Advancements in busways

Busway consists of a grounded metal enclosure, contains factory-mounted bare or insulated conductors, and is an effective means of distributing power. Busway was introduced in the early 1900s as a way to solve the automotive industry's need for a flexible power distribution system.

By Nathan Faulkner, Siemens Energy & Automation, Norcross, GA August 9, 2004
Key Concepts
  • Busway systems provide efficiency and flexibility for low and high-amperage applications.

  • Busway can be installed in most applications where cable and conduit are used.

  • Maintenance requirements are annual IR joint connection inspections and tightening affected bolts to the appropriate torque.

    Busway applications
    Busway construction types
    Busway configurations
    Other busway features
    How to start a busway installation
    Busway benefits
    More Info:

    Busway consists of a grounded metal
    enclosure, contains factory-mounted bare or insulated conductors, and is an effective means of distributing power. Busway was introduced in the early 1900s as a way to solve the automotive industry’s need for a flexible power distribution system. Since that time, busway has evolved to serve many types of loads (Fig. 1).

    Specifications for various busway lines focus on the needs of particular industries, offering high short-circuit withstand ratings, multiple ground designations, and 200% neutrals. A common misconception is that busway only efficiently serves high-current applications. Actually, busway systems provide a high degree of efficiency and adaptability for both low and high-current applications.

    Busway applications

    Busway can be installed in most applications where cable and conduit are used. Busway manufacturers produce systems that range from 100 to 6500 A. Low-current applications include high-technology firms such as computer manufacturers and test laboratories. The automotive industry and other heavy assembly industries require the use of high-current busway systems.

    Recognizing the need for adaptability, busway manufactures have developed elbows and offsets to make directional changes easy. With these fittings, busway offers extensive layout versatility. When new loads are installed, the busway system is expanded by adding tap-off units and/or new busway sections (Fig. 2).

    Busway construction types

    There are different styles of busway, including what has become the most common busway specified today, the “sandwich style.” Other designs may require the bus bars to be spaced in the air. Insulated bus bars use various materials such as Mylar, polyvinylcloride (PVC), or epoxy.

    The most popular type of busway is the sandwich style, due to its compact design and its ability to handle high short-circuit currents (Fig. 3). Most busway manufacturers offer this style using either aluminum or copper busbars enclosed in either aluminum or steel housing. Aluminum housing is not only lightweight, but it can also be used as a ground path. Depending on the design, steel housings can also be used as a ground path. However, check with the manufacturer first.

    The compact and lightweight design of busway can be especially beneficial in retrofit applications where space is limited. Most manufacturers offer both the plug-in and feeder variety of busway. Plug-in busway is used in conjunction with tap-off units, called bus plugs. This type of plug can be installed along the length of a plug-in busway system and is used to connect electrical cable to a busway distribution system. Bus plugs offer either fuse or circuit breaker protection, eliminating the need for long cable runs back to a distribution panel.

    Busway configurations

    Busway systems are available in a wide range of configurations to meet various electrical needs. Two of the most recent features are the 200% neutral (neutral conductor twice its normal size) for applications that are affected by harmonics, and the isolated ground for sensitive loads such as computerized equipment.

    Some manufacturers can offer busways with a 200% neutral, a housing ground, and an isolated ground all in one busway system. One manufacturer offers a paired phasing busbar configuration. With this design, the bus bars are grouped in pairs so that the ac current in each pair is nearly equal in magnitude and opposite in direction, producing a balanced voltage phase to phase. Two bus bars per phase are used — phase C is paired with phase A; phase A is paired with phase B; and phase B is paired with phase C. Automotive manufactures often use paired phasing busway for welder loads where a balanced voltage is critical for uniform welds.

    Other busway features

    Busway is carefully engineered and tested during product development. Tests on busway are performed to obtain specific electrical and thermal performance characteristics. These data are available to help you specify a busway system that meets your particular needs. Most manufacturers perform dielectric tests on all busway pieces to ensure the integrity of busbar insulation before it is shipped.

    Some manufacturers offer fire-rated busway systems with as much as 2 hr of protection when used in conjunction with a gypsum fire-rated wallboard, and 3 hr of protection with concrete floors.

    Expansion fittings are used when a busway system crosses a building’s expansion joint or on long straight runs where both ends of the busway are fixed. Expansion fittings incorporate a telescopic enclosure and flexible bussing. Typically, these fittings allow for

    How to start a busway installation

    Sketches are developed to route the busway. They indicate all bends, switchboards, columns, floor elevation changes, and other physical constraints. These drawings are forwarded to the busway manufacturer, who then develops final layout drawings for approval. Once approval is given, the busway is built in sections and labeled in sequence of assembly according to the drawings, simplifying the installation process.

    Some busway manufacturers also offer a final connection program that allows the plant engineer to identify certain sections of the run as unknown at the time the job order is placed. The final connection pieces are ordered when the final dimensions are known, thus eliminating dimensional errors in the run and ensuring a proper fit. For less complex installations, replacement, or system expansion, most manufacturers offer standard busway sections and fittings from warehouse stock.

    Hanger systems are offered to support both vertical and horizontal applications. These systems are engineered to provide a safe, cost effective installation solution specific to the requirements of the busway.

    Some styles of busway are shipped with a connection piece called a joint stack at one end of each section. During installation the joint stack from one piece fits on the extended busbars of the adjoining piece for a quick and easy connection. Joint stacks ensure proper electrical connection through proper torquing of the joint stack bolt. A few manufacturers offer a joint stack with a breakaway bolt head. When the proper torque value is reached, the bolt head breaks away, ensuring a solid connection. Some busway manufacturers offer elbow stacks that provide directional change (Fig. 4).

    Elbow stack benefits include:

    • User installed hardware reduced by 50%

    • Weight reduced up to 50%, aiding material handling and installation

    • Compact design, facilitating installation in tight areas

    • Fewer manufactured-to-order components, resulting in improved delivery lead time

    • Lower installed cost.

      • Some busway manufacturers are building adjustability into the joint stack to allow for additional alignment at each section if necessary. If the installer comes to the end of the run and ends up short or too long, he can go back and readjust the joint stacks, which may provide the adjustment needed, eliminating the need for a new manufactured section.

        For industrial applications where there is moving machinery, such as cranes, conveyers, hoists, and similar moving equipment, some busway manufacturers offer bus bars that are inside a housing with an opening at the bottom to provide free movement of a trolley. Brushes mounted on the trolley make contact with the bus bars, allowing power to be transferred to mobile loads. These busway types can be straight or curved. But with curved sections, current capability usually is limited to around 100 A.

        It may be desirable to have some sections of the bus bar arrangement for the trolley that aren’t powered. A section of the bus bar can be removed and replaced with a spacer. It also may be desirable to have a system that is powered only at specific locations. Less expensive casings without bus bar can be used throughout the unpowered sections.

        Busway benefits

        Busway provides an effective means of distributing power in a building. It requires little maintenance and is adaptable, making it relatively simple for accommodating changing load situations. Maintenance involves annual inspections of joint connections using an infrared (IR) thermographic camera and tightening the appropriate connecting bolts with a torque wrench if necessary. If required, power can easily be isolated to only a portion of a busway run while maintenance is completed without the risks of personal injury or equipment damage.

        The adaptability of busway is appreciated when the need to add loads to or extend power from an existing distribution system arises. With busway, additional pieces can be ordered that connect to existing runs to extend power throughout the plant. Plug-in units can be installed and/or relocated if necessary, to power additional or moved equipment.

        More Info:

        Questions about busways may be directed to Nathan Faulkner at 864-595-4719 or . For more information on busways and other power distribution technologies, go to or . Article edited by Jack Smith, senior editor, 630-288-8783, .