Bonding bushing or bonding hub at an enclosure: which fits

Short answer: standard locknuts carry the bond at a metal wall whenever the circuit runs at 250 V to ground or less, which covers every 120/208 V and 120/240 V panel. Fit a bonding bushing, a bonding hub or another listed bonding fitting, with a sized jumper, when the raceway sees more than that. The 277 V legs of a 480Y/277 V system count. The other triggers are service conductors or a grounding electrode conductor in the raceway, a nonmetallic wall, a raceway end that stops short of an enclosure, or a classified area. Size the jumper like an equipment grounding conductor: NEC Table 250.122 keys it to the overcurrent device rating, so a 200 A feeder takes 6 AWG copper.

When a standard locknut passes

Four conditions, checked together, make the plain entry correct:

• Circuit at 250 V to ground or less. 120/208Y (120 V), 120/240 (120 V) and 240 V delta all sit under the line. A 480Y/277 V feeder crosses it at 277 V.
• Metal wall, full punched knockout. One locknut outside and one inside, both pulled tight. A concentric or eccentric ring changes the fitting once the voltage crosses the line.
• Bare contact surfaces. NEC 250.12 requires paint, lacquer and enamel removed from threads and contact surfaces unless the fitting is designed to cut through them.
• Continuity the code already counts on. NEC 250.96(A) requires metal parts serving as equipment grounding conductors — raceways, cables, equipment and enclosures — to be bonded so they carry any fault current likely to be imposed on them.
• An insulating bushing on the conductors. It guards insulation against the cut edge and does no bonding, which is enough at this voltage.

Length plays no part in the choice. A 50 ft [15 m] 120/208 V feed passes on locknuts end to end, while a 6 in [150 mm] nipple into a classified area still takes a bonding fitting. The grounding fundamentals behind this list sit in the enclosure grounding and bonding guide.

The triggers that call for a bonding fitting

Each trigger changes the fitting at the raceway end, and most have nothing to do with voltage.

• Voltage above the line. 480Y/277 V (277 V to ground) and 480 V delta (480 V) both cross it, so every raceway end takes a bonding bushing with jumper, bonding locknuts or a bonding hub. The worked numbers for a 480 V EMT run sit in 480 V EMT bond bushing rules.
• A ring knockout at over-line voltage. A concentric or eccentric ring leaves a gap a locknut cannot bridge, so a jumper runs around it, or a listed enclosure and fitting combination stands in for it. The two knockout cases each have a page: bonding bushings at concentric knockouts and bonding jumpers through eccentric knockouts.
• Service conductors in the raceway. Meter to disconnect, the joint stays metal-to-metal through a listed means: bonding bushing with jumper, bonding locknuts or a bonding hub. Which hub passes at service equipment is compared in the listed conduit hub page.
• A grounding electrode conductor in ferrous metal raceway. Bond the conductor to the sleeve at both ends, or leave the metal sleeve out. The ferrous raceway around a grounding electrode conductor page walks the case.
• A nonmetallic wall. A fiberglass or polycarbonate body bonds nothing through locknuts. Each metal raceway end takes a bushing or hub with a lug and a jumper to the ground bar.
• A raceway end that stops short of an enclosure. Fit the bonding bushing at the open end and land its jumper on the equipment grounding terminal.
• A classified area. Class I, Class II, Class III and Zone installations bond every metal raceway end, at any voltage to ground.

Bushing, hub or bonding locknut: the pick

A gasketed bonding hub does two jobs at once: it bonds and it seals. On an outdoor top entry it beats a locknut and drip loop, and the trade-offs are set out in the outdoor top entry comparison.

Fittings are listed components; the enclosure is not. Our washdown bodies are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and where a listing matters, the UL 50E and NEMA 250 listing guide explains the difference.

Inside the box, treat the jumper like any equipment grounding conductor: one run from the fitting lug to the ground bar, at the table size. Routing and sizing details sit in the junction box bonding jumper article.

Worked example: a 480Y/277 V feeder into a washdown enclosure

Say you order from us a 316L wall-mount body in Type 4X for a washdown line, with one top entry for a 2 in EMT feeder. The service is 480Y/277 V, protected at 200 A. Here is the chain of checks:

1. Voltage trigger. 277 V to ground crosses the line, so both raceway ends need a listed bond. Plain locknuts are out.
2. Jumper size. NEC Table 250.122 at 200 A gives 6 AWG copper, green because NEC 250.119 colors insulated conductors 6 AWG and smaller. One piece, lug to ground bar.
3. Hole size. The 2 in hub seats in a 2.438 in [61.9 mm] hole, the knockout diameter UL 50 Table 71-1A tabulates for 2 in trade size. For scale, 1/2 in trade cuts at 0.875 in [22.2 mm] and 1-1/2 in at 1.969 in [50.0 mm]. Order the hole full punched, so no ring remains to bridge.
4. Contact surface. The #4 brushed 316L wall is bare metal, so nothing needs stripping at the hub ring. The same entry on a powder-coated carbon steel body clears the coating from the contact surface under NEC 250.12, or uses a fitting designed to bite through.
5. Before you land it. Lock out the feeder and verify absence of voltage first.

The chain reads cleanly: 277 over 250 sets the fitting, 200 A sets the jumper, 2 in sets the hole.

Cutout details that decide the fitting

A punched hole and a field-cut hole are not equals in a bonding joint. The factory hole keeps its ring true and its edge square, so the hub gasket seats flat and the locknut grips the full circumference.

• Order holes to the fitting, not the conduit. A gasketed 2 in hub wants its 2.438 in [61.9 mm] knockout; an oversize hole leaves the gasket floating. Knockout sizes by trade size sit in the cutouts, glands and HMI windows guide.
• Leave room behind the wall. The hub nut, the bushing and its lug all live inside. Set the backplate deep enough and mark rear clearance on the approval drawing before anything is cut, as the custom stainless cutouts page lays out.
• Keep bond points full-ring. A concentric or eccentric ring at a bond point adds a jumper that a punched hole would have ordered away.
• Seal the top entries. Water tracks down a raceway, so a gasketed hub on a top wall keeps it outside a Type 4X body.

What to send with the enclosure order

Your entry schedule decides the holes, and the holes decide the fitting. Send these five lines and the drawing comes back right the first time:

• Entry schedule: wall and trade size per hole, for example "top: 1 x 2 in; side: 2 x 1/2 in".
• Fitting plan: gasketed bonding hub for outdoor and washdown tops, plain punched hole where an insulating bushing takes the conductors.
• Voltage to ground per circuit: a 277 V entry notes the bonding point on the drawing; a 120 V control entry needs none.
• Material and finish: 304 stainless for indoor washdown, 316L for chlorides and brine; the bare #4 finish keeps every bonding contact coating-free.
• Enclosure type: Type 4X for hose-down areas, Type 12 for dry indoor panels.

We build each body with its cutouts from one piece and deliver in 10 business days, counted from your approval of the drawing. Start from the custom enclosures page.

The decision, case by case:

• 120/208 V or 120/240 V into a metal wall: two standard locknuts and an insulating bushing, nothing more.
• Over-line voltage, service conductors, a grounding electrode conductor in ferrous raceway, a nonmetallic wall, a free raceway end or a classified area: a bonding bushing or bonding hub, jumpered at the NEC Table 250.122 size for the overcurrent device.
• Ring knockout above the line: add the jumper around the ring, or order the hole full punched.

Send the entry schedule with the order and the holes arrive cut for the fitting you chose.

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