VFD current and panel equations
VFD selection starts with motor nameplate current, optional service-factor loading, duty, environmental derating, and the drive manufacturer's output-current table.
Imotor = Pout / (√3 × VLL × pf × ηmotor)IVFD,min = max(Inameplate, Imotor) × FSF × Fduty × Fmargin / (Ftemp × Falt)Ploss,VFD ≈ Pmotor,input × (1/ηVFD − 1)where:
- IVFD,min
- Minimum continuous VFD output-current rating[A]
- FSF
- Service factor when above-nameplate operation is intentionally included
- Fduty
- Application-duty screening factor
- Ftemp
- Ambient-temperature derating factor
- Falt
- Altitude derating factor
- Ploss,VFD
- Estimated heat dissipated by the VFD[W]
Replace every generic factor with the exact VFD manufacturer's normal-duty or heavy-duty table before selection.
How to use the VFD Sizing, Protection and Panel Calculator
Estimate the minimum VFD output-current class and screen input protection, bypass contactor, filtering, braking, and panel heat requirements. Start by choosing the operating mode, then enter values from nameplates, measurements, or project documents in the units shown. The result updates immediately and keeps intermediate quantities visible for review.
- Confirm the system and calculation mode.
- Enter measured or manufacturer-rated values.
- Review the primary result and intermediate metrics.
- Continue with the related protection or equipment-selection checks.
Calculation method and validation
The equations and symbol definitions above show the complete calculation basis. Inputs are checked before calculation so zero, negative, incompatible, or physically impossible combinations are flagged instead of silently producing a misleading result.
How to interpret the result
Use the main result as a starting value and read every supporting metric. Three-phase induction motor and PWM VFD Environmental derating is a screening sensitivity only VFD output-current rating, not kW alone, governs selection A passing or recommended result means the entered mathematical conditions are satisfied; it does not certify the complete installation or a particular product combination.
- Three-phase induction motor and PWM VFD
- Environmental derating is a screening sensitivity only
- VFD output-current rating, not kW alone, governs selection
Selection limits and next checks
Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions. Final overload, ambient, altitude, cable-length, filter, braking, fuse, breaker, RCD, and SCCR selection must follow the exact VFD manufacturer manual. Compare the output with the current applicable standard, local rules, exact VIOX product datasheet, environmental conditions, protection coordination, and the engineer's project requirements before procurement or installation.
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Assumptions
- Three-phase induction motor and PWM VFD
- Environmental derating is a screening sensitivity only
- VFD output-current rating, not kW alone, governs selection
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Final overload, ambient, altitude, cable-length, filter, braking, fuse, breaker, RCD, and SCCR selection must follow the exact VFD manufacturer manual.