Calculate busbar current, width, thickness, or allowable current density from any 3 inputs with unit conversions and estimated results.
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Busbar Current Formula
The busbar current calculator uses the cross-sectional area of a rectangular busbar and an allowable current density to estimate current capacity.
I = W*T*J
- I = busbar current, in amps
- W = busbar width
- T = busbar thickness
- J = allowable current density, such as amps/mm²
The calculator can also rearrange the same formula to solve for a missing value:
W = I/(T*J)
T = I/(W*J)
J = I/(W*T)
- Calculate busbar current: enter width, thickness, and allowable current density. The calculator multiplies them to estimate current.
- Calculate width: enter current, thickness, and allowable current density. The calculator finds the required width.
- Calculate thickness: enter current, width, and allowable current density. The calculator finds the required thickness.
- Calculate allowable current density: enter current, width, and thickness. The calculator finds the current density required for that busbar size.
Internally, width and thickness are converted to millimeters, current density is converted to amps/mm², and current is converted to amps before the calculation is made.
Typical Current Density Reference Values
Current density values vary by busbar material, cooling, enclosure temperature, allowable temperature rise, and installation method. The values below are general reference ranges only.
| Busbar material / condition | Typical current density range | Notes |
|---|---|---|
| Copper busbar, conservative estimate | 1.0 to 1.6 A/mm² | Often used where temperature rise must be limited. |
| Copper busbar, typical open-air estimate | 1.6 to 2.5 A/mm² | Depends strongly on ventilation and spacing. |
| Aluminum busbar, conservative estimate | 0.7 to 1.2 A/mm² | Lower conductivity than copper, so larger area is usually needed. |
| Aluminum busbar, typical open-air estimate | 1.0 to 1.8 A/mm² | Final sizing should be checked against applicable standards. |
Common Unit Conversions for Busbar Sizing
| Quantity | Conversion |
|---|---|
| 1 cm | 10 mm |
| 1 inch | 25.4 mm |
| 1 ft | 304.8 mm |
| 1 kA | 1000 amps |
| 1 A/mm² | 100 A/cm² |
Example Problems
Example 1: Calculate busbar current
You have a copper busbar that is 50 mm wide and 10 mm thick. The allowable current density is 1.6 A/mm².
I = W*T*J
I = 50*10*1.6 = 800 amps
The estimated busbar current is 800 amps.
Example 2: Calculate required width
You need to carry 1200 amps using a 10 mm thick busbar. The allowable current density is 1.5 A/mm².
W = I/(T*J)
W = 1200/(10*1.5) = 80 mm
The required busbar width is 80 mm.
FAQ
Is this busbar current result the final safe ampacity?
No. This result is an estimate based on cross-sectional area and allowable current density. Final busbar ampacity also depends on material, ambient temperature, enclosure type, ventilation, spacing, surface finish, temperature rise limit, and applicable electrical standards.
What current density should you enter?
Use a current density value that matches your design rules, material, and cooling conditions. For rough estimates, copper busbars are often checked around 1.0 to 2.5 A/mm², while aluminum busbars are often lower. Use a more conservative value when the busbar is enclosed, poorly ventilated, or exposed to high ambient temperature.
Why does increasing width or thickness increase current capacity?
Increasing width or thickness increases the busbar cross-sectional area. Since the formula is current = area × current density, a larger area can carry more current at the same current density.
