bus duct 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.
A plug-in bus duct system or busway can have disconnect switches and other devices mounted on it, for example, to distribute power along a long building. Many forms of busway allow plug-in devices such as switches and motor starters to be easily moved; this provides flexibility for changes on an assembly line,for example. 
Feeder busway is used to interconnect equipment, such as between a transformer and a switchgear line up. A variant type is low-impedance busway, which is designed to have lower voltage drop by virtue of close spacing of bus bars, which reduces inductive reactance.
A trolley busway provides power to equipment that must be frequently moved. The busway is open at the bottom, and a movable collector assembly "trolley" is used to connect between the fixed bus bars in the busway and the cable connected to moving equipment. 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.