Bus bars in electrical power distribution refer to thick strips of copper or aluminum that conduct electricity within a switchboard, distribution board, substation, or other electrical apparatus.
The size of bus bars is important in determining the maximum amount of current that can be safely carried. Bus bars can have a cross-sectional area of as little as 10 mm˛ but electrical substations may use metal tubes of 50 mm in diameter (1,000 mm˛) or more as bus bars.
Bus bars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. The skin effect makes AC bus bars more than about 8 mm (1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid rod, which allows a greater span between bus bar supports in outdoor switchyards.
For very heavy currents in electrical apparatus, and for heavy currents distributed through a building, bus bar can be used. Each live conductor of such a system is a rigid piece of copper or aluminum, usually in flat bars (but sometimes as tubing or other shapes). Open bus bars are never used in publicly accessible areas, although they are used in manufacturing plants and power company switch yards to gain the benefit of air cooling. A variation is to use heavy cables, especially where it is desirable to transpose or "roll" phases.