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Fundamentals of low- and medium-voltage busway
Busway as defined by the National Electrical Manufacturers Association (NEMA) is a prefabricated electrical distribution system consisting of bus bars in a protective enclosure, including straight lengths, fittings, devices and accessories. Busway transports electricity and connects to electrical gear such as switchgear, panelboards and transformers.
Busway is an excellent alternative to cable and conduit in commercial and industrial applications because it’s not as complex to configure, less expensive to install and easier to replace, especially in applications where load locations are likely to change.
Sandwich-style busway has phase bars stacked together inside a non-ventilated aluminum or steel enclosure. This busway can be indoor, sprinkler proof or outdoor rated and can be used for delivering power between gear using feeder pieces. Additionally, tap-off locations can be provided on the busway itself. This style is called plug-in busway and is typically used to power equipment below the busway. Capable of providing high ampacity with equally high short-circuit withstand ratings, sandwich-style busway is a cost-efficient solution in multiple markets
Common market segments: commercial construction, general and heavy industry and data centers. Sandwich-style busway is used in applications up to 600 V and up to 5000 A.
Track busway is a continuous rail design where tap-off boxes can be installed or removed nearly anywhere along the busway system. The tap-off boxes come equipped with cord whips and fixed receptacles allowing for power distribution to equipment below the busway. Capable of providing low voltage and ampacity solutions with simple onsite reconfigurations, track busway is common in data center markets but is also very effective in laboratories and warehouse applications.
Common market segments: data centers, machine shops and labs. Track busway is rated for indoor use and is available up to 1000 A and 600 V.
Non-segregated phase bus has copper phase bars located in one enclosure separated by air and epoxy insulation. Environmental ratings include indoor, outdoor and dust tight. Housing or enclosure material can vary from aluminum, steel or stainless steel. Capable of providing both medium and low voltage solutions, non-segregated phase bus duct is well suited for harsh environments.
Common market segments: electrical utilities, petrochemical sites and heavy industry. Non-segregated phase bus duct is used in applications up to 38 kV and 6000 A.
Busway is used as an alternative to cable and conduit and is commonly found throughout mission critical facilities, data centers, industrial facilities, large residential complexes, petrochemical and electrical utilities.
Standards for busway are defined by NEMA, UL, CSA, ANSI and IEEE in North America; CCC in China; and IEC in Europe and other parts of the world.
Busway and cable and conduit both provide the same basic function: delivering power between electrical equipment.
However, busway uses less material since each piece is engineered-to-order for the job and typically has a smaller cross-sectional area. Busway has a lower voltage drop than cable and conduit due to the solid phase bars which results in lower impedance. Perhaps most importantly, installation of cable and conduit is typically more labor intensive than installation of busway, and the higher the ampere rating of the system, the greater the labor differential between the two approaches.
Fusible and circuit breaker disconnect bus plugs come in both plug-in and bolt-on applications, depending on the busway. Typical ranges for fusible bus plugs are from 30 A to 1200 A and up to 600 V. Typical ranges for circuit breaker plugs are from 15 A to 1600 A and up to 600 V.
The plugs can handle a multitude of ground and neutral options, and they come in multiple configurations for different style busway:
Interrupt rating is typically specified on a symmetrical current basis and refers to the magnitude of current that the overcurrent protective device (circuit breaker or fused switch) can safely interrupt without damaging itself or the busway. Interrupting ratings apply only to the actual overcurrent protective devices that are interrupting the circuit under fault conditions, not to the busway assembly itself. Typical fusible interrupt ratings are 200 kAIC and breakers range from 10, 65, 150 or 200 kAIC, depending on voltage.
The short circuit current rating of the busway is the maximum amount of current that the busway can safely withstand, that is allowed to pass through it, without damage to the busway.
The interrupting device in busway interrupts the flow of electricity. This is the type of overcurrent protection device found in the bus plug (either fuse or breaker).
A fuse is a device that interrupts excessive current flow. The current is interrupted by the melting of an electrical wire or strip designed to melt at a prescribed time/temperature rating.
A circuit breaker is a device that interrupts excessive current flow. Circuit breakers can have multiple trip unit types that detect the overflow such as thermal, magnetic or electronic.
