Bus duct

5000 ampere copper and 4000A aluminium bus ducts

Bus duct inside an electrical service building at a Canadian oil refinery.

Bus duct inside an electrical closet in Mississauga Civic Centre, firestopped along with electrical conduit using firestop mortar.

1992 Bus duct segment with internal factory firestop that was replaced prior to fire testing with firestop mortar. To supply such systems, the bus duct manufacturer had to know where the duct penetrated fire separations required to have a fire-resistance rating.

In electrical power distribution, a bus duct (also called busway), is a sheet metal duct containing either copper or aluminium busbars for the purpose of conducting a substantial current of electricity. It is an alternative means of conducting electricity to power cables or cable bus.

Construction

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  Section of copper bus duct with sandwiched bus bars
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  Section of aluminium bus duct with sandwiched bus bars
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  Innards of bus duct showing plastic wrappings of sandwiched busbars and hollow plastic tube spacers.
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  Individual busbar wrapping

Similar to cable tray, bus ducts have thicker, cold-formed steel side rails and thinner sheet metal coverings. Busbars inside may be separated with distinct and even gaps between them, or “sandwiched” together.

Typically, individual busbars are wrapped or coated with a non-conducting, covalent material, such as plastic or (in older systems) electrical tape.

At the connection point, busbars flare out to enable connection to the next segment.

Use and application

Bus ducts are used in commercial and industrial settings, both indoors and outdoors. Manufacturers deliver them assembled in large segments for electrical contractors to connect and support.

Through-penetration firestopping

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  Firestop test sample including 2 bus ducts
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  ASTM E814 test sample after external firestop mortar installation
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  ASTM E814 test sample after external firestop insulation installation
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  Bus duct firestop test, furnace view
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  End of bus duct on unexposed side of firestop test sample, as smoke generation begins
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  Smoke generation at end of fire test
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  Bus duct firestop test sample comes out of the furnace
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  Bus duct firestop test sample during hose stream test
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  Bus duct firestop test sample unexposed side after fire and hose stream test
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  Copper bus duct innards after fire testing, with plastic components burned off
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  Aluminium bus duct innards during post-mortem investigation following ASTM E814 fire testing

Bus ducts are building service penetrants that are required to be externally firestopped where they penetrate fire separations required to have a fire-resistance rating. Any internal firestops that are supplied by the manufacturer may be tested as integral components to either UL857 or IEEE C37.23 for North American models, and then via ASTM E814, UL1479 or ULC-S115. Bus duct-internal firestops mitigate the transmission of internal fires, smoke and heat between fire compartments through the combustion of bus duct-internal covalent wrappings, spacers and brackets.

See also

• Bus (computing)
• Busbar
• Firestop
• Fire-resistance rating
• Duct (flow)

References

• Merriam Webster Definition of Bus Duct
• YouTube Bus Duct Installation
• YouTube Bus Duct Construction and Installation
• Underwriters Laboratories Standard UL857 abstract
• IEEE C37.23 Standard For Metal Enclosed Bus
• Interpretations for IEEE Std C37.23-1987 IEEE Guide for Metal-Enclosed Bus and Calculating Losses in Isolated-Phase Bus
• IEEE P3004.7 - Recommended Practice for the Protection of Power Cables and Busway Used in Industrial and Commercial Power Systems
• IEEE Busway Short Circuit Testing Procedures
• IEEE Mathematical model of high voltage sandwich bus duct
• UL Certification Listing C-BJ-8028, Firestop Configurations C & D show examples of external bus duct through-penetration firestopping