It was introduced in Article 365 of the National Electrical Code (NEC) in 1969. Currently, it is covered in NEC Article 370. Although the current 20th edition and 2006 Canadian Electrical Code (CEC) does not still have any specific regulation on cable bus systems, however, the required conditions to attain a free air rating of conductors and circuit loadings which are design parameters for an engineered cable bus system are specified in Section 4-004 (ampacities of wires and cables), Section 12-2210 (ampacities of conductors in cable trays) and Section 8 -104 (maximum circuit loading).
A cable bus system can economically transmit large quantities of power between a power source and primary load centers such as from a generator to transformer, transformer to switchgear or from switchgear to switchgear. Cable bus systems can also be used as feeders for large motors, machineries, primary and secondary feeders in industrial plant electrical installations.
Some commercial and institutional facilities such as large shopping malls, convention centers, office buildings, sports complex and hospitals use cable bus for its service entrance and power distribution feeders.
Cable Bus – An engineered system
Cable bus is a completely engineered system and is a congregation of conductors insulated in a metal construction. A typical cable bus system consists of fully insulated single conductor power cables that are mounted on support blocks within a ventilated enclosure and has the required accessories for a complete power feeder system. It has the widest available range of ampere rating and voltage class in addition to having a high fault capacity. Electrical conductors carry currents of between 400 to 6000 amperes per phase at typical voltage levels from 600 volts up to 35KV and higher.
Cable bus manufacturers have the capability to customize a design to suit the specific requirements of an electrical installation. They have knowledgeable and experienced technical staff to design a cable bus system from scratch or work with existing prints for industries like medical, marine, electronics, web, construction, petrochemical , high vacuum, power plants, high voltage, etc. In some instances, customers only provide the details of the equipment termination and power route and manufacturers will do the detailed engineering design of the most economical and suitable cable bus electrical system for the installation.
Some customer-oriented manufacturers that value no less than 100% client satisfaction provide their customers with field engineering services in addition to their design expertise. Clients are provided with field engineering services prior to and during the equipment installation up to the time it is tested and commissioned.
A complete cable bus system includes all the necessary fittings, entrance fittings, tap boxes, enclosure connectors, insulated conductors, electrical connectors, terminating kits and other accessories that will be required for a specific installation. All parts of the cable bus enclosure are designed to ensure that there are no projections or sharp edges that could damage the conductor insulations and cause injuries to personnel during its handling/transporting and installation.
The top and bottom of the cable bus enclosure sections are designed to provide mechanical strength and slots are provided for the ventilation of conductors. The bottom section is factory installed by welding. The top cover will be fastened with stainless steel hardware.
The Canadian Electrical Code and the National Electrical Code requirement on grounding is adequately meet in the design of the cable bus system by the use of high pressure splice joints between bus sections. These splice joints eliminate the need to use bonding jumpers across bus sections. Insulation testing is conducted in a cable bus system prior to energizing. Megger tests are done for 600 volts systems and for higher voltage systems insulation testing is usually done using the DC high potential tests.
The single conductor cables in a cable bus system are installed in a fully ventilated enclosure to keep the conductors within temperature limits. This engineered design allows for an excellent cooling of the conductors and a free air rating of the power conductors is achieved. In addition to eliminating hot spots in the conductors, the ventilated enclosure also protects the conductors from physical damage and guards the entry of foreign objects.
The fully insulated conductors are installed continuously in the field from the power source to the load thereby eliminating intermediate splices that could be potential weak points in the power system. These conductors are pulled in after the cable bus enclosure is installed. The only conductor connections are at the equipment terminations. Since connections are the cause of most electrical failures, minimizing the conductor connections substantially increases the system reliability.
All current carrying conductors have an insulation rating of 90 degrees centigrade for the specified ampacity and voltage. The conductor spacing and ventilation is maintained in the cable bus enclosure. These conductors are phased and supported with the support blocks to maintain low impedance and ensure the mechanical strength necessary to prevent the damage or movement of the conductors under short circuit conditions of up to 100,000 RMS symmetrical amperes. The phasing arrangements of the conductors in the cable bus system result in low power loss and therefore lower operating cost.
Cable bus systems having low impedance reduce the power consumption and minimize the voltage drop of the system. Conductors used may be made of copper or aluminum. The conductor insulation is usually cross-linked polyethylene (XLP) and for higher voltage the recommended insulation is ethylene-Propylene rubber (EPR).
Cable bus manufacturers usually provide the standard cable termination hardware such as compression type cable connectors, terminal lugs and heat shrink or cold shrink termination kits for high voltage systems (5KV, 15 KV and higher).
CABLE BUS Support Blocks
The support blocks of a cable bus system are used to maintain conductor spacing and phasing arrangement. These are installed every 18” in vertical runs and a maximum of 36" in horizontal applications to ensure proper arrangement and bracing in the event of a short circuit. These support blocks are designed in segments to maintain a minimum of one conductor diameter in both the vertical and horizontal planes to attain the free air conductor rating. The bottom support blocks are factory pre-assembled in the bottom section of the housing assembly and the next layers of support blocks are then fastened using stainless steel bolts.
Ventilated Cable Bus Enclosure
The ventilated aluminum enclosure of the cable bus system is likewise designed to withstand the mechanical forces encountered from short circuit currents. Manufactured straight sections are factory cut in lengths of ten and twenty feet long. Just like a cable tray system, the cable bus system has fittings for use in directional changes such as horizontal elbows, vertical inside elbows and vertical outside elbows with bends for 90, 60, 45 and 30 degrees. Top covers are removable while bottom covers are factory welded into place.
The cable bus enclosure is grounded at sufficient points for the purpose of preventing a potential above ground on the enclosure in the event of a fault. There are cable bus system enclosures that are made of aluminum alloy and that use high pressure splice joints which provide excellent ground continuity. This particular cable bus design eliminates the use of additional ground wire for most systems.
Cable bus enclosure made of stainless steel is also an option. However, due to the superior material properties of aluminum, it is highly recommended and suitable for cable bus systems intended for installation in extreme low temperature conditions.
The cable bus system has been proven to be the most cost effective power feeder. It can generate the following cost savings compared to the rigid bus bar duct system:
1. Lower Installation Cost
Bus bar duct systems have pre-manufactured sections that need to be bolted in place and are usually heavy that require the use of heavy lift equipment that could add to the project cost. Cable bus systems are pre-cut to fit specific location. Sections are relatively light and its bottom cable support blocks are factory pre-installed. Sections can easily be handled by two men without the aid of heavy equipment such as booms, lifts or cranes.
2. Reduced Cost of Enclosure
Outdoor type bus bar duct systems which are totally enclosed require larger bus bars due to derating because the exterior portion has insufficient cooling and anti-condensation devices like space heaters must be installed. These space heaters and larger enclosure add up to the project cost. Cable bus systems with its ventilated top and bottom covers can be installed both indoors and outdoors which allow the use of the free air rating of the single conductor cables.
Additionally, the cable bus systems do not have to be precisely aligned with the bus bars in the switchgears or transformers which is necessary in the bus bar systems. Cable bus systems also do not require heater strips to keep moisture out of the system because it is designed to be ventilated for both outdoor and indoor conditions.
Cable Bus Accessories and Fittings
The cable bus manufacturers have the following standard accessories for their cable bus systems:
- BOX CONNECTORS
Box connectors are used to terminate cable bus at switchgear or other metal structures and are furnished with all the necessary hardware.
- WATERTIGHT SEALS
These fittings are used to provide a weatherproof seal with walls and can also be used for floor penetrations that need to be watertight. Entrance fittings are furnished with all the necessary hardware.
- TAP BOXES
Tap boxes are necessary for intermediate load tapping. These are required to make taps from main bus runs to bus ties and intermediate loads.
- CABLE BUS SUPPORTS
The cable bus systems are supported according to NEMA standards and supports are installed typically 12 feet on center. Manufacturers supply a variety of structural and trapeze supports, including T-type, frame support, pedestal support and wall support.
- HORIZONTAL ELBOW
These are used to make changes in direction of from 15 to 90 degrees.
- VERTICAL ELBOW
These are used to make changes in the vertical direction ranging from 15 to 90 degrees.
The following additional engineering features make the cable bus the most technically advanced electrical power feeder system:
- Low Installation Cost
- Uses the standard cable tray installation and support methods
- Highest System Reliability
- Compliance with NEC Article 365/370
- Utilizes fully rated insulated copper or aluminum conductors and typically supplied with an insulation temperature rating of 90 degrees centigrade
- Conductors are installed at the site and no splicing required
- Conductor spacing is maintained allowing the conductors to carry higher ampacities compared to the random lay of conductor or the conduit and wire system
- No conductor transposition required and insulated conductors are FT 4 rated with CSA and UL Certification.
- All welded construction
- Light weight and durable 6063-T5 aluminum construction for ease in installation and suitability for extreme low temperature conditions.
- Stainless steel tie-bolts for conductor support blocks
- Flexible and adaptable making unforeseen dimensional changes during the site installation easy to resolve as straight cable bus sections can be cut-to-fit onsite
- Enclosures are made of lightweight aluminum alloy that allows simplified installation with no special lifting equipment required and can easily be handled by two men
- No special tools or expertise are required during the installation