Listed conduit hubs for bonding: which hub passes at service equipment

Short answer: a threaded hub certified only as a conduit fitting is not, by itself, the bond the NEC service-bonding rule calls for at service equipment. Order the same hub in its grounding-and-bonding version — bonding screw or lug, serrations that bite the wall, UL category KDER — or back a plain hub with a bonding locknut or a grounding bushing and a jumper sized from NEC Table 250.122, which is 6 AWG copper at a 200 A service. The two hub versions look identical on the shelf. UL lists them in two different categories, Conduit Fittings (DWTT) and Grounding and Bonding Equipment (KDER), and only the second one carries the bond on its own. Here is how to tell them apart, what each listing covers, and what to draw on the cutout.

What does the NEC ask for at the service?

The NEC service-bonding rule calls for the normally non-current-carrying metal parts of a service — the enclosure and every metal raceway that carries service conductors — to be bonded to each other and to the grounded conductor, so fault current has one continuous metal path back to the source.

The bond is what clears a fault fast. NEC 250.4(A)(5) names this the effective ground-fault current path and states that the earth is not part of it. A fault into the enclosure must drive enough current through metal to trip the device ahead within seconds. Every extra joint sits exactly where that current flows, so a loose or coated joint hides inside the one path that has to work.

NEC 250.96(A) adds the panel-side rule: metal parts meant to act as equipment grounding conductors, raceways and enclosures included, are bonded together so they can carry the fault current likely to be imposed on them. The service is where this bites hardest, because the grounded conductor is bonded to the enclosure only there, and the raceway bond usually lands on the neutral bar. The fundamentals sit in the grounding and bonding guide, and the question of whether the rigid raceway itself counts as the grounding path is covered in rigid conduit as grounding conductor with NEMA 4X boxes.

Does an ordinary threaded hub count as a listed hub?

The code accepts a 'listed' hub as one of the standard bonding methods, and an ordinary threaded hub is listed — as a conduit fitting. That wording gap is what trips inspections: UL keeps two hub listings, and only the grounding-and-bonding one qualifies on its own.

UL's guide to electrical equipment, the White Book, splits hub listings in two: Conduit Fittings under category code DWTT, and Grounding and Bonding Equipment under category code KDER. A hub carried under DWTT alone is a mechanical fitting. It secures the raceway and, where marked, seals a wet location, and that is the whole of its promise. In UL's wording, a grounding and bonding hub is a certified hub provided with a certified grounding and bonding lug; the same hub body earns the KDER listing once that lug and the serrated bonding means join the evaluation. Both families are evaluated to UL 514B, the Standard for Safety for Conduit, Tubing, and Cable Fittings.

A plain DWTT hub on a service raceway therefore needs company: a bonding locknut, a bonding wedge, or a grounding bushing with a jumper. The service-bonding rule reads the same way, because bonding jumpers stay in the picture unless the standard methods are met or every concentric and eccentric knockout ring is fully removed.

How do you tell a grounding-and-bonding hub from a plain one?

Look for the bonding means and the marking: the grounding-and-bonding version carries a grounding screw or lug on its nut and serrations on both faces that bite the enclosure wall, and its catalog page and package say grounding and bonding — UL's database files it under KDER, not DWTT.

The bodies, threads and gaskets match; the differences are the screw, the bite and the paperwork. Serrations do the real work: they cut through mill scale and thin coatings to reach bare metal, the same principle behind bonding armoured cable glands with serrated washers. The same hub line usually appears in both versions side by side in a catalog, and the plain one never advertises a bonding screw. Read the package marking first. If it is silent, look the catalog number up and read the listing category: Conduit Fittings (DWTT) means fitting only, Grounding and Bonding Equipment (KDER) means the hub itself is a bond. An inspector with the White Book runs exactly that lookup.

What does each listing actually cover?

Both hub families are evaluated to UL 514B, the Standard for Safety for Conduit, Tubing, and Cable Fittings; the grounding-and-bonding listing adds the bonding screw or lug, the serrated contact seats and the fault-current path through the hub to the wall.

A DWTT-only listing speaks to securement and sealing. Nothing in it says the threaded joint can carry fault current to the enclosure wall. The KDER listing adds the pieces a bond needs: the lug, the serrated seats and the screw thread that takes the jumper. Keep the evaluations separate in your head when you spec a service box in stainless. The water rating belongs to the enclosure standard, tested on the assembled box; our wall-mount boxes are rated NEMA 4X / IP66 by the manufacturer and are not UL listed, while a UL 50E listing is its own evaluation of the box as a whole. The hub you choose has to hold that rating too: a gasketed hub marked for wet locations keeps the entry at Type 4X performance, and an unrated hole accessory drags the whole assembly down to its own figure.

What changes on a bare stainless enclosure wall?

Bare 304 or 316L saves you the scraping: NEC 250.12 wants paint, lacquer and enamel off the threads and contact surfaces at bonding connections, and an unpainted stainless wall has nothing to remove, so the hub's serrations land on clean metal on the first turn.

Painted steel needs the coating cut back under the flange and around the bonding screw before anything bonds; stainless skips that work, which is one quiet reason panels in wet plants go together faster. The contact point still matters. Keep the hole area free of labels and gasket squeeze, and wrench-tighten the nut so the serrations seat into the wall. Then there is the hole itself. Our stainless boxes have no knockouts, because the wall is cut to your drawing, so each hub's hole dimension goes on the cutout drawing. UL 50's Table 71-1A pairs knockout diameters with fitting dimensions, and the hub datasheet prints the exact hole. The custom stainless cutout page lists what to send, and the cutouts, glands and windows guide covers the hole-to-fitting fit. The same contact-surface logic governs glands; see cable gland bonding requirements for stainless junction boxes.

What jumper size does a grounding bushing need?

Size each raceway bonding jumper from the overcurrent device ahead of the raceway using NEC Table 250.122: a 200 A service disconnect takes 6 AWG copper, 100 A takes 8 AWG, and every metal entrance on that service uses the same size.

Worked example. A 200 A service disconnect lives in a stainless wall-mount enclosure and takes one 2 in rigid metal conduit through the bottom. Fit a grounding bushing on the raceway inside and run 6 AWG copper from its lug to the neutral bar, or use the grounding-and-bonding hub, whose bonding screw takes the same 6 AWG. The mapping runs straight across the table: 200 A lands on 6 AWG copper. Two metal entrances mean two jumpers, each 6 AWG, both landing on the same neutral bar, because the grounded conductor and the enclosure are one at the service. One conductor per lug, torqued to the lug maker's marking. The aluminum column runs larger: 4 AWG aluminum at the same 200 A device.

Away from the service, bonding jumper sizing follows its own two rules; the sub panel bonding jumper size article walks them.

What goes on the order and the cutout drawing?

Write the grounding-and-bonding hub into the order for every metal service entrance, sized to the conduit — a 2 in entrance takes a 2 in hub — with copper jumpers per Table 250.122 and every hole drawn to its hub's dimension: three raceway entries mean three bonding points.

• Enclosure: a stainless wall-mount box sized for the disconnect, its wiring space and the hubs; see the stainless wall-mount line.
• Hubs: grounding-and-bonding versions, UL category KDER, gasketed and marked for wet locations, one per metal raceway entry — a 2 in service entrance plus any 3/4 in or 1 in metal entries.
• Jumpers: copper, sized from NEC Table 250.122 by the device ahead of the raceway, one per entry, landed on the neutral bar.
• Cutouts: each hub's hole dimension per its datasheet or UL 50's Table 71-1A, positioned clear of stiffeners and the back panel; send positions and sizes with the drawing.
• Timing: delivered in 10 business days anywhere in the world, counted from your order — or from your approval of the drawing when the cutouts are custom.

Decisions by case. A metal raceway at the service: the grounding-and-bonding hub with its jumper is the clean single-fitting answer. A plain DWTT hub already installed: add a bonding locknut or a grounding bushing with a Table 250.122 jumper, and one fitting makes the entrance right. A nonmetallic entrance: no raceway bond exists, the enclosure takes its bond through the grounded conductor connection, and the fitting only has to keep weather out — for outdoor entries, the sealing side of that choice is covered in conduit hub vs sealing locknut for outdoor top entries. Entries after the service take their own jumper rules, covered in the sub-panel article linked above.

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