Copper busbars are widely used in electrical distribution systems to carry current and connect various electrical devices. They are especially common in power distribution units due to their high conductivity and durability. Many people have asked about the current-carrying capacity of copper busbars, so today we’ll take a closer look at how to calculate it.
Understanding the current capacity of copper and aluminum busbars is essential for proper electrical design. A simple estimation method can be used:
- **Single copper busbar current capacity** = width (mm) × thickness coefficient
- **Double busbar current capacity** = width (mm) × thickness coefficient × 1.5 (experience factor)
For general use, copper typically carries **5–8 A per square mm**, while aluminum is around **3–5 A per square mm**.
The current carrying capacity of copper busbars at 40°C can be calculated using the following formula:
- **Copper busbar current at 40°C** = width × thickness coefficient
Thickness coefficients vary depending on the busbar thickness:
- 12 mm thick → 20
- 10 mm thick → 18
- 8 mm thick → 16
- 6 mm thick → 14
- 5 mm thick → 13
- 4 mm thick → 12
So the sequence is: [12-20, 10-18, 8-16, 6-14, 5-13, 4-12].
For multi-layer copper busbars at 40°C:
- Double layer = 1.56–1.58 times single layer
- Triple layer = 2 times single layer
- Quadruple layer = 2.45 times single layer (not recommended; consider using a special busbar instead)
Also, note that:
- **Copper busbar at 40°C** = copper busbar at 25°C × 0.85
- **Aluminum busbar at 40°C** = copper busbar at 40°C ÷ 1.3
For example, for a TMY100×10 copper busbar:
- Single layer: 100 × 18 = 1800 A (manual value is 1860 A)
- Double layer: 1860 × 1.58 = 2940 A (manual value is 2942 A)
- Triple layer: 1860 × 2 = 3720 A (manual value is 3780 A)
All calculations are accurate and close to manual values.
Another quick formula for rectangular copper busbars is:
- **Single rectangular copper busbar current** = width × (thickness + 8.5) A
Example: 15×3 at 40°C = 15 × 11.5 = 172.5 A
Example: 100×8 at 40°C = 100 × 16.5 = 1650 A
For multiple layers:
- Double layer = 1.5 × single layer
- Triple layer = 2.0 × single layer
Now, for aluminum busbars, there's a traditional saying: "Thick three rows wide by ten, last four rows wide by twelve." This means that for a given width, the current capacity increases as the thickness increases.
Here’s a table for aluminum busbars:
| Thickness (mm) | Current Capacity (A) |
|----------------|----------------------|
| 3 | Width × 10 |
| 4 | Width × 12 |
| 5 | Width × 13 |
| 6 | Width × 14 |
| 8 | Width × 16 |
| 10 | Width × 18 |
Note: 7 mm and 9 mm thicknesses are not commonly available.
Example: For a 40×4 aluminum busbar, current capacity = 40 × 12 = 480 A
Example: For a 60×6 aluminum busbar, current capacity = 60 × 14 = 840 A
There's also a saying: "Copper busbar is multiplied by a little three," meaning that copper has about 30% higher current capacity than aluminum of the same size. So when calculating copper busbar capacity, first estimate as if it were aluminum, then multiply by 1.3.
Example: For a 50×13 copper busbar, current capacity = 50 × 13 × 1.3 = 845 A
Other factors like ambient temperature, number of busbars, and installation orientation (horizontal or vertical) can also affect the current capacity. If the ambient temperature is above 25°C or multiple busbars are placed side by side, you should reduce the capacity by about 10%. When using 2, 3, or 4 busbars in parallel, they can be rated at 8x, 7x, or 6x respectively.
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