Cable installation is one of the activities that demands the most man-hours from electricians in building construction and renovation projects. The use of a reliable wire management system makes the installation simpler and safer, and cable bus is very useful when dealing with medium-voltage conductors. The NFPA 70 National Electrical Code dedicates Chapter 3 to Wiring Methods and Materials, including raceway systems for conductors above 600V.

Cable bus is used with conductors much larger than those delivering power to lighting fixtures, power outlets and HVAC equipment in your home or office. The rated voltage exceeds 600V, while the current range is typically from 1,000A to 8,000A. There are other raceway options for large conductors, such as busway and cable tray, but cable bus offers a middle point that brings advantages of both.


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Wireways for Conductors Above 600V

When dealing with conductors and their raceway systems, the NEC establishes additional requirements when rated voltage exceeds 600V in a given application:

  • Conductors above 600V must not share enclosures and wireways with circuits below 600V, unless a specific code article makes an exception.
  • Circuits above 600V must use suitable enclosures to prevent accidental contact or physical damage.
  • The smallest bend radius allowed is 8 times the diameter for unshielded conductors, and 12 times the diameter for shielded conductors. Multi-conductor cables are limited to 12 times the individual conductor diameter or 7 times the total diameter, whichever is less.
  • Metallic raceways must be protected from induction heating.
  • Consider there are differences between the requirements for above-ground and buried conductors.

To compare busway, cable bus and cable tray, first we  must define each type of raceway:

RACEWAY TYPE

DESCRIPTION

Busway

Grounded metal enclosure, which comes with factory-installed conductors. These can be bare or insulated; made from aluminum or copper; and shaped as bars, rods or tubes.

Cable bus

Assembly of insulated conductors that includes fittings and terminations, all enclosed in a ventilated metallic housing for protection. Unlike busway, which is fully factory-assembled, cable bus is partially assembled at the project site. Cable bus is designed to withstand the magnetic forces present during faults, reliably holding conductors in place.

Cable tray

A structural assembly with multiple sections and fittings, designed to fasten wiring or smaller raceways.

Allowed Usage and Limitations of Each Wireway System

Like in any engineering decision, each configuration brings advantages and disadvantages. The allowed and prohibited uses of busway, cable bus and cable tray are as follows:

RACEWAY TYPE

ALLOWED

NOT ALLOWED

Busway   







1. Exposed installation, with special requirements for busways through walls or floors.
2. Concealed, but only if the busway is of totally enclosed and non-ventilated type, and if the joints and fittings are accessible for maintenance.



1. Exposed to severe physical damage, or in hoistways.
2. Exposed to corrosive vapors.
3. Hazardous locations, unless a specific busway is rated for that use.
4. Outdoor, wet or damp locations, unless a specific busway is rated for that use.
5. Less than 2.5 meters above the floor or a working platform, unless it comes with a suitable cover.

Cable bus




1. Only exposed work, at any voltage or current for which the conductors are rated. Special requirements apply for crossing walls and floors.
2. Branch circuits, feeders and service entrances.
3. Outdoor, corrosive, wet or damp locations, but only if the cable bus model is rated for that use.

1. Hoistways
2. Hazardous locations, unless a specific cable bus is rated for that use.



Cable tray



1. Physical support for service conductors, feeders and branch circuits. Also allowed for communications, control and signaling cable.
2. Outdoor and exposed to sunlight, as long as the conductors are rated as sunlight-resistant.

1. Hoistways or anywhere exposed to severe physical damage.
2. Ducts, plenums and air-handling spaces in general, unless used for a type of wire rated for those spaces and following special instructions provided by the NEC.

What Advantages Does Cable Bus Offer?

Cable bus is characterized by its safety, since it is built to withstand the magnetic forces that occur when one of the conductors carries a fault current. Cable bus also offers a middle ground between the fully prefabricated busway and the site-assembled cable tray.

Cable bus also saves on supporting fixtures, by using some prefabricated components while retaining flexibility in conductor layout. In a few words, cable bus offers a rigid construction like busway, while having wiring flexibility and open ventilated construction like cable tray. Some common applications of bus duct include connections between the following equipment:

  • Generators and transformers
  • Generators and switchgear
  • Transformers and switchgear
  • Different switchgear units
  • Switchgear to motor control centers (MCC)
  • MCCs and high horsepower motors

Electrical safety is important in any installation, but additional protection measures are required as voltage level increases, since accidents have a greater potential to cause damage. By working with qualified electrical design engineers you can ensure your installation meets the NFPA 70 National Electrical Code, in addition to local standards like the NYC Electrical Code or the NJ Uniform Construction Code, Electrical Subcode.

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