built-in threaded hub or tapped boss | bare metal threads, fully engaged | No |
| Standard Myers hub (no lug) | nothing listed | Yes, plus a jumper |
| Concentric or eccentric knockout left in the wall | nothing; the NEC calls it an impaired connection | Yes, bond around it |
| Reducing washer in the opening | nothing; impaired connection | Yes, bond around it |
| PVC or other nonmetallic raceway | no metal path to bond | No bond; the hub only seals |
A concentric or eccentric knockout breaks the contact path, and a reducing washer does the same: the NEC treats both as impaired connections that need bonding around them, not through them. The piece on when a bonding bushing is required at a concentric knockout shows the fix. A PVC raceway has no metal path to bond at all, so the hub only seals and carries the conduit; how a Myers hub seals on PVC conduit covers that sealing job.
Pick the grounding-type hub
Two kinds of Myers hub hang on the fitting rack. The standard hub clamps and seals; the grounding type adds the lug and the bonding listing, and it is the one that counts as a bond. They look alike from the outside, so buy the grounding type by name. Both attach with machine-threaded screws into the hub body, and the grounding lug lands on the bus inside your enclosure.
When the raceway is a mast riser, the NEC has required since its 2014 edition that the mast fitting be identified for use with service entrance equipment, so order a hub marked for mast use. The rundown of which listed hub passes at service equipment goes deeper on the markings.
Install the hub, step by step
- Match the hub to the raceway's trade size and specify the grounding type.
- Cut or order the hole at the knockout diameter for that trade size (table below; the dimensions come from UL 50 Table 71-1A, see the cutouts, glands and HMI windows guide), and keep it round with a deburred edge.
- Bare the seat. NEC 250.12 requires paint, lacquer and enamel to come off threads and contact surfaces; a #4 brushed 304 or 316L wall ships bare, a painted steel box does not. The piece on grounding screw threads and lug fit shows what a clean seat looks like.
- Bolt the hub on with its machine screws, threads fully engaged.
- Land the grounding lug on the ground bus inside.
- If the opening still shows concentric rings or sits on a reducing washer, add the bonding bushing and jumper around it.
- Bond the far end of a raceway that carries a grounding electrode conductor.
- On a mast riser, fit the hub that is identified for service entrance equipment.
| Trade size | Hole (knockout) diameter |
|---|---|
| 1/2 in | 7/8 in [22.2 mm] |
| 3/4 in | 1-3/32 in [28.2 mm] |
| 1 in | 1-3/8 in [34.9 mm] |
| 2 in | 2-15/32 in [62.7 mm] |
| 4 in | 4-11/16 in [119.1 mm] |
Worked example: a 200 A service riser on a grounding-type hub
Inputs. A 200 A service overcurrent device (the worked example's assumption, typical for a small service); 2 in rigid metal conduit with a 2.375 in [60.3 mm] outside diameter (standard conduit dimension); a 2 in grounding-type bolt-on hub (the example's selection); a hole at 2-15/32 in [62.7 mm], the 2 in knockout diameter from UL 50 Table 71-1A.
Rule. A grounding-type hub is one of the NEC's service-raceway bonding methods, so this end takes no bushing. Any jumper in the standard-hub alternative follows NEC Table 250.122, which sizes the equipment grounding conductor from the rating of the overcurrent device ahead of it. The hole clearance keeps the hub grommet seated on bare metal.
Arithmetic. Clearance per side: (2.469 − 2.375) ÷ 2 = 0.047 in [1.2 mm]. Jumper for the standard-hub alternative: 200 A maps to 6 AWG copper under NEC Table 250.122. That conductor runs about 0.395 Ω per 1000 ft at 20 °C, so an 18 in [457 mm] jumper measures 0.395 × 1.5 ÷ 1000 = 0.0006 Ω, or 0.6 mΩ. Note the contrast: 6 AWG carries 65 A continuous at 75 °C in the NEC ampacity table, yet the sizing table pairs it with a 200 A device, because a bond is sized to the fault path, not to the load.
Result. The hub is the bond at the enclosure end; the hole leaves 0.047 in per side for the grommet; a sound hub path reads on the 0.6 mΩ scale.
Decision. Grounding-type hub over a clean 2-15/32 in hole: no bonding bushing at this end. Standard Myers hub: add a bonding bushing with a 6 AWG copper jumper. Any concentric ring or reducing washer in the opening: bond around it, whatever hub you used.
Checks when done
De-energize the service and verify absence of voltage before any meter work. Then:
- Meter from the raceway to the enclosure's grounding lug and expect milliohms, the 0.6 mΩ scale from the example. A reading in ohms means paint under the seat or a standard hub; find which and fix it.
- Confirm the far end of a grounding-electrode raceway has its own bond.
- Seat the hub grommet square on the conduit so the grommet, not the threads, keeps the water out.
- Give the door its own bond path; the enclosure-door bonding rules cover hinges and covers.
Mistakes that cost the bond
- A standard Myers hub on a service raceway because it was the only one on the shelf: swap it for the grounding type before inspection.
- A hub over a partially punched knockout: drive the rings fully out so the hub lands on one clean hole, or bond around what remains.
- Coating under the seat on a painted steel box: bare the contact surfaces per NEC 250.12.
- The far end of a grounding-electrode raceway left unbonded: bond it at the other cabinet or electrode.
- A mast clamp with no service-entrance identification on a riser: fit the identified fitting.
- Loose machine screws: snug them so the hub body and grommet clamp metal to metal.
Order the opening with the box
We cut hub openings and tapped bosses into 304 and 316L walls as part of the enclosure order. Lay the hub out with the cutout designer, or send the whole job through the custom stainless cutouts page. The factory cut keeps the hole round and the seat bare, and the box is delivered in 10 business days, counted from your approval of the drawing for custom work. Ratings stay honest too: the assembly keeps its rating only while every opening keeps its own seal, and our bodies are rated NEMA 4X / IP66 by the manufacturer; not UL listed.
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 |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 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 |
Questions
Does a bolt-on hub count as service raceway bonding?
Yes, when the hub is the grounding type: the NEC lists threaded hubs among the methods that bond a metal service raceway to the service equipment, so you fit no bonding bushing at that end. Land the lug on bare metal. A standard Myers hub without a grounding lug seals only, and a 200 A service then needs a bonding bushing with a 6 AWG copper jumper.
Does a GEC raceway that lands on a hub need a bond at both ends?
Yes: a metal raceway that carries a grounding electrode conductor is bonded at both ends. The grounding-type hub is the bond at the enclosure end, and the far end keeps its own bond at the other cabinet or the electrode. On systems over 250 V to ground, use listed bonding parts such as bonding bushings at both raceway ends.
What size hole does a 2 in hub need in a stainless wall?
Cut 2-15/32 in [62.7 mm], the knockout diameter UL 50 Table 71-1A gives for 2 in trade size. The conduit outside diameter is 2.375 in [60.3 mm], so the hub keeps 0.047 in [1.2 mm] of clearance per side and its gasket seat lands on bare 316L.
Is a built-in hub better than a bolt-on hub for bonding?
Neither bonds better: a built-in threaded hub bonds through bare threads the way a threaded coupling does, and a grounding-type bolt-on hub bonds through its listed body and lug. The built-in boss keeps the opening round and skips the field cut; we cut or tap it into 304 or 316L walls on request, delivered in 10 business days from drawing approval.
