Busbar sizing looks like a lookup: find the current, read the cross-section, order the bar. In practice the catalogue figure is a starting point that several site conditions then reduce.

The base figures

As a working rule, EC-grade copper carries about 1.6 A per square millimetre and aluminium about 0.8 A per square millimetre in typical panel conditions. Aluminium therefore needs roughly twice the cross-section for the same current, which changes the size of the busbar chamber and of the whole panel.

What reduces the figure

Those numbers assume a reference ambient. Indian installations frequently do not match it.

  • Ambient temperature. A panel rated at 45 degrees Celsius ambient in a boiler house or rooftop plant room is working harder than the same panel in a conditioned substation.
  • Enclosure ventilation. Higher IP ratings restrict airflow. An IP55 enclosure traps more heat than an IP42 one, and the busbar de-rates accordingly.
  • Bar arrangement. Bars mounted close together heat each other. Spacing and orientation change the effective capacity.

Why this shows up as a field problem

An undersized busbar does not fail immediately. It runs hot, which accelerates oxidation at the joints. Oxidised joints have higher resistance, which produces more heat, which oxidises faster. The failure arrives years later at a bolted joint that has been quietly degrading since commissioning.

The fix is unglamorous: size the bar for the actual ambient and enclosure, torque the joints to specification, and check them during scheduled maintenance. Thermographic scanning finds developing joint problems long before they become outages.