What Are Lock Nuts & Bushings?

Lock nuts are threaded fastening rings used to secure conduit connectors, threaded hubs, chase nipples and similar fittings to boxes, pull cans, cabinets and panel enclosures. Bushings are installed at the raceway entry to help protect conductor insulation from contact with metal edges and, in some cases, to provide a bonding connection. In day-to-day trade use, lock nuts handle the mechanical hold while bushings address conductor protection and, where required by design, electrical continuity. Common variants include standard steel lock nuts, grounding or bonding lock nuts, insulated bushings, metallic threaded bushings and bonding bushings with a lug.

Where Are Lock Nuts & Bushings Used?

These fittings are used anywhere conduit or a threaded raceway fitting terminates at an enclosure. Typical applications include EMT connectors into steel boxes, rigid or IMC entries into switchboards and distribution equipment, chase nipples between cabinets, service raceways, motor control centres, rooftop equipment, industrial machinery, generator connections and retrofit panel work. They are also common in maintenance jobs where existing enclosures need a secure raceway entry without replacing the whole fitting system. In corrosive or washdown areas, material choice becomes more important, and in vibration-prone equipment rooms or mechanical spaces, fit and thread quality matter more than many buyers expect.

How To Choose Lock Nuts & Bushings

Start with the raceway and fitting type. EMT connectors often use lock nuts at the enclosure wall, while threaded rigid and IMC entries may use lock nuts, threaded bushings or bonding bushings depending on the assembly. Next, match the trade size exactly. Then consider conductor protection. If conductors are being pulled through a threaded raceway end or nipple and there is any chance of insulation rubbing on metal, an insulated bushing is often the safer practical choice. If the bonding path depends on the fitting arrangement, look at bonding bushings or other listed bonding means rather than assuming a standard lock nut is enough. Also consider enclosure material, indoor or outdoor exposure, corrosion, vibration and whether the job is new construction, service replacement or retrofit into existing gear.

Trade Rules Of Thumb

As a practical rule of thumb, standard lock nuts are fine for many ordinary box terminations where the connector or fitting is already listed for that use and no special bonding accessory is called for. When moving into larger raceways, heavier conductors or threaded rigid and IMC entries, many electricians step up attention to bushings because conductor damage becomes more costly and harder to spot after pull-in. Another common trade practice is to avoid relying on a marginal thread engagement at painted or uneven enclosure walls. If the enclosure finish, knockout condition or fitting stack-up looks questionable, use the fitting arrangement that gives the cleanest mechanical seat and the clearest bonding method. These are practical habits, not code substitutes, and listed installation instructions still govern.

Sizing Guidelines

Lock nuts and bushings are selected by the trade size of the conduit or threaded fitting, not by conductor size alone. A 1 in raceway takes 1 in accessories, a 2 in raceway takes 2 in accessories, and so on. The conductor side matters when deciding whether a plain metallic bushing is enough or whether an insulated throat is the better choice. As a typical guideline, the larger the conductors, the tighter the bend, and the longer the pull, the more valuable insulation protection becomes at the raceway entry. For bonding bushings, also confirm lug range and conductor compatibility where applicable. Do not assume all bushings of the same trade size accept the same bonding conductor or fit every enclosure condition. Always verify the product listing and manufacturer data for the exact assembly.

Common Installation Practices

Good installation practice starts with clean threads, a properly seated connector or nipple, and a lock nut tightened square to the enclosure wall. On steel boxes and cabinets, electricians commonly ensure paint, burrs or distorted knockouts do not interfere with the fitting seat. Where bushings are used, they should thread fully and sit properly without cross-threading. On larger raceways, installers often fit the raceway and bushing before conductor pull-in so the throat protection is in place from the start. For bonding bushings, the bonding conductor should be routed and terminated so it is protected from strain and does not interfere with cover removal or future maintenance. Follow the fitting listing, torque guidance where provided and site-specific bonding design.

Common Mistakes

One common mistake is treating all lock nuts and bushings as interchangeable. A standard lock nut, an insulated bushing and a bonding bushing solve different problems. Another is selecting by visual fit only and missing the exact trade size or thread type. Installers also run into trouble when they rely on a lock nut alone for bonding in assemblies that need a more deliberate bonding method, or when they omit conductor protection on threaded entries carrying larger wire. Cross-threading, under-tightening, over-tightening and installing over damaged knockouts are also frequent causes of callbacks. In retrofit work, a rushed fitting choice can create poor thread engagement or interference with dead fronts and covers.

Brand Comparisons

Bridgeport is widely recognized in the fittings market and is commonly chosen when buyers want a known trade brand with broad acceptance across commercial and industrial work. Hubbell is also a strong name, especially where buyers are standardizing across a wider electrical product range and want dependable fit and finish from a major manufacturer. Ortech can be a practical value option for many standard applications where the job does not call for a specific legacy brand match. In the broader market, T&B and Iberville are also commonly cross-shopped, especially on projects where existing installed material already uses those lines. If you are matching an existing specification, maintenance stock or installed system, staying with the same brand family may simplify fit, inspection and replacement. If you are buying for general stock, Bridgeport, Hubbell and Ortech can each be suitable depending on budget, application and required fitting style.

Related Products

Buyers looking at lock nuts and bushings often also need EMT connectors, rigid and IMC connectors, chase nipples, reducing washers, grounding and bonding fittings, conduit bodies, locknut sealing accessories, box connectors, hubs and enclosure hardware. On larger raceway jobs, it is also worth checking conductor pulling lubricant, bonding jumpers, panel accessories and replacement knock-out seals. For service and maintenance teams, keeping a range of common trade sizes on hand can reduce downtime on cabinet modifications and equipment changeouts.

Frequently Asked Questions

When should I use an insulated bushing instead of a standard metallic bushing?

Use an insulated bushing when conductor insulation needs better protection at the raceway entry, especially on threaded rigid or IMC terminations, larger conductors, tighter bends or pulls where rubbing at the throat is more likely. The exact requirement depends on the fitting listing and the installation design.

Is a lock nut enough for bonding?

Sometimes, but not always. A standard lock nut provides mechanical retention, but the bonding method must match the raceway system, enclosure and fitting listing. Where bonding continuity is critical, a bonding bushing or other listed bonding means may be the correct choice.

Are lock nuts and bushings selected by wire size or conduit size?

They are primarily selected by the trade size of the conduit or threaded fitting. Wire size then affects whether you should move from a plain fitting to an insulated or bonding style based on conductor protection and installation method.

Can I mix brands of lock nuts, bushings and connectors?

In many routine applications, mixed brands are used, but fit, thread quality and listing compatibility still matter. On specified projects, maintenance stock programs or existing installations, matching the installed brand may reduce fit issues and simplify approvals.

What is the difference between a bonding bushing and an insulated bushing?

A bonding bushing is intended to help maintain the bonding path, often with provision for a bonding conductor connection. An insulated bushing is mainly intended to protect conductor insulation from abrasion at the raceway entry. Some jobs may need one function, the other, or a fitting arrangement that addresses both.

Do I need bushings on every conduit termination?

No. The need depends on the raceway type, fitting style, conductor size, enclosure entry and the listed use of the fitting. Many standard connectors already provide the intended termination method, while other assemblies benefit from or require a separate bushing.