Busbar current carrying capacity calculator
Calculate the maximum continuous current-carrying capacity of copper or aluminum busbars based on size, material, ambient temperature, ventilation, and installation conditions. Supports rectangular and round shapes.
How to Calculate Busbar Current-Carrying Capacity?
The calculation is based on thermal equilibrium principles, where heat generated by current equals heat dissipated to the environment. The core formula used in this calculator follows IEC 60890 and IEEE 738 standards:
I = K × √(A × ΔT) × f₁ × f₂ × f₃
- I = Maximum continuous current (A)
- K = Material factor (Copper ≈ 1.0, Aluminum ≈ 0.6)
- A = Cross-sectional area (mm²)
- ΔT = Allowable temperature rise (°C), typically 30–50°C
- f₁ = Ventilation factor (e.g., open air vs. enclosed)
- f₂ = Position factor (horizontal, vertical, stacked)
- f₃ = Surface factor (unpainted, dark painted)
Step-by-Step Calculation Example
Copper flat bar: 100 mm × 10 mm, ambient 35°C, ΔT = 30°C, open air, horizontal, unpainted
- Area A = 100 × 10 = 1000 mm²
- K = 1.0 (copper)
- √(A × ΔT) = √(1000 × 30) = √30,000 ≈ 173.2
- f₁ = 1.0 (open air), f₂ = 0.95 (horizontal), f₃ = 1.0 (unpainted)
- I = 1.0 × 173.2 × 1.0 × 0.95 × 1.0 ≈ 1645 A
What Is the Rule of Thumb for Busbar Sizing?
While precise calculation is recommended, engineers often use these **rules of thumb** for quick estimation:
| Rule | Application | Accuracy |
|---|---|---|
| 1.25 Rule | Size busbar at 125% of full-load current (common in US NEC) | Low accuracy; for general design only |
| Thermal Rule | Use IEC/IEEE formulas with actual parameters | High accuracy; recommended for final design |
| Material Factor | Aluminum carries ~60% of copper’s current per mm² | Medium accuracy; useful for material comparison |
How Many Amps Can a Bus Bar Handle?
The maximum current depends on multiple factors. Here are typical values for common configurations:
| Busbar Type | Dimensions | Max Current (A) | Notes |
|---|---|---|---|
| Copper Flat Bar | 100×10 mm | ~2800 A | Open air, 30°C rise |
| Aluminum Flat Bar | 100×10 mm | ~1700 A | Same conditions as above |
| Round Copper | Φ50 mm | ~1200 A | Less efficient than flat bar |
Frequently Asked Questions (FAQ)
How to decide busbar size?
Follow these steps:
- Determine required current (from load calculation)
- Select material (copper for high performance, aluminum for cost)
- Choose shape (flat bars are more efficient)
- Apply correction factors for temperature, ventilation, and position
- Use IEC or IEEE formulas to compute ampacity
- Verify against standard sizes (e.g., 100×10, 120×12)
What is the IEC standard for busbar sizing?
The primary standard is IEC 60890, which defines methods for calculating the current-carrying capacity of bare conductors, including busbars. It considers material, geometry, ambient temperature, and cooling conditions.
What is the NEC code for busbars?
In the US, NEC Article 366 covers busways and busbars. It specifies minimum conductor sizes, insulation requirements, and installation rules. However, it does not provide direct ampacity tables—engineers must use IEC/IEEE methods or manufacturer data.
What is the formula for earthing busbar size?
Earthing (grounding) busbars are sized based on fault current and duration. The formula is: S = √(I²t / k) Where: - S = Cross-sectional area (mm²) - I = Fault current (kA) - t = Duration (s) - k = Material constant (e.g., 137 for copper, 84 for aluminum)
What is the 1.25 rule in electrical?
The "1.25 rule" means sizing conductors or equipment at **125% of the full-load current** to account for continuous operation (over 3 hours). This is required by NEC for motors and some loads, but not for busbars unless specified.
Why is it called a busbar?
The term "busbar" comes from the word "bus," meaning a central distribution point. In electrical systems, a busbar acts as a **central power distribution hub**, connecting multiple circuits and distributing electricity like a bus route.
