VFD Sizing Calculator

A VFD sizing calculator converts motor nameplate current into a target drive output current so an entered variable frequency drive rating can be checked before procurement or panel layout is finalized. The calculator takes Motor FLA, an Application multiplier, and a Derating factor, then produces a Required VFD current value that reflects both the operating margin the application demands and the deratings the installation imposes.

Inputs and Their Function

Motor FLA is the motor’s full-load amps or nameplate current basis — the starting point for any drive current calculation, not the VFD’s own rating.

Application multiplier is entered as a percentage and scales FLA upward for duty conditions such as overload margin, starting torque demand, or process variability. A value above 100% builds in headroom; a value at 100% assumes no additional margin.

Derating factor, also entered as a percentage, reduces the drive’s usable output to account for temperature, enclosure type, altitude, or other project-specific limits on continuous output current. A derating factor below 100% means the drive must be oversized relative to the raw multiplied FLA to still deliver the required current under those conditions.

Entered VFD output rating is the nameplate or catalog output current of the drive under consideration, entered strictly for comparison — it does not feed into the Required VFD current calculation itself.

System voltage and Phase (single-phase or balanced three-phase) are used only to compute Motor load in kVA for load-review context; they do not alter the current-based sizing result.

Required VFD Current Formula

The calculator applies the multiplier first, then divides by the derating factor:

\(\text{Required VFD current} = \frac{\text{Motor FLA} \times \text{Application multiplier}}{\text{Derating factor}}\)

This sequence means the derating factor is applied to the already-adjusted current, not to raw FLA — a drive rated for continuous duty under one set of ambient conditions may need a higher nameplate rating once altitude or enclosure derating is factored in.

Calculation Example

With Motor FLA of 42 A, Application multiplier of 110%, and Derating factor of 90%:

\(\text{Required VFD current} = \frac{42 \times 1.10}{0.90} = 51.3333\ \text{A}\)

Against an Entered VFD output rating of 60 A, the calculator reports an Entered VFD margin of 8.6667 A (60 − 51.3333), and a Rating status of within_entered_rating, confirming the entered drive’s output current exceeds the calculated requirement.

Motor load is computed separately using System voltage (480 V) and Phase (balanced three-phase) with standard three-phase kVA math, returning 34.9181 kVA — a figure used for feeder and load-schedule cross-checks rather than for drive current sizing.

Reading the Rating Status

The within_entered_rating output is an arithmetic comparison between two numbers already entered or derived: Required VFD current versus Entered VFD output rating. It does not confirm that the drive’s continuous duty rating, overload rating, carrier frequency derating, or output waveform characteristics meet the actual installation’s ambient and enclosure conditions. Manufacturer output current tables typically include their own temperature and altitude derating curves that must be checked directly against catalog data, separate from this calculator’s percentage-based Derating factor input.

Field and Application Limits

This calculation screens output current only. It does not size branch-circuit conductors, disconnect devices, or overload protection for the motor circuit, and it does not evaluate short-circuit current rating, harmonic content, or motor cable length limits that affect voltage reflection at the motor terminals. Conductor ampacity, terminal ratings, and insulation temperature ratings for the feeder supplying the VFD, and for the output conductors to the motor, must be sized under separate ampacity and adjustment-factor calculations and verified against the authority having jurisdiction’s adopted code edition. The Motor load kVA figure is useful for feeder loading review but does not replace a full load calculation for the panel or service supplying the drive.