VFD Size Calculator

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

VFD: A variable frequency drive, which is an electronic controller that changes motor speed by changing the power sent to the motor.

Motor nameplate amps and service factor: The motor's full-load current in amps, paired with the service factor printed on the motor nameplate.

Service factor: A multiplier for extra allowed motor loading under nameplate conditions. A service factor of 1.15 makes an 8 A motor count as 9.2 A in this calculator.

Motor load type: The type of job the motor does. It sets the planning margin: 10% for variable torque, 15% for constant torque, and 25% for heavy start or shock load.

Extra safety margin: An optional added percent used when a project, electrician, or drive maker asks for more room.

Available VFD output amp ratings: The list of drive output current ratings the calculator can choose from after it finds the required minimum.

Minimum VFD output amps needed: The calculated output current target after motor service factors, load type margin, and extra safety margin are applied.

Next available VFD output amp rating: The first rating in your available ratings list that is at least as large as the minimum required output amps.

Mistake: Typing horsepower or kilowatts into Motor nameplate amps and service factor.
Fix: Use motor full-load amps in A for each row, not motor power.

Mistake: Leaving a motor's service factor blank in Motor nameplate amps and service factor.
Fix: Enter the printed service factor, or use 1.00 only when you are sure no extra service factor applies.

Mistake: Choosing Motor load type as Variable torque, easy start for a conveyor, crusher, or other hard-start load.
Fix: Choose Constant torque, normal starting or Heavy start or shock load when the load needs more starting margin.

Mistake: Using a negative Extra safety margin to force a smaller Minimum VFD output amps needed.
Fix: Keep Extra safety margin at 0 or a positive percent; do not use it to reduce the calculated size.

Mistake: Leaving Available VFD output amp ratings at the default when your drive series has different output ratings.
Fix: Replace the list with the actual output amp ratings from the VFD models you can buy.

Mistake: Treating Next available VFD output amp rating as a full electrical design.
Fix: Use Multi-motor sizing note and the drive manual to check overload protection, wiring, voltage, setup, and manufacturer rules.

• This calculator sizes the VFD by output current in amps. It does not size input breakers, fuses, branch-circuit conductors, grounding, or motor overload protection.
• The load type margins are planning margins. They are not a code rule and may not match a specific manufacturer's heavy-duty, normal-duty, or overload rating system.
• A shared VFD running multiple motors can need extra application checks, separate motor overload protection, and drive setup rules from the manufacturer [1].
• The result assumes the motors are compatible with one shared VFD and that voltage, phase, motor insulation, cable length, cooling, and drive output filter needs have been checked separately.
• The Available VFD output amp ratings list is generic by default. Replace it with the exact output current ratings from the VFD product line you plan to use.
• If no listed rating is high enough, the rounded recommendation cannot be chosen. Add larger ratings or choose a larger drive series.

How the motor rows are counted

Each complete motor row uses the motor full-load amps and service factor entered in Motor nameplate amps and service factor. Blank rows are ignored, but a row with a nonpositive amp value or nonpositive service factor is an error.

adjusted_motor_amps_i = motor_fla_i * service_factor_i

The adjusted motor amps are then added together.

total_adjusted_motor_amps = sum(adjusted_motor_amps_i)

How the minimum drive amps are found

Motor load type sets the load margin used in the calculation. Variable torque uses 10%, constant torque uses 15%, and heavy start or shock load uses 25%. Extra safety margin is added after the load type margin.

minimum_vfd_output_amps = total_adjusted_motor_amps * (1 + load_margin_pct / 100) * (1 + extra_margin_pct / 100)

How the rounded drive rating is selected

The Available VFD output amp ratings values are read as a comma-separated list, cleaned, sorted from low to high, and duplicate ratings are removed. The calculator chooses the first rating that is at least as high as the minimum required output amps.

next_vfd_output_amps = first sorted available rating where rating >= minimum_vfd_output_amps

Headroom shows how much larger the selected rating is than the calculated minimum.

rating_headroom_amps = next_vfd_output_amps - minimum_vfd_output_amps

rating_headroom_percent = rating_headroom_amps / minimum_vfd_output_amps * 100

Mini example

Suppose the motor rows are 8.0 A with a 1.15 service factor and 5.5 A with a 1.00 service factor. The adjusted motor amps are 8.0 * 1.15 = 9.2 A and 5.5 * 1.00 = 5.5 A, so Total motor amps after service factors is 14.7 A. With Motor load type set to constant torque, the 15% margin gives Minimum VFD output amps needed = 14.7 * 1.15 = 16.905 A. With the default ratings list, the next listed rating at or above 16.905 A is 20 A. The selected rating has 3.095 A of headroom, which is about 18.31%.