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VFD Sizing Calculator

Work out vfd sizing instantly with clear inputs, formula shown and shareable results.

About VFD Sizing Calculator

A VFD is sized on current, not motor kilowatts, because it must supply the motor's actual full load current plus the overload the application needs. Above 40 degrees C ambient most drives derate about 2% per degree, and switching frequency above the default derates them further.

Formula

VFD sizing

drive current >= motor FLC / ambient derating; overload capability >= FLC x required overload

Worked example

With the default values below, VFD Sizing Calculator returns Drive continuous current required of 30.06. Change any input above and every figure here is recalculated in your browser.

Inputs used

Motor rating
15 kW
Supply voltage
400 V
Required overload
150 %
Ambient temperature
45 °C

Results

Drive continuous current required
30.06 A
Motor full load current
27.05 A
Ambient derating
90.0% %
Overload current required
40.58 A

Step by step

  1. 1Values used
  2. 2VFD sizing
  3. 3Drive continuous current required= 30.06 A
  4. 4Motor full load current= 27.05 A
  5. 5Ambient derating= 90.0 %
  6. 6Overload current required= 40.58 A

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why size on current rather than kW?

Because kW ratings assume a standard 4-pole motor at a standard power factor. High-pole-count, high-slip or older motors draw more current for the same kW.

What overload do I need?

110% for pumps and fans (variable torque), 150% for conveyors and positive displacement pumps, and up to 200% for high-breakaway loads.

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