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Busway: fundamentals of low and medium voltage busway

What is 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. 

 Busway components

Busway components include:
  • Bus bars, or conductors, conduct electricity; they are made from aluminum or copper and vary in size
  • Housing, an aluminum or steel enclosure to contain the busbars
  • Insulating system made of a combination of air, epoxy and mylar; it separates the conductors from each other to prevent electrical faults
  • Fittings such as elbows, offsets and tees help to properly route busway from one electrical connection or termination
Busway fitting elbow
Components of a busway
Busway fitting tee

 Where is busway used?

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.

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Data center

 How does busway compare to cable and conduit?

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.

Man does bills and finance with calculator
Calculator

Different types of busway

Non-segregated busway 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.

 

 

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 plug-in design where plugs can be installed or removed nearly anywhere along the busway. These plugs can have cord drops or 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 225 A and 600 V.

Air-insulated busway has either copper or aluminum phase bars held in place with polymer supports contained in an aluminum enclosure. The plastic clips keep the conductors equally spaced and isolated, providing the air insulation. This busway is indoor rated and is available in plug-in pieces and feeder designs. Capable of providing medium ampacity and low voltage solutions with robust safety ratings.

Common market segments: data centers, machine shops and labs. Air-insulated busway is rated for indoor use and is available up to 600 A and 600 V.

 Types of bus plug

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:

  • Standard – mechanical lugs included for cabling to equipment; bolt-on or plug-in
  • Receptacles – typically available in a single, double or quad plug-in arrangement. Fixed mounted means the receptacle is mounted to the base of the plug-in unit. Cord Mounted means the cord drop is mounted to the bottom of the unit with a receptacle attached.
Bus plug
Fusible with corded receptacle bus plug

 Ratings and devices in a busway

Voltage classes & maximum voltage ratings

ANSI standards define voltage classifications for low voltage (up to 600 V), medium voltage (between 600 V and 69 kV) and high voltage (between 69 kV and 230 kV). Extra-high voltage and ultra-high voltage classes are also defined in the ANSI standards. (Note that NEC 2014 expanded the definition of low voltage to include up to 1,000 V.)

Interrupt rating defined

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.

Interrupting device basics

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. 

Interested in seeing an installation? Watch this video. 

 

Busway installation

Power-R-Way III accessories and plugs installation

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