Busbar Size Calculator (Current Density Method)

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Updated November 2, 2026
Busbar Size Calculator (Current Density Method): formula and free online calculator

A first estimate of busbar size divides the current by a safe current density, typically about 1.2–1.6 A/mm² for copper and 0.8–1.0 A/mm² for aluminium inside switchboards. This busbar size calculator gives the cross-section, the bar width for a chosen thickness and the resistive loss per metre — see the busbar sizing calculation guide for temperature-rise and short-circuit checks.

A/mm²
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W/m
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Type in any field to change it. Press × on a value to calculate that field instead.

How to use the busbar size calculator

Enter the rated current, choose copper or aluminium, set the design current density, the bar thickness and the number of bars per phase. The calculator returns the cross-section required, the width of each bar and the resistive loss per metre of each phase at 20 °C DC. Round up to the next standard bar size.

Busbar sizing formulas

A = I ÷ J  ·  w = A ÷ (n × t)  ·  P/L = I² ρ ÷ A

Example

1,600 A copper busbar at 1.6 A/mm², 10 mm thick bars, one bar per phase:

  • A = 1600 ÷ 1.6 = 1,000 mm²
  • w = 1000 ÷ (1 × 10) = 100 mm → one 100 × 10 mm bar per phase
  • Loss = 1600² × 1.724 × 10⁻⁸ ÷ 0.001 = 44.1 W/m per phase

Current estimate for standard flat bars

Size Area Copper at 1.2 A/mm² Copper at 1.6 A/mm² Aluminium at 0.8 A/mm²
20 × 5 mm 100 mm² 120 A 160 A 80 A
30 × 5 mm 150 mm² 180 A 240 A 120 A
40 × 5 mm 200 mm² 240 A 320 A 160 A
40 × 10 mm 400 mm² 480 A 640 A 320 A
50 × 10 mm 500 mm² 600 A 800 A 400 A
60 × 10 mm 600 mm² 720 A 960 A 480 A
80 × 10 mm 800 mm² 960 A 1,280 A 640 A
100 × 10 mm 1,000 mm² 1,200 A 1,600 A 800 A
120 × 10 mm 1,200 mm² 1,440 A 1,920 A 960 A
160 × 10 mm 1,600 mm² 1,920 A 2,560 A 1,280 A
200 × 10 mm 2,000 mm² 2,400 A 3,200 A 1,600 A

These are current-density estimates, not certified ratings — large bars carry less per mm² than small ones.

Design notes

  • AC effects: above about 10 mm thickness the skin effect means the centre of a copper bar carries little current at 50 Hz, so use several thinner bars with gaps rather than one thick bar.
  • Multiple bars per phase do not share current perfectly; derate by about 10–20% for 2–4 bars.
  • Painting bars matt black improves heat dissipation by radiation; enclosure ventilation and ambient temperature matter as much as cross-section.
  • Check the short-circuit withstand: copper needs about A ≥ Isc × √t ÷ 143 mm² for a 1 s fault from 90 °C to 250 °C (k ≈ 143).

Frequently asked questions

How do I calculate busbar size?

Cross-section = current ÷ current density. For 1,600 A at 1.6 A/mm², a copper busbar needs 1,000 mm² — for example one 100 × 10 mm bar per phase.

What current density should I use for a copper busbar?

About 1.2–1.6 A/mm² for enclosed switchboards and up to about 2 A/mm² for small, well-ventilated bars. Large bars need lower densities because their surface area per mm² is smaller.

Why is aluminium busbar larger than copper?

Aluminium has about 1.6 times the resistivity of copper, so it needs roughly 60% more cross-section for the same loss and temperature rise — but it is much lighter and cheaper.

Is the current density method enough for final design?

No. Final sizing should be verified by temperature-rise tests or tables (IEC 61439, DIN 43671), and the bars and supports checked for short-circuit thermal and mechanical stress.

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