Busbar Size Calculator
Size a copper busbar for a given current, checking current density and temperature rise.
About Busbar Size Calculator
Busbars are sized on current density rather than tabulated ampacity, because their cooling depends heavily on the enclosure. Loss per metre matters because it becomes heat inside a switchboard, where removal is difficult. Multiple thinner bars per phase carry more current than one thick bar of the same area, because the skin effect and surface-to-area ratio both improve.
Formula
Busbar Size
Current density = current ÷ (width × thickness × bars); loss per metre = I² R
- target density
- Typically 1.2 to 2 A/mm² for air-cooled copper busbar
Worked example
With the default values below, Busbar Size Calculator returns Actual Current Density of 1.5000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Continuous currentminimum 0 A
- 1,200 A
- Bar widthminimum 1 mm
- 80 mm
- Bar thicknessminimum 0.5 mm
- 10 mm
- Bars per phaseaccepts 1–4
- 1
- Target current densityaccepts 0.1–5 A/mm2
- 1.6 A/mm2
Results
- Actual Current Density
- 1.5000 A/mm2
- Total Cross-Sectional Area
- 800.00 mm2
- Area Required at the Target Density
- 750.00 mm2
- Power Loss per Metre
- 37.238 W
- Verdict
- Within the target current density
Step by step
- 1Values used
- 2Busbar Size
- 3Actual Current Density= 1.5000 A/mm2
- 4Total Cross-Sectional Area= 800.00 mm2
- 5Area Required at the Target Density= 750.00 mm2
- 6Power Loss per Metre= 37.238 W
- 7Verdict= Within the target current density
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Busbar Size calculated?
Current density = current ÷ (width × thickness × bars); loss per metre = I² R Busbars are sized on current density rather than tabulated ampacity, because their cooling depends heavily on the enclosure. Loss per metre matters because it becomes heat inside a switchboard, where removal is difficult.
Why does Busbar Size matter?
Multiple thinner bars per phase carry more current than one thick bar of the same area, because the skin effect and surface-to-area ratio both improve.
What values do I need to enter?
This calculator takes 5 inputs: Continuous current, Bar width, Bar thickness, Bars per phase, Target current density. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.