A 1000 kVA transformer's own open-circuit and short-circuit test results, used to find its full-load efficiency, the loading point where efficiency actually peaks, and its voltage regulation at rated load.
| Rated capacity | 1000 kVA |
| No-load loss (open-circuit test) | 1800 W |
| Load loss at rated current (short-circuit test) | 11,000 W |
| Impedance (nameplate, from SC test) | 6.0% |
| Loading condition | 100% of rated load, 0.85 power factor lagging |
Maximum efficiency occurs where variable (copper) loss equals fixed (iron) loss — not necessarily at full load.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Full-load efficiency | n/a (informational) | 98.52% | ✓ PASS |
| Peak efficiency (occurs at 40.5% loading) | n/a (informational) | 98.96% | ✓ PASS |
| Voltage regulation at full load | n/a (informational) | 4.14% | ✓ PASS |
Key insight: A transformer's efficiency curve peaks where fixed loss equals variable loss, not at 100% loading — which is why a transformer that's oversized relative to its typical load (running well below full rating most of the time) isn't necessarily wasting efficiency, and can sometimes be operating closer to its actual efficiency sweet spot than a tightly-sized unit running near full load constantly.
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Open Generator & Transformer Analysis calculator →No-load loss (1800 W) is constant 24/7 regardless of load, while load loss scales with the square of loading fraction — so a transformer's annual loss cost depends on its typical average loading over the year (this example uses 60% as a representative average), not just its instantaneous efficiency at any one snapshot like full load or the maximum-efficiency point.
Nameplate impedance directly determines available short-circuit current on the secondary side during a fault (roughly rated current / z%, before any source impedance is added) and also sets the reactive voltage-drop component (vx) used in the regulation calculation — a lower-impedance transformer gives tighter voltage regulation under load but also allows higher fault current, which is exactly the kind of trade-off protection studies have to account for.