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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Design considerations beyond the basic rule
Skin effect and bar proportions
The amps-per-square-inch rule treats the cross-section as uniformly carrying current. In practice, at 60 Hz AC, skin effect concentrates current near the surface. For thick bars — say 1 inch or more — the centre of the bar carries less current than the surface, reducing effective capacity below what the simple area formula predicts. The practical response is to prefer wider, thinner bars (greater perimeter relative to area) rather than thick square cross-sections, or to use multiple thinner bars per phase with gaps to allow airflow.
Plating and surface emissivity
A bright bare copper bar has low thermal emissivity — it does not radiate heat effectively. Tin or silver plating, anodising, or even a coat of flat black paint raises emissivity and improves radiation cooling, worth roughly 10–15% more current capacity at the same temperature rise. Plating also improves joint resistance, which matters for both heating and voltage drop at connections.
Short-circuit withstand
The continuous ampacity rule says nothing about fault current. A bus bar must also withstand the peak and RMS asymmetrical fault current for the required duration (typically 0.03–3 seconds depending on the protection scheme). This is a separate mechanical and thermal calculation: the bar must not deform or melt during the fault event. Short-circuit ratings come from the switchgear standard (UL 891 for switchboards, IEC 61439 for assemblies) and usually require manufacturer data rather than a rule of thumb.
Voltage drop
Even a correctly ampacity-rated bar can cause significant voltage drop if it is long. Bus bars in large switchgear may run several meters; the resistance of the bar multiplied by the current gives the resistive voltage drop. At 1,000 A in a copper bar 2 m long with 0.5 in² cross-section, the resistance is roughly 0.3 mΩ, giving 0.3 V drop — small. But in a system with tight voltage tolerance, or a bar carrying motor starting currents, checking voltage drop separately is worthwhile.