Busbar Selection on the Low-Voltage Side

Low-voltage busbars should be selected based on current rating, short-circuit capacity, material, temperature rise, and IEC 61439 compliance.

Key Considerations for Low-Voltage Busbar Selection

1. Current Rating (Ir): The busbar must carry the continuous load current without exceeding the permissible temperature rise. The IEC 61439 standard provides guidelines for determining the rated current, taking into account the diversity factor of connected loads, which reduces the total calculated current for multiple circuits . 2. Short-Circuit Withstand (Isc): Busbars must withstand prospective short-circuit currents for a specified duration (typically 1–3 seconds) without mechanical or thermal failure. This ensures safety during fault conditions and protects connected equipment . 3. Material Selection: Copper and aluminum are the most common busbar materials. Copper offers higher conductivity and smaller cross-sectional area but is more expensive, while aluminum is lighter and cheaper but requires a larger cross-section to carry the same current . 4. Temperature Rise and Ambient Conditions: The permissible temperature rise is usually 70°C for copper and 55°C for aluminum above ambient temperature. Ambient temperature, enclosure type, and mounting orientation affect heat dissipation and must be considered in sizing . 5. Cross-Sectional Area Calculation: A practical formula for copper busbars is 1 mm² = 2 A. For example, a 4000 A busbar requires 2000 mm² of cross-sectional area. Width and height can be combined to achieve the required area, and multiple busbars can be paralleled if needed . 6. Compliance with IEC 61439: IEC 61439-1 and -2 define design verification, thermal performance, dielectric strength, and short-circuit withstand for low-voltage assemblies. Busbars must be validated as part of the complete assembly, not as standalone conductors . 7. Installation and Environmental Factors: Consider open-air vs. enclosed installation, airflow, and potential derating factors. Proper spacing, insulation, and mechanical support are essential for safe operation .

Practical Steps for Selection

1. Determine the continuous load current and apply the diversity factor.
2. Calculate the required cross-sectional area based on material and current rating.
3. Verify short-circuit withstand using prospective fault current data.
4. Check temperature rise limits for the chosen material and installation conditions.
5. Ensure IEC 61439 compliance for the complete assembly.
6. Adjust busbar dimensions or parallel multiple bars if necessary to meet current and mechanical requirements. By following these steps, engineers can ensure that low-voltage busbars are safe, efficient, and compliant with international standards, minimizing overheating, voltage drops, and equipment damage while optimizing cost and space.