VFD Sizing Calculator

About the VFD Sizing Calculator

This VFD sizing calculator recommends a Variable Frequency Drive output rating based on motor HP, voltage, service factor, and load type. Right-sizing the VFD prevents costly oversizing while ensuring the drive can handle real-world starting current, service-factor overload, and load variability without nuisance tripping.

Variable-torque loads (centrifugal pumps and fans) need minimal headroom. Constant-torque loads (conveyors, compressors, hoists) need more. Heavy/shock loads (crushers, mills, punches) need the most.

How VFD sizing works

Min VFD output A ≥ Motor FLA × Service Factor × Load Type Factor

Start with the motor’s full-load amps (FLA). Multiply by the service factor (1.0 for SF 1.0 motors, 1.15 for typical industrial). Multiply by a load-type factor: 1.0 for variable-torque (pumps, fans), 1.1 for constant-torque (conveyors), 1.25 for heavy/shock loads. Round up to the next standard VFD size.

Worked example

A 10 HP, 480V three-phase motor (FLA ≈ 14 A) with 1.15 service factor, driving a centrifugal pump: minimum VFD = 14 × 1.15 × 1.0 = ~16 A output. A standard 10 HP / 14 A VFD wouldn’t quite handle the SF overload — specify the next size up (15 HP / 21 A) or a 10 HP heavy-duty VFD.

When to use this calculator

• Selecting VFDs for new motor installations
• Replacing a motor starter with a soft-start VFD
• Adding speed control to existing pump or fan installations
• Specifying VFDs for energy-saving retrofits
• Preventing nuisance VFD overload trips on shock loads
• Verifying that an existing VFD can support a motor change

VFD sizing rules of thumb

• Variable torque (pumps/fans): VFD HP = motor HP. The torque demand drops with speed squared, so the VFD never sees more than nameplate.
• Constant torque (conveyors, mixers, compressors): VFD HP = motor HP × 1.1. Torque is constant across speed range, requiring full current at low RPM.
• High inertia/shock loads (cranes, crushers): VFD HP = motor HP × 1.25 minimum. Specify “heavy duty” rated VFDs.
• Multi-motor on one VFD: sum motor FLAs and add 10–20% margin.

Frequently Asked Questions

Why can’t I just use a VFD with the same HP as my motor?

For variable-torque loads (pumps, fans), you can. For constant-torque or shock loads, the VFD has to deliver full motor current at low RPM — without RPM, it can’t develop the back-EMF that limits current. Oversizing 10–25% prevents nuisance trips.

What is service factor (SF)?

SF is the overload margin built into a motor. A 10 HP motor with SF 1.15 can run at 11.5 HP continuously without overheating. The VFD must be able to deliver this overload current.

Should I use a soft starter instead of a VFD?

If you only need to reduce starting current (not vary speed during operation), a soft starter is cheaper. If you need speed control or energy savings, choose a VFD. VFDs cost 2–5× more but deliver much more value for variable-flow applications.

What VFD HP do I need for a 25 HP pump motor?

For a centrifugal pump (variable torque), a 25 HP VFD works. For a positive-displacement pump (constant torque), step up to 30 HP. Always verify against the motor’s actual FLA, not just HP.

Does VFD output affect motor sizing?

Slightly. VFD-driven motors run cooler at full speed (no inrush) but hotter at low RPM (reduced cooling fan speed). For continuous operation below 50% speed, consider an inverter-duty motor with separate cooling.

What about harmonics — do I need a filter?

VFDs generate harmonic currents that can affect the upstream supply. For installations >50 HP or where multiple VFDs share a transformer, line reactors or harmonic filters are recommended to meet IEEE 519 limits.

Related calculators and Electracore products

Specifying a VFD-driven motor? Browse Variable Speed motors, IEC frame motors, or General Purpose motors built for VFD operation.

Related calculators: HP to Amps Calculator · Motor FLA Calculator · Motor Torque Calculator · Motor Energy Cost Calculator.