Busbar Current Rating

Busbars — copper or aluminium bars — distribute current within switchboards and distribution panels. Their current rating is determined by the balance between I²R heat generation and heat dissipation through convection, radiation, and conduction. Temperature rise is the critical parameter: IEC 61439-1 limits the temperature rise of bare copper busbars to 105K above ambient, giving a maximum temperature of 140°C at 35°C ambient. Factors affecting busbar rating include material conductivity (copper carries about 60% more current than aluminium for the same cross-section), bar dimensions (wider bars dissipate heat more effectively), surface treatment (painted bars radiate heat better than polished ones, paradoxically improving ratings by 15–20%), spacing between bars (closer spacing reduces convective cooling), and enclosure ventilation. For multiple bars per phase, spacing between bars is critical — bars touching each other share heat and carry less than double a single bar's rating. Design verification can be by testing (measuring temperature rise under rated current), calculation (detailed thermal analysis), or derivation from tested designs using established rules. For high-current applications, forced-air or liquid cooling can significantly increase busbar capacity within the same enclosure dimensions.