Does a bonding bushing replace a locknut?

No. The bushing bonds the raceway and protects conductors, but NEC 300.12 and 314.17(B) still require the raceway to be mechanically secured to the enclosure, which is the locknut or hub's job.

Can a Myers hub be used instead of a locknut?

Yes. A listed threaded hub is designed to secure and bond the raceway on its own and is commonly required for rain tight top entries, subject to the listing and NEC 110.3(B).

Can you use a bonding bushing without a locknut on a panel?

Q: When is a bonding bushing required at a panel?

NEC 250.92(B) requires bonding methods at service raceways, and 250.97 requires bonding for metal raceways containing circuits over 250 volts to ground where concentric or eccentric knockouts are present.

Generally no. A bonding bushing and a locknut do two different jobs. The bushing provides the bonding connection and protects the conductors. The locknut, or a listed hub, is what mechanically secures the raceway to the enclosure and makes the metal to metal connection tight. Removing the locknut and relying on the bushing alone leaves the raceway loose in the knockout.

What secures the raceway

Section 300.10 of the 2023 NEC requires metal raceways, cable armor, and other metal enclosures for conductors to be metallically joined together into a continuous electrically conductive system and to be connected to all boxes, fittings, and cabinets so as to provide effective electrical continuity. Section 300.12 requires raceways to be mechanically continuous between cabinets, boxes, fittings, and other enclosures or outlets. Section 314.17(B) requires the raceway or cable to be secured to the box for metal boxes. A threaded bushing spun onto a nipple from the inside of the can, with nothing on the outside, does not hold the raceway in place if the assembly is pulled or the enclosure flexes.

The normal, listed assembly for a threaded rigid nipple entering a panel is a locknut outside, the enclosure wall, and a locknut or a bonding bushing inside, all made up wrench tight. Section 344.42 and Section 358.42 require couplings and connectors to be made up tight, and for raceways buried in masonry or concrete they must be of the concrete tight type.

What the bonding bushing is for

Section 250.92(B) covers bonding at service equipment and permits threadless couplings and connectors made up tight, threaded couplings and hubs, bonding jumpers, and bonding type locknuts, bushings, or other listed devices. Note the phrase bonding type locknuts. That is a bonding locknut, and it is a locknut, so it satisfies the securing function and the bonding function at once. Section 250.97 requires bonding for metal raceways containing circuits over 250 volts to ground, and it points to the 250.92(B) methods with the concentric or eccentric knockout condition. Section 250.8(A) lists the permitted bonding connection means.

Where a locknut really is not used

There are two legitimate exceptions to the locknut habit. The first is a listed threaded hub such as a Myers style hub, which is designed to be the securing and bonding means on its own and is often required to keep a rain tight seal on the top of an outdoor enclosure. The second is a factory threaded boss or hub on the enclosure itself, where the raceway threads directly into the equipment. In both cases the manufacturer instructions and 110.3(B) govern.

Conductor protection

Section 300.4(G) requires an insulating fitting or an insulated bushing at raceway terminations where 4 AWG or larger insulated conductors enter an enclosure. A metal bonding bushing with an insulated throat satisfies this. A bare metal bushing on a large conductor entry does not.

Field takeaway

Keep the locknut. Use a bonding type locknut or a bonding bushing with a lug where 250.92(B) or 250.97 requires bonding, and use a listed hub if you want to eliminate the outside locknut on a wet location entry.