VFD Size Calculator
Calculate the recommended VFD current rating, kW/HP class, overload duty and derating margin for AC motor drive selection.
β‘ Field Rule: Size a VFD mainly by motor nameplate FLA, not only by kW/HP. For pumps and fans normal duty is often enough; conveyors, crushers, mixers and heavy starting loads usually need heavy-duty rating and extra current margin.
π VFD sizing flow: Supply β VFD β Motor load
Motor Nameplate FLA
Supply Voltage
Phase
Duty Type
Spare Current Margin
Derating Factor
Use this tab when you have the motor nameplate current. This is normally the best method for selecting a VFD.
Presets:
Motor Power
Supply Voltage
Phase
Power Factor
Motor Efficiency
Duty Type
Spare Margin
Use this only when nameplate FLA is not available. Always confirm with the real motor nameplate before ordering the drive.
Motor presets:
Number of Motors
FLA per Motor
Simultaneous Load
Largest Motor Starting Add
Spare Margin
Derating Factor
Supply Voltage
For one VFD feeding multiple motors, each motor usually needs its own overload protection after the VFD. Check manufacturer rules for output contactors and cable length.
Examples:
π Formula Reference
VFD Current from FLA
I_vfd = FLA Γ duty factor Γ margin Γ· derating
3-Phase FLA Estimate
I = Pout Γ· (β3 Γ V Γ PF Γ Ξ·)
1-Phase FLA Estimate
I = Pout Γ· (V Γ PF Γ Ξ·)
Multi-Motor Current
I_total = Ξ£FLA Γ load% + starting allowance
π Quick Reference
Duty Selection
Pump / fanNormal
ConveyorHeavy
Crusher / mixerSevere
Sizing Priority
Best inputNameplate FLA
Second optionkW/HP
Final checkDrive manual
Common Checks
Supply1Ο / 3Ο
MotorInsulation
PanelTemp/altitude
π Practical Notes
Current rating comes firstTwo VFDs with the same kW rating can have different current ratings. Match the VFD output current to motor FLA and duty rating.
Normal duty vs heavy dutyNormal duty often supports variable-torque loads like pumps and fans. Heavy duty is safer for constant-torque loads and frequent acceleration.
Derating mattersHigh cabinet temperature, high altitude, switching frequency and single-phase input can reduce the usable current of a drive.
Multi-motor warningOne VFD can run multiple motors only when the drive manual permits it and each motor is protected correctly.
What is a VFD Size Calculator?
A VFD size calculator helps select a variable frequency drive for an AC motor by estimating the required drive output current, kW or HP class, duty rating and derating margin.
How to size a VFD for a motor
The most reliable method is to use the motor nameplate full-load current. Select a VFD whose rated output current is equal to or higher than the motor FLA after duty type, derating and safety margin are considered.
Should VFD be sized by kW or amps?
Use amps as the final selection check. kW or HP is useful for quick estimation, but the drive output current rating must be suitable for the actual motor nameplate current.
β Frequently Asked Questions
Normally no. Start with motor nameplate FLA and drive manufacturer rules. A lightly loaded motor may still need acceleration, overload and thermal margin.
Moderate oversizing is usually acceptable, but too much oversizing can affect current sensing, protection settings and cost. Configure motor nameplate data correctly in the drive.
Many VFDs include electronic motor overload protection, but multi-motor applications or local code may require individual overload devices. Follow the drive manual and site standards.
Yes, in some applications. Size the VFD for total simultaneous current and provide suitable overload protection for each motor.
Normal duty is usually for easier variable-torque loads. Heavy duty allows higher overload current and is preferred for constant-torque or high starting load applications.
Yes. Some drives can accept single-phase input only with derating, while others require three-phase input. Always check the selected VFD datasheet.