1. Cross-sectional area of ​​copper busbar: This is the most direct factor. The larger the cross-sectional area, the higher the current carrying capacity. It is usually expressed as “width x thickness”, such as 30mm x 4mm.
2. Allowable temperature rise: Busbars generate heat when carrying current. The set maximum allowable temperature (usually the temperature rise relative to ambient temperature, such as 60°C or 70°C) directly determines the current carrying capacity. The stricter the temperature rise requirement, the lower the allowable current carrying capacity.
3. Busbar arrangement:

· Horizontal: Large heat dissipation area, higher current carrying capacity.

· Vertical: Better heat dissipation effect, because hot air naturally rises to form a chimney effect, the current carrying capacity is about 5%-10% higher than horizontal.

· Multiple in parallel: When using multiple busbars to transmit large currents in parallel, due to mutual thermal influence, the total current carrying capacity is not a simple summation, but needs to be multiplied by a parallel factor (less than 1).

4. Ambient temperature: The standard reference ambient temperature is usually 35°C. If the actual ambient temperature is higher (e.g., 45°C), the current carrying capacity must be reduced; conversely, it can be appropriately increased.
5. Surface Treatment:

• Bare Copper: Good heat dissipation.
• Tin Plating: Stronger oxidation resistance, but slightly worse heat dissipation than bare copper; under the same conditions, the current carrying capacity is approximately 95%-98% of that of bare copper.
• Silver Plating: Best conductivity and high-temperature resistance; commonly used in high-performance applications.

General Reference Data Table

The following is a table estimating the current carrying capacity of common rectangular copper busbars under the conditions of an ambient temperature of 35°C, busbar laid flat, and an allowable temperature rise of 70°C. This is a commonly used empirical reference value in engineering.

Copper busbar specifications (width x thickness) mm, cross-sectional area (mm²), approximate current carrying capacity (A):

15 x 3 45 156

20 x 3 60 204

25 x 3 75 252

30 x 4 120 380

40 x 4 160 494

40 x 5 200 556

50 x 5 250 680

50 x 6 300 740

60 x 6 360 855

80 x 6 480 1070

100 x 6 600 1275

60 x 8 480 1025

80 x 8 640 1300

100 x 8 800 1555

120 x 8 960 1810

Important Note: This table provides reference values. Actual design should refer to national standards (such as GB/T 7251, IEC 60439) or the specific product’s technical manual.

How to Calculate and Select?

In practical engineering, besides consulting tables, empirical formulas can be used for estimation.

Empirical Formula (Applicable to Copper Busbars):

When laid flat: I ≈ (Thickness mm) × (Width mm) × Current Carrying Factor

This current carrying factor will vary depending on the temperature rise requirement:

· When the temperature rise is 60°C, the current carrying factor is approximately 10-11 A/mm²

· When the temperature rise is 70°C, the current carrying factor is approximately 12-13 A/mm²

Example: A 30mm x 4mm copper busbar requires a temperature rise of 70°C.

Current carrying capacity I ≈ 4mm × 30mm × 12 A/mm² = 1440 A? Wait, this result clearly doesn’t match the table above.

Note: The “cross-sectional area” in this formula needs correction; a more accurate term is “current density.” For rectangular busbars, the current density is typically 1.5 ~ 2.5 A/mm² (depending on factors such as temperature rise and layout).

A more reliable calculation method is:

1. Initially select a specification based on the table.
2. Perform temperature rise verification: Based on the busbar’s heating and cooling power, calculate whether it can stabilize within the allowable temperature rise under rated current. This involves complex heat transfer calculations, usually done using specialized software or manuals.
3. Consider derating: If the ambient temperature is above 35°C, or if multiple sections are used in parallel, adjustments need to be made according to the derating factor.

Summary and Recommendations

1. Primary principle: There is no single answer. Your current requirements, allowable temperature rise, installation method, and environmental conditions must be clearly defined.
2. Quick Estimation: Use the reference data table provided above, combined with the concept of “current density 1.5 ~ 2.5 A/mm²”, for quick estimation. For general power distribution (temperature rise around 70°C), a conservative estimate of 1.8 ~ 2.0 A/mm² can be taken.

· For example: To carry 1000A of current, the required cross-sectional area ≈ 1000A / 2.0 A/mm² = 500 mm². A 60×8 (480mm²) or 100×5 (500mm²) busbar can be selected.

3. Rigorous Design: For critical applications, high current, or special environments, it is strongly recommended to consult national standards (such as GB/T 7251.1) or consult professional electrical engineers or busbar manufacturers. They will provide the most accurate technical data and selection support.
4. Safety Margin: In actual selection, a certain safety margin should usually be left (for example, selecting a specification with a current carrying capacity 20% higher than the calculated value) to ensure long-term stable and reliable operation of the system.

Hopefully, this detailed explanation will help you understand the complexity of copper busbar current carrying capacity and make an initial judgment.

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