Busbar size is calculated by determining the required cross-sectional area based on the current rating and material, often using a rule-of-thumb like 1 Amp per mm² for copper, and considering short-circuit withstand using thermal stress formulas with material constants. After calculating the area, you find a standard busbar width and thickness that meets this area and the specific project requirements. 
1. Determine the Required Current Rating 

• Identify the maximum continuous current (Amperes) the busbar needs to carry.

2. Calculate the Cross-Sectional Area (A)

• Method 1: Rule of Thumb
  • This is a common starting point for general-purpose busbars. 
  • A widely used rule is 1 Ampere per 1 square millimeter (mm²) for copper busbars, though this can be adjusted based on specific applications and tolerances. 
• Method 2: Thermal Stress (for Short-Circuit)
  • This formula is used when the busbar must withstand a fault current for a specified duration. 
  • Formula: A = √(I² x t) / k 
    • A: Cross-sectional area (mm²) 
    • I: Fault current (A) 
    • t: Fault duration (seconds) 
    • k: Material constant (e.g., Copper: 205, Aluminum: 126) 

3. Select Standard Dimensions (Width and Thickness)

• Once the required cross-sectional area (A) is determined, select a standard busbar size by finding a combination of width and thickness that provides the calculated area. 
• For example, if you need an area of 100 mm², you could choose a 10 mm x 10 mm busbar, or a 20 mm x 5 mm busbar. 

4. Consider Derating Factors and Other Design Considerations

• Derating Factors:
  Apply derating factors for factors like ambient temperature, grouping of busbars, and ventilation to adjust the current carrying capacity. 
• Electromagnetic Forces:
  For high currents, calculate the forces between busbar phases and ensure the supports can withstand these forces. 
• Material & Standards:
  Always refer to relevant industry standards (e.g., IEC 61439, IS 808) and select appropriate materials like copper or aluminum based on cost and conductivity. 
• Panel/Enclosure Design:
  Consider how the busbar will be mounted and how its shape (width/height ratio) affects inductance and capacitance.