A busbar size calculator for copper and aluminium
A busbar must carry its rated current without getting hotter than the bar, its supports and its joints can tolerate. This busbar size calculator gives a first estimate: it takes the current you need, adds a design margin, corrects for a hot ambient, applies a current density for copper or aluminium and picks the smallest standard flat bar with enough cross-section. It also checks the short-circuit area, the bar’s mass and its heat loss.
Current density is the traditional rule of thumb for panel work. Published practical guides quote roughly 1.2 to 1.6 A/mm² for copper and 0.8 to 1.2 A/mm² for aluminium, with lower values for enclosed and grouped bars and higher values in open air. The calculator offers those as presets and lets you enter your own.
This is a screening tool, not a verification. Real temperature rise depends on bar spacing, orientation, enclosure ventilation, joints and plating, and a switchboard must be verified to IEC 61439-1 or your local standard. The manufacturer’s ampacity table for the actual section always takes precedence, and a qualified engineer should sign off the design.
How to use the Busbar Size Calculator
- Enter the rated current and choose copper or aluminium.
- Choose a current density for an enclosed or open arrangement, or type your own in A/mm².
- Set the design margin, the ambient temperature and the maximum bar temperature; leave the reference at 35 °C unless your source states otherwise.
- Optionally enter the prospective short-circuit current, duration and the k constant for your standard.
- For several bars per phase enter their number and the supplier’s group factor.
- Read the required area and the list of nearest standard flat bars.
Busbar Size Calculator features
Copper and aluminium
Different current density, resistivity, temperature coefficient and mass for each.
Ambient derating
Scales the rating with the square root of the available temperature rise.
Short-circuit area
Adiabatic minimum area from fault current, time and k.
Standard sizes
Common flat bars from 12 × 3 mm to 200 × 20 mm with mass, resistance and loss.
The sizing method
The calculator multiplies the rated current by the design margin (125% is a common choice), divides by a derating factor kθ = √((θmax − θamb) ÷ (θmax − θref)), then divides by the current density: A = I × margin ÷ (kθ × J). For a short circuit the minimum area is A = Isc × √t ÷ k, and the larger of the two governs.
Example: 630 A of copper in an enclosure with J = 1.2 A/mm², a margin of 1.25, a 40 °C ambient, a 35 °C reference and an 85 °C limit. Design current 787.5 A, kθ = 0.949, so the current at reference conditions is 830 A and the area is 692 mm². The smallest listed bar above that is 120 × 6 mm (720 mm²). Aluminium for the same job at 0.8 A/mm² needs about 1,038 mm², roughly 1.5 times the copper.
| Material | Resistivity at 20 °C | Density | Typical J (enclosed → open) |
|---|---|---|---|
| Copper | 1.724 × 10⁻⁸ Ω·m | 8,960 kg/m³ | 1.2 → 1.6 A/mm² |
| Aluminium | 2.82 × 10⁻⁸ Ω·m | 2,700 kg/m³ | 0.8 → 1.2 A/mm² |
What this does not cover
- Skin and proximity effects at higher currents: several bars in parallel carry less than the sum of their areas, which is why a group factor from the supplier is needed.
- Mechanical short-circuit forces and support spacing, which can decide the section in high fault-level boards.
- Joint and termination heating, plating, painting and the limits of connected equipment.
- The temperature-rise limits and verification routes of IEC 61439-1, which apply to the complete assembly.
Busbar Size Calculator FAQ
How do I calculate busbar size from amps?
Divide the current, including a margin, by a current density in A/mm². For 630 A with a 125% margin and 1.2 A/mm² copper that is about 656 mm² before ambient derating.
What current density should I use for copper busbars?
Practical guides quote about 1.2 A/mm² for enclosed or grouped copper and up to 1.6 A/mm² in open air. Use the manufacturer’s ampacity table for the real section.
How much bigger must an aluminium busbar be?
About 1.5 to 1.6 times the copper cross-section for similar resistance, which is why the aluminium densities are lower.
How is the short-circuit area calculated?
A = Isc × √t ÷ k, an adiabatic heating estimate. k depends on material and the starting and final temperatures, so confirm it in IEC 61439-1 or IEC 60865.
Why use a group factor for several bars?
Parallel bars crowd each other’s cooling and push current to their outer faces, so two bars carry less than twice one bar. The factor comes from tests in the supplier’s tables.
Does this replace IEC 61439 verification?
No. It is an early sizing estimate. The assembly must be verified by test, calculation or approved design rules.