COPPER & BUSBARS / AMPACITY
Copper Busbar Ampacity Calculator
Estimate current-carrying capacity from bar size, temperature rise and mounting orientation. Check current density, voltage drop and power loss.
Calculation results
EXAMPLE RESULTEstimated ampacity · selected cooling model
94.5A- Current density
- 3.94 A/mm²
- Operating temperature
- 105.0 °C
- Cross-section
- 24.00 mm²
- DC resistance
- 95.83 μΩ
- Drop at estimated ampacity
- 9.05 mV
- Loss at estimated ampacity
- 0.855 W
Temperature rise & ampacity comparison
Kt = 4.70 W/(m²·K)
Dimensions, ambient temperature and Kt held constant. Check assembly temperature limits separately.
Uniform-temperature, single-bar DC model. Joints, radiation, forced airflow and AC effects are not resolved. This estimate is not a product current rating.
Worked example: a 12 × 2 mm copper busbar
Use copper, flat mounting, a 12 mm width, 2 mm thickness, 100 mm length, 40 °C ambient and a 65 °C allowed rise. The assumed operating temperature is 105 °C and Kt is 4.7 W/(m²·K).
With the copper preset ρ20 = 1.724 × 10−8 Ω·m and α = 0.00393 /°C, the model gives 94.5 A, 3.94 A/mm², 95.83 μΩ, 9.05 mV and 0.855 W. The voltage drop and power loss are evaluated at the estimated ampacity.
| Width × thickness | Area | Estimated current | Current density |
|---|---|---|---|
| 12 × 2 mm | 24 mm² | 94.5 A | 3.94 A/mm² |
| 20 × 3 mm | 60 mm² | 191.5 A | 3.19 A/mm² |
| 30 × 5 mm | 150 mm² | 373.5 A | 2.49 A/mm² |
These are calculated examples, not tested product ratings or standard ampacity values. A 105 °C operating temperature may exceed the limits of connected parts.
How the busbar current rating is estimated
The model equates electrical heating I²R with cooling Kt × surface area × temperature rise. For a uniform rectangular bar, the length cancels when end effects are neglected:
I = √[Kt × 2(W + H) × W × H × ΔT / ρ(T)]
ρ(T) = ρ20 × [1 + α(T − 20)]
T = ambient temperature + ΔT
W and H are width and thickness in metres, ΔT is the temperature rise in K (numerically equal to a rise in °C), and ρ(T) is resistivity in Ω·m. Inputs in mm are converted internally. The presets use Kt = 4.7 for flat mounting and 6.9 for mounting on edge, in W/(m²·K).
Kt is an assumed coefficient, not a measured property of your assembly. The calculation uses DC resistance and uniform temperature. It does not separately model radiation, forced airflow, enclosure geometry, nearby conductors, supports, joints, skin effect or proximity effect. Validate cooling and temperature limits for the actual installation.
| Material | ρ at 20 °C (Ω·m) | α (1/°C) |
|---|---|---|
| Copper | 1.724 × 10−8 | 0.00393 |
| H62 brass, approximate | 6.2 × 10−8 | 0.002 |
| Aluminum, approximate | 2.82 × 10−8 | 0.00429 |
Use alloy-specific supplier data for detailed design. These are this tool's nominal presets; other tools or tables may use different material and cooling assumptions.
Busbar sizing and current-carrying capacity questions
How do I choose a copper busbar size for a given current?
Start with the load current, maximum ambient temperature and the temperature limit of the assembly. Enter a candidate width and thickness, then compare the model estimate with the load. Increase the section or improve verified cooling if needed, and check voltage drop, joints and short-circuit requirements separately. Use the copper busbar ampacity table as a comparison only after matching its assumptions.
Why not use a fixed amps-per-square-millimetre rule?
Current density is the estimated current divided by cross-section. Cooling depends on exposed surface as well as section area, so a single A/mm² value does not describe every size, orientation or enclosure. The example table shows how current density changes with bar dimensions in this model.
Why does changing length affect voltage drop but not ampacity?
In this uniform model, both electrical heating and exposed cooling surface increase in proportion to length. Their length terms cancel in the ampacity equation. Resistance, voltage drop and total loss still increase with length. End connections and nonuniform cooling can change that behavior in a real assembly.
Can I use this for AC busbars or parallel bars?
This is a single-bar estimate using DC resistance. AC skin and proximity effects, and the mutual heating and current sharing of parallel bars, require additional analysis. The skin depth calculator and current density and temperature rise guide provide related background.
How can I check voltage drop at my actual load current?
The outputs here use the estimated ampacity. To evaluate a known current, open the copper busbar resistance and power loss calculator and enter the load and expected conductor temperature. Check the material presets when comparing results between tools.
Further reading: Copper for Busbars — Guidance for Design and Installation, section 2, discusses thermal design and AC effects. The fixed Kt presets here are not ratings taken from that guide.