A busbar is a solid metal bar that distributes current inside switchgear and distribution boards. Sizing it has two parts: the bar must carry the continuous current without exceeding its allowable temperature rise, and it must survive the short-circuit current for the time the protection takes to clear, without melting or deforming. The continuous part is handled with an allowable current density (amps per mm²); the short-circuit part with the adiabatic thermal equation. The larger of the two areas governs.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: IEC 61439, IEC 60865 (short-circuit effects), and copper-association busbar guides.
Engineering notice. Current density is a planning estimate; the real ampacity of a bar depends on its surface finish, orientation, spacing, enclosure and ambient temperature. Final busbar selection must use the manufacturer's de-rated tables and a verified short-circuit study. See our disclaimer.
Continuous current — the current-density method
The simplest first estimate divides the current by an allowable current density:
Typical conservative design densities for bare bars in still air are about 1.2–1.6 A/mm² for copper and 0.8–1.0 A/mm² for aluminium. Larger bars run a little cooler per unit area, and forced ventilation raises the figure, so always confirm against the manufacturer's ampacity tables for the actual bar size, mounting and temperature.
Short-circuit withstand — the adiabatic equation
During a fault the bar heats almost instantly, so the check assumes no heat escapes (adiabatic). The minimum cross-section is:
Here Isc is the RMS fault current in amps, t is the clearing time in seconds, and k is a material constant (about 143 for copper and 94 for aluminium for common start/finish temperatures). Get the fault current from the fault current calculator. The bar you choose must satisfy both the continuous-current area and this thermal minimum.
Worked example — 800 A switchboard incomer
Scenario: An 800 A copper busbar, fault level 30 kA for 0.5 s.
The continuous area (615 mm²) governs, so a 80 × 8 mm bar (640 mm²) is the next standard size that satisfies both checks. The short-circuit requirement is comfortably met.
Frequently Asked Questions
Divide current by an allowable density: area (mm²) = current (A) ÷ density (A/mm²). Bare copper in still air is about 1.2–1.6 A/mm²; aluminium 0.8–1.0. For 630 A copper at 1.2: 525 mm², so a 100 × 6 mm bar (600 mm²) is the next standard size.
Roughly 1.2–1.6 A/mm² for bare copper in still air, higher with forced cooling. Aluminium carries about two-thirds of copper (0.8–1.0 A/mm²). Always confirm against the manufacturer's de-rated tables.
Use Amin = Isc × √t ÷ k, with Isc in amps, t in seconds, k ≈ 143 (copper) or 94 (aluminium). The chosen bar must meet both the continuous-current area and this thermal minimum.
Copper has higher conductivity, so it needs a smaller cross-section and runs cooler for the same current. Aluminium is lighter and cheaper but needs about 50–60% more area and careful jointing against oxidation.
Yes. It is set by the allowable temperature rise (typically 50–65 °C over ambient). Higher ambient, enclosed mounting or closely-stacked bars all reduce the allowable current, so de-rating factors apply.