A bus bar is the heavy conductor that distributes current inside switchgear and panelboards. Sizing it by cross-section keeps the temperature rise within limits. This calculator estimates the continuous ampacity of a rectangular copper or aluminum bar using the standard amps-per-square-inch rule, adjusted for real installation conditions.
How it works
The base estimate multiplies the bar’s cross-sectional area by a material constant:
ampacity = area (in^2) x amps-per-in^2- Copper: about 1000 A/in² (roughly 2 A/mm²).
- Aluminum: about 700 A/in² (roughly 1.0–1.1 A/mm²).
These figures assume a single bar in free air with about a 30 °C rise above ambient. Three adjustments bring it closer to reality:
- Stacking — each bar after the first adds only about 80% of its area’s worth of capacity, because inner bars run hotter.
- Enclosure — still air inside a panel cuts cooling by roughly 20%.
- Finish — a dull, painted, or plated bar radiates heat better than bright bare metal, worth about 15% more.
Worked example
A copper bar 2 in × 1/4 in, single bar, inside an enclosure, bright finish:
- Cross-section:
2 × 0.25 = 0.5 in². - Base:
0.5 × 1000 = 500 A. - Enclosure derate:
500 × 0.80 = 400 A.
So expect roughly 400 A continuous. Switch to a plated/dull finish and it rises to about 460 A.
Notes and tips
- At high currents and AC, skin effect pushes current to the bar surface, so thin wide bars outperform thick square ones of the same area.
- Joint quality dominates real-world heating — clean, torque, and plate bolted connections so they do not become the hot spot.
- This rule of thumb is for sizing intuition. Always confirm the final bar against manufacturer ampacity tables and the temperature-rise test limits of the applicable standard.
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