Is Busway The Best Way?

June 1, 1999
Busway, which is a grounded metal enclosure containing factory-mounted bare or insulated conductors, can be an effective means of distributing power. Need to add load to or extend power from an existing distribution system? Busway systems may be the answer. First introduced in 1932, busway solved the automotive industry's need for a flexible power distribution system to serve its linear layouts. Since

Busway, which is a grounded metal enclosure containing factory-mounted bare or insulated conductors, can be an effective means of distributing power.

Need to add load to or extend power from an existing distribution system? Busway systems may be the answer. First introduced in 1932, busway solved the automotive industry's need for a flexible power distribution system to serve its linear layouts. Since that time, this product has grown to serve many other types of loads.

Specifications for various busway lines focus on the needs of particular industries, offering high short-circuit interrupting ratings (AIC), multiple-ground designations, and 200% neutrals; just to name a few. Despite this versatility, there's still one common misconception in the industry: Busway only serves high-amperage applications efficiently. However, these systems actually provide a high degree of efficiency and flexibility for both low- and high-amperage applications.

Where and how to apply busway. You can install busway in most applications where you'd normally use cable and conduit. Busway manufacturers produce systems ranging from 100A to 6500A. Low-amperage applications include high-technology firms, such as computer manufacturers and test laboratories. The automotive industry and other heavy assembly industries require high-amperage busway systems.

Recognizing the need for flexibility, manufacturers developed elbows and offsets to make directional changes easy. With these fittings, busway offers extensive layout versatility. When new loads develop, it's easy to meet changing conditions by adding tap-off units and/or new sections. However, busway is not the best solution for every application. For example, if the situation requires low current to a specific source, you're better suited for cable and conduit, which is best for underground applications. In Sec. 364-4(b), the National Electrical Code (NEC) says you can't install busway where it's subject to severe physical damage or corrosive vapors.

Construction techniques. There are different styles of busway. The "sandwich style" has become the most common type. Its popularity stems from its compact design and ability to handle high short-circuit currents. Other designs may require you to space bus bars in the air. Like the sandwich style, insulated bus bars use various materials, such as Mylar, polyvinylchloride (PVC), or epoxy. Most manufacturers offer this style using either aluminum or copper bus bars enclosed in aluminum or steel housing. Aluminum housing is not only lightweight, but it is suitable as a ground path. Depending on the design, steel housings are also suitable as a ground path. But, be sure to check with the manufacturer first.

The compact and lightweight design of busway is especially beneficial in retrofit applications where space is limited. Most manufacturers offer both the plug-in and feeder variety.

You can use plug-in busway with tap-off units called bus plugs, which you can install along the length of a plug-in busway system. Basically, you use bus plugs to connect electrical cable to a busway distribution system. They are available with either fuse or circuit breaker protection, eliminating the need for long cable runs back to a distribution board.

Most manufacturers offer a wide range of options to meet various electrical needs. Examples include a 200% neutral for applications affected by harmonics, or an isolated ground for such loads as sensitive computer equipment. For special applications, some offer busways with a 200% neutral, a housing ground, and an isolated ground; all in one system. One manufacturer makes a "paired phasing" bus bar configuration, in which it groups the bus bars in pairs. This allows the AC current in each pair to be nearly equal in magnitude and opposite in direction, producing a balanced voltage phase-to-phase. This configuration uses two bus bars per phase, with Phase C paired with Phase A, Phase A with Phase B, and Phase B with Phase C. Ford and Chrysler often use "paired phasing" busway for welder loads because a balanced voltage is critical for uniform welds.

Since testing provides information like short-circuit ratings, voltage drop, and mechanical strength, manufacturers carefully engineer and test busway systems during product development. They also perform UL dielectric tests on all pieces to ensure the integrity of bus bar insulation before product shipment to a construction site.

Many also provide fire-rating data. Some systems offer as much as 2-hr protection when used with a gypsum board firewall and 3-hr protection with concrete floors. Due to its compact design, sandwich-style busway does not allow air or smoke flow inside the housing. You need an additional sealant only where you cut a hole in a floor or wall for the busway to pass through.

You should use expansion fittings whenever a busway system crosses a building's expansion joint, or on long straight runs where both ends of the busway are held in a permanent fixed position. Expansion fittings contain a sliding expansion enclosure and phase bars. These allow a certain amount of movement, depending on the design. Usually, allowable expansion is in the 52-in. range.

How to get started. Once you've decided to use busway, you should develop sketches of the route the busway must follow; indicating all bends, switchboards, columns, floor elevation changes, etc. Then, forward these drawings to the manufacturer, who will then develop final layout drawings for approval. Once it gets approval, the manufacturer begins building the busway in sections and labels them in sequence, according to the drawings. This greatly simplifies installation.

Some manufacturers also offer a final connection program, which allows you to mark certain sections of the run as "unknown" at the time you place the order. This way, you can start the installation and order the final connection pieces when you find out the final dimensions. This eliminates dimensional errors in the run and ensures proper fit.

For uncomplicated applications, most manufacturers offer standard busway sections and fittings from warehouse stock. Just like other standard products, you order them by catalog numbers for direct shipment to the job site.

B ecause of the weight of individual high-ampacity busway sections, you may have some difficulty installing them. One solution is to use mechanical lifts often found at construction sites. Typically, you suspend busway by hangers attached to the building structure. Busway manufacturers offer these hangers as standard. After you install the hangers, lift the busway into place. Then, you can easily connect the sections together.

Some styles of busway arrive with a connection piece called a "joint stack" at one end of each section. (See Fig. 5.) During installation, the joint stack from one piece fits on the extended bus bars of the adjoining piece for a quick and easy connection. You must torque the joint stack bolt to ensure a proper connection. Some manufacturers offer a joint stack with a breakaway bolt head; when you reach the proper torque value, the bolt head breaks away.

Another excellent feature some manufacturers offer is built-in adjustability in the joint stack. This allows for additional alignment at each section, if necessary. If you come to the end of the run and see you're too short or too long, you can go back and readjust the joint stacks. This feature eliminates the need for a new manufactured section.

NEMA recommends checking the integrity of busway joint stack connections as a means of connecting busway segments six months after initial installation and once a year thereafter. You can check the torque of joint stacks easily because the joint stack bolt is at ground potential. So, you can check the torque during daily operations without cutting power to the system.

For industrial applications where there is moving machinery, such as cranes, conveyers, hoists, and similar moving equipment, some manufacturers offer special busways where the bus bars are inside a housing, with an opening at the bottom to provide free movement of a trolley. Mounted on the trolley, brushes make contact with the bus bars. This allows power transfer to mobile loads. These types of busway can be straight or curved. With curved sections, however, you're limited to a current capability of around 100A. If you want power to some sections of the bus bar arrangement, just remove those sections and replace them with a spacer.

Benefits to end users. Busway provides an effective means of distributing power in a building. Since it requires easy maintenance procedures and is flexible, it's relatively simple for accommodating changing load situations. Maintenance primarily consists of annual inspections of joint fittings using an infrared heat gun. Then, using a torque wrench, tighten the appropriate connecting bolts. If required, you can cut power to only a portion of a busway run so you can perform any required maintenance without risk of injury or equipment damage.

About the Author

Robert W. Morgan

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