What size wire should be used for the small busbar at the top of the cabinet

The wire size for a small busbar depends on the busbar's current rating, material, and installation conditions, typically calculated using the busbar's cross-sectional area and current density.

Determining Wire Size

To select the correct wire size for a busbar:

1. Identify the Busbar Current Rating: Determine the maximum continuous current the busbar will carry. For example, a small cabinet busbar may carry anywhere from 100 A to 600 A depending on the application .
2. Select Material and Current Density: Copper and aluminum have different current-carrying capacities. For copper busbars, IEC 61439-1 recommends a current density of 1.2–1.6 A/mm² for air-cooled, enclosed busbars. Aluminum busbars are rated lower, around 0.8–1.2 A/mm² .
3. Calculate Cross-Sectional Area: Use the formula: $$A=\frac{I}{J}$$ where $A$ is the cross-sectional area in mm², $I$ is the rated current in amperes, and $J$ is the current density in A/mm². For example, a 200 A copper busbar at 1.5 A/mm² requires: $$A=\frac{200}{1.5}\approx 133\text{ mm²}(]\; [^2^].)$$
4. Consider Short-Circuit Withstand: Verify the busbar can handle short-circuit currents using the adiabatic formula: $$A\ge \frac{I_k\times \sqrt{t}}{k}$$ where $I_k$ is the short-circuit current, $t$ is the fault duration, and $k$ is a material constant (143 for copper). This ensures the busbar and connected wires do not overheat during faults .
5. Wire Selection: The wire connecting to the busbar should match or exceed the busbar's cross-sectional area to prevent overheating. For a 133 mm² busbar, a 2 AWG copper wire (approx. 133 mm²) would be suitable. Always round up to the next standard wire size for safety and code compliance.

Additional Considerations

• Temperature Rise: Ensure the wire and busbar do not exceed the allowable temperature rise (typically 65–70°C above ambient for bare copper in IEC standards), .
• Derating Factors: Account for ambient temperature, enclosure type, and multiple conductors in proximity, which reduce effective ampacity .
• Mechanical Support: Ensure proper busbar supports and spacing to withstand electromagnetic forces during short circuits .
• Compliance: Follow IEC 61439 or NEC ampacity tables for final verification . By following these steps, you can select a wire size that safely matches the small busbar at the top of your cabinet, ensuring both thermal and mechanical safety.