Since the national standards do not specify the cross section of bus ducts, the same variety of bus ducts labelled with the same rated current value usually have different manufacturer's cross sections. This situation has the following reasons.

(1) The routine specimen cross-section is different from the actual product. The previous set of test reports did not specify the cross section of the busway being tested. Reducing the cross-section of the busway can reduce the cost and also submit the routine test report, so a busway with a large cross-section is used to pretend to be a busway with a small cross-section for the routine test of the busway.

(2) Product cross-section and specifications are inconsistent. For samples that do not show the cross-section of the bus duct, the designer must be aware of the manufacturer's forgery.

The busway connects the conductors of each disconnecting branch circuit in an electrical installation. It is the carrier of the collector distribution and is also known as a converging bus duct. Customarily, the conductors of the current-carrying branch circuits in each distribution unit are referred to as bus ducts. The role of the bus duct is to collect, distribute and transport electrical energy. Busway in operation, there is a huge amount of electrical energy through the short circuit, bear a huge heat and power effect, must be a reasonable choice of busway material, cross-section shape and cross-sectional area, to meet the requirements of safe and economic operation.

In the selection and application of bus ducts under different environmental conditions, should consider the selection of different types and materials of bus ducts. In addition, various bus ducts should usually be selected according to the maximum long-term operating current of the bus duct cross-section. According to the maximum short-circuit current conditions, check the short thermal stability and action stability of bus ducts. Bus ducts heat up when passing current due to factors such as conductors having resistance and the tendency effect of AC currents when multiple conductors are in close proximity. Copper, aluminium bare busway long-term work when the allowable temperature of heat are 70 ℃, but its contact surface has a reliable tin cover (such as ultrasonic enamel, etc.) allowable temperature increased to 85 ℃.

Subject to the limitations of this continuous heating allowable temperature, different materials, cross-section of the bus duct gives the corresponding long-term allowable current value, in the selection of bus duct cross-section, the bus duct of the actual maximum long-term operating current (counting more than 30 minutes over the negative centre of gravity) than the selection of the section of the bus duct of the long-term allowable current value of the small.

In the case of large average annual loads, conductors and bus duct headmasters (e.g., outdoor distribution unit bus ducts), bus ducts are usually selected according to the economic current density method. The economic current density is the current flowing per unit of cross-sectional area of the bus duct, taking into account the losses of the bus duct, and the lowest annual maintenance and depreciation costs of the bus duct and ancillary equipment. Economic current density divided by the long-term operating current without taking into account the overload, the cross-section of the busway. Economic current density method of selecting the cross-section of the busway, generally larger than the maximum long-term operating current supported by the cross-section of the busway.

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