What Is Busway / Bus Duct? Types and Uses Explained

Busway, also called bus duct or a busbar trunking system, is a prefabricated electrical distribution system that carries power through enclosed copper or aluminum busbars instead of individual conductors pulled through conduit. It is built in factory sections that bolt together, complete with joints, fittings, hangers, and either fixed or plug-in connection points, and it's covered in the United States by NEC Article 368 and the product standard UL 857. Selecting the right feeder method also means correctly sizing the circuit breakers and disconnect switches that protect and isolate the run. This guide covers how busway compares to conventional wiring methods, the construction and connection types available, the code rules that govern it, and where it makes sense on a project.

Busway vs. Conduit-and-Wire vs. Cable Tray

Busway is one of three common ways to run a large power feeder, and each method trades off cost, flexibility, and installation labor differently.

Busway generally has the smallest cross-section for a given ampacity because manufacturers engineer the housing tightly around the busbars to dissipate heat, and UL 857 permits ampacity ratings based on temperature rise rather than the 40 percent conduit-fill rule that governs pipe and wire.

Types of Busway Construction

• Sandwich busway: busbars are stacked tightly together with thin insulation (typically mylar or epoxy) between them inside a metallic housing; this construction gives the lowest reactance, the best voltage drop performance, and the smallest cross-sectional area of any busway type.
• Air-insulated busway: busbars are spaced apart with air as the primary insulation, which is simpler to manufacture but has a larger footprint and higher reactance than sandwich construction.
• Track busway: an open-slot form of plug-in busway that allows a compatible plug-in unit to tap in at virtually any point along the run, rather than only at fixed tap-off windows.

Feeder Busway vs. Plug-In Busway

Busway serves two distinct roles in a distribution system, and the two are often combined on the same project:

• Feeder busway moves power between major pieces of equipment, such as from a switchboard up through a building's electrical closets, with no tap-off points along its length. Because it has no openings, feeder busway can achieve higher weatherproof ratings, up to IP65 in some designs.
• Plug-in busway adds tap-off windows or an open track along the run so loads can connect directly to the bus without routing a separate feeder back to a panel. This is the busway most associated with data centers, manufacturing floors, and modular office layouts where equipment gets added, moved, or reconfigured over time.

Codes and Standards: NEC Article 368 and UL 857

Two documents govern busway in the United States:

• UL 857, the product safety standard, defines busway as a prefabricated electrical distribution system with enclosed bus bars, joints, fittings, and accessories, and sets the listing requirements manufacturers must meet.
• NEC Article 368 governs how busway is installed and applied in the field, including support spacing, overcurrent protection rules, and where busway is or isn't permitted, such as restrictions in wet or hazardous locations unless the busway is specifically listed for that use.

Article 368.10(C)(2) also requires a 4-inch waterproof curb around a floor penetration for vertical busway, and some jurisdictions layer on additional local requirements for weatherproofing beyond what UL 857 defines.

Ampacity, Overcurrent Protection, and Tap Rules

Busway is manufactured and listed at a specific ampacity, and NEC 368.17 sets the overcurrent protection rules for feeder busway:

• Overcurrent protection for a busway feeder must match the busway's current rating; a breaker sized well below the busway's rating doesn't violate code, but a breaker or tap sized above the busway's rating does.
• Where a busway's ampacity is reduced along its run, overcurrent protection is generally required at the point of reduction.
• An industrial-establishment exception allows omitting that additional overcurrent device where the reduced-ampacity section runs no more than 50 feet, carries at least one-third the rating of the upstream device, and is kept clear of combustible material.
• Plug-in taps for feeder or branch circuits must include their own overcurrent protection, using an externally operable circuit breaker or fusible switch at the plug-in point.

Where Busway Is Used

• Data centers and mission-critical facilities, where overhead busway feeds power to server rows with plug-in units that can be added or relocated without shutting the system down.
• High-rise buildings, where vertical feeder busway runs floor to floor far more compactly than an equivalent conduit-and-wire riser.
• Manufacturing plants, where plug-in busway lets machinery and workstations be moved or added along a production line without rerouting conduit.
• Retrofit and expansion projects, where the lower field-labor requirement of bolted, modular sections can shorten an installation timeline compared with pulling new feeder wire.

Advantages and Limitations of Busway

Busway's main advantages are a smaller footprint for a given ampacity, lower field-installation labor, excellent voltage-drop performance in sandwich designs, and the ability to add or relocate taps without new wiring runs. The tradeoffs worth planning for are a higher material cost than conduit and wire for smaller feeders, the low reactance that can increase available fault current at downstream taps (which affects equipment fault-current ratings), and the need to follow manufacturer-specific support and weatherproofing details, since UL 857 does not assign the same NEMA enclosure ratings used elsewhere in electrical distribution.

The Bottom Line

Busway distributes power through enclosed, prefabricated busbar sections rather than conductors pulled through conduit, governed by UL 857 for the product and NEC Article 368 for its installation. Sandwich construction gives the most compact footprint and best voltage drop, air-insulated construction is simpler, and track busway offers the most flexible tap-off access. Feeder busway moves power point to point with no taps, while plug-in busway lets loads connect anywhere along the run, which is why it dominates data centers, manufacturing floors, and high-rise risers where flexibility and a smaller footprint matter more than the higher upfront material cost.