A 100 x 10 mm copper busbar carrying 1200 A, checked by solving the actual heat balance between I²R loss and convective/radiative cooling — not a table lookup.
| Busbar | 1 bar, 100 mm wide x 10 mm thick, copper |
| Mounting | Open, vertical, bare finish (emissivity 0.3) |
| Ambient temperature | 35°C |
| Load current | 1200 A |
| Maximum allowed temperature rise | 65 K |
At thermal equilibrium, resistive (I²R) heat generation equals heat lost to convection and radiation — but resistance itself rises with temperature, so this has to be solved iteratively rather than in one step.
Starting from an initial guess and refining until the temperature rise stabilizes.
Solving the same heat balance in reverse, for the current that produces exactly the maximum allowed 65 K rise.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Temperature rise at 1200 A | ≤ 65 K | 17.61 K | ✓ PASS |
| Maximum continuous current at 65 K rise | n/a (informational) | 2478 A | ✓ PASS |
Key insight: A busbar's continuous ampacity isn't a single number pulled from a manufacturer table — it depends on the specific installation's mounting, orientation, finish (emissivity), ambient temperature and even nearby airflow, all of which change how effectively it sheds heat. This first-principles heat-balance approach recomputes the actual thermal limit for the specific installation rather than relying on a generic published rating that may not match the real conditions.
Every input in this example is editable in the live calculator — free, no signup.
Open Busbar & Switchgear Rating calculator →Radiative heat loss scales with emissivity, and a bare copper bus has a fairly low emissivity (about 0.3) compared to a painted finish (about 0.9) — painting an otherwise-identical busbar black can meaningfully increase its radiative cooling and therefore its continuous ampacity at the same temperature-rise limit, which is why the calculator exposes emissivity as an adjustable input rather than hiding it as a fixed constant.
No — this is explicitly a transparent engineering approximation for preliminary sizing, with every coefficient (emissivity, McAdams constant, proximity/skin-effect factors) exposed as an adjustable input rather than hidden. For final equipment specification or formal compliance, a manufacturer's actual IEC 60890 type-test data for the specific busbar configuration should be used.