Originally a busway consisted of bare copper conductors supported on inorganic insulators, such as porcelain, mounted within a nonventilated steel housing. This type of construction was adequate for the current ratings of 225Ð600 A then used.
As the use of busways expanded and increased loads demanded higher current ratings, the housing was ventilated to provide better cooling at higher capacities. The bus bars were covered with insulation for safety and to permit closer spacing of bars of opposite polarity in order to achieve lower reactance and voltage drop.
Feeder busway is used to transmit large blocks of power. It has a very low and balanced circuit reactance to minimize voltage drop and sustain voltage at the utilization equipment Feeder busway is frequently used between the source of power, such as a distribution transformer or service drop, and the service entrance equipment.
Industrial plants use feeder busway from the service equipment to supply large loads directly and to supply smaller current ratings of feeder and plug-in busway, which in turn supply loads through power take-offs or plug-in units.
Available current ratings range from 600 - 5000 A, 600 Vac or Vdc. By paralleling runs, higher ratings can be achieved. The manufacturer should be consulted for dc ratings. Feeder busway is available in single-phase and three-phase service with 50% and 100% neutral conductor. A grounding bus is available with all ratings and types.
Available short-circuit current ratings are 42 000 - 200 000 A, symmetrical rms (see 13.8.2). The voltage drop of low-impedance feeder busway with the entire load at the end of the run ranges from 1-3 V/100 ft, line-to-line, depending upon the type of construction and the current rating .
Feeder busway is available in indoor and outdoor construction. Outdoor construction is designed so that exposure to the weather will not interfere with successful operation.
Plug-in busway is used in industrial plants as an overhead system to supply power to utilization equipment. Plug-in busway provides tapoff provisions at regular intervals (approximately every 2 ft) over the length of the run to allow safe connection of a switch or circuit breaker to the busway. Load side cable connections can then be short and direct. Plug-in tapoffs (bus plugs) can be connected to their loads by conduit and wire or flexible bus drop cable.
Bus plugs can be removed, relocated, and reused. The use of flexible cable permits the bus plug and machine it serves to be relocated and put back into service in a minimum of time Bus plugs are available in several types.
They include fusible switches, circuit breakers, static voltage protectors (potentializer), ground detectors (indicating), combination motor starters and lighting contactors, transformers, and capacitor plugs. Many can be equipped with additional accessories, such as control power transformers, relays, indicating lights (blown fuse), and terminal blocks for remote control and indication.
Busway is totally enclosed and can be of the ventilated or non-ventilated design. Plug-in busways have current ratings ranging from 100-5000 A. Plug-in and feeder busway sections of the same manufacturers above 600 A are usually of compatible design and are interchangeable, allowing for a section of plug-in to be installed in a feeder run where tapoffs are desired.
Bus plugs are generally limited to maximum ratings of 800 A for fused-switch type plugs and 1200 A for circuit-breaker type plugs. Short-circuit current ratings vary from 10 000Ð200 000 A symmetrical rms. The voltage drop ranges are approximately from 1- 3 V/100 ft, line-to-line, for evenly distributed
loading. If the entire load is concentrated at the end of the run, these values double.
A neutral bar may be provided for single-phase loads such as lighting. Neutral bars usually are of the
same capacity as the phase bars.The bus housing may be used as an equipment grounding path.
However, grounding bus bar is often added for greater system protection and coordination under ground fault conditions. The grounding bus bar provides a low-impedance ground path and reduces the possibility of arcing at the joint under high-level ground faults if the housing is used as a ground path.
Lighting busway is rated a maximum of 60 A, 300 V-to-ground, with two, three, or four conductors. It may be used on 480Y/277 V or 208Y/120 V systems and is specifically designed for use with fluorescent and high-intensity discharge lightingTapoffs for lighting busway are available in various types and include those with built-in circuit protection by either fuse or circuit breaker.
Accessories include special mounting brackets and tapoffs for surface or close coupling attachment of fluorescent lighting fixtures to the busway. Lighting busway can be surface-mounted, recessed in dropped ceilings, or suspended from drop rods. Hangers are available to accommodate each method.
Lighting busways provide power to the lighting Þxture and also serve as the mechanical support for the fixture. Auxiliary supporting means called strength beams are available for increasing supporting intervals.
The strength beams provide supports for the lighting busway as required by the National Electrical Code (NEC) (ANSI/NFPA 70-1993). Lighting busway is also used to provide power for light industrial applications.
Trolley busway is constructed to receive stationary or movable take-off devices to power overhead cranes, monorail systems, industrial doors, and conveyor lines. Trolley busways are not suitable for outdoor application.
They are used on a moving production line to supply electric power to a motor or a portable tool moving with a production line, or where operators move back and forth to perform their specific operations.
Trolley busway is available in current ratings ranging from 60Ð800 A, up to 600 V ac or dc, and 3, 4, and 5 wire. The steel casing serves as the ground. Tapoffs (moving trolleys) range from 15Ð200 A and can be equipped with circuit breakers, fusible protection, starters, contactors, and relays.