3-Wire Vs. 4-Wire Bus Duct Plugs

3-Wire Busways

• Three wire buses have two phase bars and a neutral line. According to "Mechanical and Electrical Equipment for Buildings," "light duty busways are rated from 20 to 60 A at 300 V. in 2- and 3-wire construction." Three-wire busways provide flexible and mobile high power connections without the need to constantly splice and move power cables to accommodate industrial equipment moves. Three-wire busways are used for connecting motors and single-phase loads.

4-Wire Busways

• Four-wire bus ducts provide direct electrical connection for high-powered industrial equipment. According to "Mechanical and Electrical Equipment for Buildings," "a somewhat heavier design rated 60 A to 100 A at 600 V is available in 3- and 4-wire construction." Circuit breakers are located on the busways to prevent overloads. Four-wire busways are used with lighting panels and hand power tools. Four-wire feeder bus ducts commonly have plug-in points on alternative sides to provide power to assembly line work station tools.

Bus Duct Plugs

• Bus bars are coated with silver at bus plug-in points, called bus duct plugs. Bus duct plug fingers, the equivalent of power plug prongs, create an electrical connection on contact with the silver-coated areas. Plug-in devices are the same, regardless of the number of bus duct wires. According to "2005 National Electrical Estimator," bus duct might be 3-phase, 3-wire service or 3-phase, 4-wire service. The device should be the same. The connection is broken when the plug-in device is unplugged from the bus duct, similar to unplugging a cord from a conventional power plug.

Regulations

• The National Electric Code defines electrical design regulations for safety and industry-wide standards. The NEC requires power plugs with cords connected to the feeder bus duct if it is less than 6 feet long. The power cord must also have a tension take-up device to prevent slack in the power cord. "Audel Guide to the 2011 National Electrical Code" gives the only exception to this rule as "the cord is supported every 8 feet and the entire installation is under the supervision of qualified people." The regulations are the same, regardless of whether the bus way plug connects to a 3-wire or 4-wire bus duct.