How to Size a VFD: Motor Power, Current & Overload

Size a VFD by motor current, not just motor power. As a practical rule, choose a drive whose rated output current is at least 110–125% of the motor's full-load current, then check the overload requirement and any derating factors for temperature, altitude and cable length. This guide walks through the four steps of VFD sizing with the nameplate data you need and the mistakes that cause the most field failures.

The Golden Rule: Match Current, Not Just Power

A 55 kW motor from one manufacturer can draw a noticeably different full-load current than a 55 kW motor from another, especially across voltage ratings and efficiency classes. The VFD output current is the real constraint: if the drive's rated current is lower than the motor's full-load current, the drive will overheat or trip on overload even though the kilowatt figures look correct. Always size from the motor nameplate current (FLC) at the actual supply voltage, then apply a margin for starting and short-term overloads.

Step 1: Collect the Motor Nameplate Data

Step 2: Apply the Overload Margin

Standard industrial VFDs provide 110% current for 60 s (or 150% for 60 s on heavy-duty rated models) to cover starting torque and short peaks. If the application needs more – e.g. a crusher, centrifuge or high-inertia fan – select a heavier-duty frame or a larger drive. For constant-torque loads, use the "heavy duty" rating of the drive; for variable-torque loads (pumps, fans), the normal-duty rating is usually sufficient. When in doubt, the cost of one frame size up is far lower than a field failure.

Step 3: Check the Derating Factors

Altitude and ambient-temperature derating follow IEC 61800-2. In a tropical plant or an enclosure with several drives, temperature derating is the most commonly missed factor.

Step 4: Select the Drive Model and Verify

• Pick the drive whose rated output current ≥ the current found in steps 1–3
• Verify the input supply: voltage tolerance, available short-circuit current, and required input protection (fuses/MCB)
• Confirm the control mode needed (V/f, sensorless vector, closed-loop) for the load
• Check braking: high-inertia or overhauling loads need a braking chopper/resistor

Common Sizing Mistakes

• Sizing by kW only and ignoring full-load current at the actual voltage
• Forgetting that a 50 Hz motor nameplate at 60 Hz supply changes the duty
• Oversizing "to be safe", then running the motor at very low speed where self-cooled motors overheat
• Ignoring starting torque peaks on conveyors and high-friction machines

Frequently Asked Questions

Can a VFD be larger than the motor? Yes – one frame up is normal and safe for derating, cable length or future motor changes. Very large oversizing reduces efficiency and can make low-current protection less precise.

Can a VFD be smaller than the motor? Only if the application current never exceeds the drive rating – e.g. a low-demand variable-torque load. For general use, match or exceed the motor FLC plus margin.

What is the difference between normal and heavy duty rating? Heavy duty gives higher overload capability (e.g. 150% for 60 s) for constant-torque and high-inertia loads; normal duty suits pumps and fans with lower peaks. See our VFD selection guide and the step-by-step selection article for the full process.

For help sizing a drive for your motor – or to check derating for your site altitude and cable run – contact our engineering team. Browse our low-voltage VFDs and medium-voltage VFDs ranges, or start with our VFD vs. soft starter guide.