A 160 A soft starter looks big enough for a 140 A motor until ambient, altitude and starting duty are applied. How the derated capacity is worked out and which size passes.
| Motor | 75 kW, 400 V, full-load current 140 A |
| Site | Ambient 50 °C, altitude 1500 m |
| Duty | 12 starts per hour |
| Candidate soft starters | 160 A and 200 A, both rated 400 V |
The derating rates used (1% per °C above 40 °C, 1% per 100 m above 1000 m, and a starts-per-hour factor) are typical manufacturer figures. The chosen manufacturer's own derating curves take precedence.
11 to 20 starts per hour reduces the thermal capacity available for running current.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Voltage rating | ≥ 400 V | 400 V | ✓ PASS |
| 160 A unit, derated | ≥ 140 A | 123.1 A | ✗ FAIL |
| 200 A unit, derated | ≥ 140 A | 153.9 A | ✓ PASS |
Key insight: The three derating factors multiply. Each looks small on its own, but together they remove 23% of the rating here.
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Open Soft Starter Sizing Calculator calculator →An internally or externally bypassed soft starter carries running current through the contactor, so many manufacturers allow higher ratings or more starts. Use the bypassed rating tables in that case.
Each start heats the thyristors with several times rated current. More starts leave less time to cool, so less continuous current can be carried.