The maximum continuous current for a small copper busbar typically ranges from 1.5 to 2.5 A/mm² of cross-sectional area, depending on material, mounting, and ambient conditions.

Key Factors Affecting Busbar Current Capacity

1. Material: Copper has higher conductivity than aluminum, so copper busbars carry more current for the same cross-section. Aluminum requires a larger cross-section to achieve the same ampacity .
2. Cross-sectional area: The current-carrying capacity is proportional to the busbar's cross-sectional area. For example, a 10×100 mm (1,000 mm²) copper busbar can carry up to 2,500 A in open-air at 40°C ambient, derated to about 2,125 A in enclosed panels .
3. Mounting orientation and enclosure: Horizontal flat mounting allows better heat dissipation than vertical or enclosed arrangements. Enclosed panels reduce ampacity by 15–30% depending on ventilation .
4. Temperature rise limits: IEC 61439-1 limits bare copper busbars to a maximum temperature rise of 70 K above 35°C ambient. Connection points are more restrictive (typically 55–95°C) to protect insulation and contacts .
5. Current density: Typical design values for copper busbars are 1.0–1.6 A/mm² in low-voltage switchgear, while aluminum busbars are rated 0.7–1.2 A/mm². Higher current density reduces cross-section but increases thermal rise .

Practical Example

• A 100×10 mm copper busbar (1,000 mm²) in horizontal flat mounting at 40°C ambient can carry approximately 1,550 A DC or 1,504 A AC with a 50°C temperature rise. In enclosed switchgear, the continuous current drops to around 1,085 A, and applying a 1.25× safety factor gives a design current of 1,240 A .
• Smaller busbars, such as 50×10 mm (500 mm²), would carry roughly half the current, depending on installation conditions and derating factors.

Summary

The maximum normal current for a small busbar depends on its material, cross-section, mounting, and ambient conditions. For copper, small busbars typically carry 1–2 A/mm², while aluminum carries 0.7–1.2 A/mm². Always verify ampacity using manufacturer tables and IEC 61439-1 limits, and apply derating for enclosure, temperature, and safety margins to ensure reliable operation .