Answers 'what VFD rated output current do I need?' before a specific drive has been selected — starting from motor FLA, this applies a duty-type multiplier (Constant Torque loads need proportionally more rated current headroom than Variable Torque loads on the same physical drive frame) plus ambient temperature and altitude derating, to give a minimum required nameplate rated current to shop against.
Standards followed
Generic manufacturer-typical derating rates (≈1%/°C above 40°C ambient, ≈1%/100m above 1000m altitude) — the same figures used across this site's other power-electronics sizing screens (soft starters), broadly consistent with IEC 60947-4-2-style thermal-capacity screening but not a literal reproduction of any specific manufacturer's table.VT/CT duty multiplier (~1.15x for Constant Torque vs Variable Torque) — a widely cited rule of thumb reflecting that most drive frames publish a lower Constant-Torque current rating (~85-87% of the frame's Variable-Torque rating) for the same hardware.
What it can do
- Required VFD output current from motor FLA, duty type and a safety margin.
- Temperature and altitude derating applied to translate that into a minimum VFD nameplate rated current.
- Optional pass/fail check against a specific candidate VFD's rated current and voltage.
Example problem
65A FLA motor, 400V, Variable Torque duty (centrifugal pump), 10% safety margin, 40°C ambient, 1000m altitude.
- Duty multiplier (VT) = 1.0; temperature and altitude factors = 1.0 (both at the no-derating threshold).
- Required output current = 65 × 1.10 × 1.0 = 71.5A.
- Minimum required VFD rated current = 71.5 / (1.0 × 1.0) = 71.5A.
Shop for a VFD with a Variable-Torque rated output current of at least 71.5A at 400V.
Want the full step-by-step version?
This screens rated current only — motor cable length/dV-dt filtering, carrier (switching) frequency derating, and the specific drive model's exact VT/CT current tables all need separate confirmation against the manufacturer's application data before final selection. For checking a VFD you've already chosen against a motor's nameplate (voltage match, current loading, V/Hz ratio, slip), see the VFD Parameter Selection Calculator instead.