Checks whether a candidate soft starter's nameplate rated current is adequate for a given motor, after derating for ambient temperature above 40°C, altitude above 1000m, and duty cycle (starts per hour) — the three most commonly published soft-starter derating factors — plus a voltage-rating adequacy check.
Standards followed
Generic manufacturer-typical derating rates (≈1%/°C above 40°C, ≈1%/100m above 1000m, starts-per-hour bands) — broadly consistent with the kind of thermal-capacity screening IEC 60947-4-2 AC-53a duty ratings address, but not a literal reproduction of any specific manufacturer's or IEC's published table
What it can do
- Temperature, altitude, and starts-per-hour derating factors applied to the candidate soft starter's rated current.
- Voltage adequacy check (soft starter rated voltage ≥ motor rated voltage).
- Overall pass/fail against the motor's full-load current.
Example problem
85A FLA motor, 400V, candidate soft starter rated 105A/400V, 40°C ambient, 1000m altitude, 5 starts/hour.
- All three derating factors = 1.0 (within the no-derating thresholds).
- Derated capacity = 105 × 1.0 × 1.0 × 1.0 = 105A ≥ 85A FLA — adequate.
ADEQUATE — 105A derated capacity comfortably covers the 85A motor FLA.
These derating rates are generic screening figures, not a substitute for the specific manufacturer's datasheet — always verify the final soft starter selection (including its AC-53a duty rating for your actual starts-per-hour and start time) against the chosen model's published data.