6.1 Introduction

A conductor carrying a current will experience a resistive loss that gets converted to heat and raises the temperature of the conductor. A conductor, whose temperature is higher than that of its ambient, will dissipate heat and attain a steady-state temperature when

Heat Generated = Heat Dissipated

• The governing criteria for assigning a current rating to a Busbar in a Busbar System is the maximum current at which it attains a maximum temperature imposed by the specification. The current rating can be under a steady-state or specified short time. The maximum temperature limit for the conductor is set based on
• The ability of the conductor and joints to retain the physical and chemical properties during their working life. The ability of the insulation system, in contact with the conductor, to withstand the temperature continuously without any degradation to its properties or life.
•  Meeting the health & safety norms.

The heat generated by the conductor carrying a current I and having a resistance R, at its operating temperature, is I²R Heat is dissipated from the conductor by

The ampacity of a conductor is normally assigned for a bare conductor/conductors in a defined configuration, at a specified ambient temperature and temperature rise, located indoors in still air. The assigned value is for DC or AC at 50 Hz or 60 Hz.
Very rarely do Busbar Systems operate under these ideal conditions. They are subjected to different manufacturing processes and service conditions such as:

• Varying ambient temperatures and atmospheric conditions
• Painted, coated, or sleeved
• Installed open or in a metal enclosure with air or gas as primary insulation.
• Cast or impregnated with resin
•  Installed indoors or outdoors
• Subjected to natural or forced air cooling