Bonding bushing without a locknut: can it hold the raceway?

Short answer: yes. A metal bonding bushing — one with set screws, a bonding-wire lug, or both — holds an EMT connector in its knockout with no locknut behind it, and bonds the joint at the same time. The NEC lets a raceway be secured by its bushing when the bushing is not 'wholly of insulating material', and UL investigates these bushings as bonding devices in their own right. What follows is the rule, the listing behind it, five fixes when the threads run short, and the ring arithmetic to run before you order fittings.

Why the locknut runs out of room

The stack inside a shallow box is tight. A set-screw EMT connector enters the knockout on a short threaded neck. The locknut clamps it from inside, and an insulated bonding bushing behind it eats more thread. Plenty of connectors run out of neck before the bushing seats, so the joint stays loose with everything torqued. Die-cast zinc connectors are the usual culprit: their necks carry fewer usable threads than steel ones, and a steel connector in the same trade size usually recovers the turns you are missing. The same shortage hits cable glands, and the answer is the same there — pick the longer-threaded part first, as in cable gland too short for junction box threads.

What lets a bushing secure the raceway

The bond is a code duty on its own. NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors — raceways, cables, equipment, enclosures — to be bonded together so they can safely carry any fault current likely to be imposed on them. That matters most where the raceway itself is the grounding path, which metal raceway as equipment grounding conductor: when it counts sorts out. The enclosure grounding and bonding guide walks the whole path.

Securing is the second duty. The NEC wants the raceway end fastened where it enters the box, and it lets the bushing do that job when the bushing is not 'wholly of insulating material'. A metal-bodied insulated bushing qualifies: its flange presses the wall and its set screws grip the connector neck. The locknut behind it adds no bond and no hold. UL's guide information for grounding and bonding equipment (category KDER) describes bushings that bond through set screws, through a provision for a bonding wire, or through means for mounting a wire connector — the bushing's own means carry the joint. One contact point to respect: NEC 250.12 requires paint and other nonconductive coatings removed from threads and contact surfaces at bonding connections unless the fitting is designed to bite through them, so scrape a painted box at the flange. Stainless walls need no scraping; grounding screw threads and lug fit shows how to judge a surface.

What is tested, and what the liner takes

UL investigates the bushing as a bonding device: set screws that bond it reliably to the enclosure, a lug for a bonding conductor, or both. One set screw per bushing is the common supply, and the liner is the insulating half — it keeps conductor insulation off the thread ends while the metal body does the work. A wholly plastic bushing has no listed role holding a connector in a wall.

Boxes are covered too. UL's guide information for boxes states that boxes with concentric or eccentric knockouts have been investigated for bonding and are suitable for it, so the jumper duty lands where the NEC puts it: on services and on circuits over 250 V to ground. The details, and the point where the bushing becomes mandatory at such a hole, sit in when a bonding bushing is required at a concentric knockout. Heat is the easy side. A polyetherimide liner carries a 150 C maker rating, and a 75 C conductor terminal leaves a 75 C margin. Terminal warmth never picks this fitting; fit and metal contact do.

Five fixes when the thread runs short

Work down the list; the first line that fits your box is the line to order.

1. Drop the locknut, keep the metal bonding bushing. A metal-bodied bushing with set screws or a lug secures and bonds on its own. Tighten each set screw until it bites, then try to rock the flange by hand. A joint that rocks is not done.
2. Swap in a steel connector. Steel necks carry more usable threads than die-cast ones, and that usually recovers the whole stack.
3. Use a thin-pattern locknut with a plain insulating bushing. When your bushing is wholly plastic it secures nothing, so a reduced-height locknut keeps the stack inside the available thread.
4. Fill an oversize knockout with a reducing washer. It sits between the bushing flange and the wall; the worked example below sizes it.
5. Use a hub at service entrances. A bolt-on hub lands on the box with its own gasket and screws, and bolt-on and built-in hubs for service raceway bonding lays out that choice.

Working it out: a 3/4 in connector in a 1 in knockout

Say a 3/4 in connector has to land in a knockout punched for 1 in, the usual find inside an older panel. The hole sizes come from UL 50 Table 71-1A, which sets knockout diameters and fitting dimensions: 1-1/8 in [28.6 mm] for a 3/4 in boss, 1-3/8 in [34.9 mm] for a 1 in hole. The guide to cutouts, glands and HMI windows walks through that table. The rule: the bushing flange must bear on flat metal all round, because an uncovered ring lets the joint tilt and the set screws carry the load sideways.

The arithmetic: (34.9 - 28.6) / 2 = 3.15 mm of open ring per side, call it 3.2 mm [1/8 in]. The result: a flange that stops partway off that ring leaves the joint free to rock, so plan on the washer. The decision: fit a 1 in x 3/4 in reducing washer over the connector ahead of the bushing, so the bevel fills the ring and the flange clamps flat metal. The threshold to carry with you: daylight between flange and wall means the washer goes in; metal-to-metal all round means the bushing alone can secure and bond.

What changes in the panel

You gain a fitting's worth of depth and one fewer part in the stack. Water is where the rating lives. A locknut-and-bushing joint is not a water seal, and the weakest fitting in the wall sets the assembly's rating: on a box rated NEMA 4X / IP66 by the manufacturer — as ours are, not UL listed — washdown work gets a gasketed hub, and a bare EMT connector stays on indoor Type 12 duty. De-energize and verify absence of voltage before you open the box or drive a knockout. Leave the set screws reachable as well; a technician who changes a connector should re-tighten the bond the same way.

What to specify

Put these lines on the drawing or the order:

• Trade size and connector style: a steel set-screw connector at every short-wall entry.
• A metal insulated bonding bushing, set-screw type with bonding lug, at each raceway end that owes the bond.
• A reducing washer for every oversize knockout, with both trade sizes stated (1 in x 3/4 in in the example above).
• A bonding conductor sized from the overcurrent device ahead of the raceway, per NEC Table 250.122: 20 A takes a 12 AWG copper jumper, 100 A takes 8 AWG.
• A gasketed hub wherever the assembly must keep water out.

The decision, case by case. Metal bonding bushing in hand: it secures and bonds alone, no locknut. Wholly plastic bushing: keep a thin locknut under it. Die-cast connector in a shallow wall: change to steel before you stack parts. Oversize knockout: reducing washer under the flange, sized to the two holes. Service or washdown entry: a gasketed, bonded hub. On our stainless boxes the knockouts arrive cut to your drawing — see custom stainless cutouts — so the wall meets the fitting with the right hole from day one.

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