An 800 kW load with growth margin, hot-climate derating, and full N-1 redundancy — each of the two installed transformers has to be able to carry the entire load alone.
| Connected load | 800 kW at 0.9 power factor |
| Growth margin | 15% |
| Safety margin | 10% |
| Ambient temperature | 45°C (reference is 40°C per IEC 60076) |
| Altitude | 1000 m (reference/threshold) |
| Cooling class | Oil-immersed, natural cooling (ONAN) |
| Redundancy | 2 identical units, N-1 (either one must carry full load alone) |
IEC 60076 references 40°C max ambient for a transformer's nameplate rating; above that, capacity is commonly derated at roughly 1.25% per °C (an engineering rule of thumb, not the standard's own hot-spot thermal model).
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Per-unit required capacity | n/a (this is the computed sizing target) | 1199.4 kVA | ✓ PASS |
| Recommended standard transformer size | n/a (this is the result) | 1500 kVA x 2 units | ✓ PASS |
Key insight: N-1 redundancy for two units means each one is sized for 100% of the load, not 50% — doubling unit count doesn't halve each unit's required size the way it might intuitively seem to, because the whole point of N-1 is that the system must survive losing any single unit and still carry the full load on what remains.
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Open Transformer Sizer calculator →With three identical units and N-1 redundancy, any two of the three must be able to carry the full load between them, so each unit is sized for effectiveRequired / 2 rather than effectiveRequired / 1 — a meaningfully smaller per-unit size than the two-transformer case, though total installed capacity (3 x that smaller size) doesn't necessarily drop as much as per-unit size alone suggests.
40°C max ambient (with a 30°C monthly average and 20°C yearly average) is IEC 60076's own standard reference service condition that a transformer's nameplate rating is defined against — a transformer isn't 'derated' for operating at exactly its reference conditions, only for operating in a hotter environment than the nameplate rating assumes, which is exactly the 45°C site condition in this example.