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    Interactive calculator

    Soft Starter Sizing

    In-line current to seek, assumed current limit and candidate checks.

    EstimatePreliminary result, not a design verification.
    Before you calculate

    In-line soft starter: the current to seek and what stays unassessed

    A preliminary three-phase screening. It gives the continuous in-line current to look for and an assumed current-limit setting; the starter model, the acceleration of the load and the thermal duty need the manufacturer's data.

    Learn more — complete guide →
    Step 1

    Motor

    A preliminary in-line, three-phase soft starter screening. The motor current comes from the nameplate when you enter it, otherwise from the built-in reference.

    Three-phase motor. This tool covers three-phase motors only.

    The mechanical rating on the nameplate, not the electrical input power. 1 HP = 0.745699872 kW.
    Advanced: nameplate and motor data (all optional)

    Anything you enter here replaces the built-in reference model for that quantity. Leave a field empty to use the reference where it applies.

    Empty = 350 %, an assumed setting, not a predicted start current.
    The starting method does not prove how the motor is wound for running. Follow the nameplate.
    Advanced: check a candidate soft starter (optional, manufacturer data)

    Enter only what the manufacturer documents. Each check is independent; anything missing stays NOT ASSESSED, and there is never an overall PASS.

    Only a factor from the manufacturer; none is assumed.
    Product-specific, for example 350 for one manufacturer series.
    Step 2

    Results

    Motor rated current28.5 AReference assumption or your efficiency and power factor
    Minimum continuous in-line starter current to seek≥ 28.5 AAt least the motor rated LINE current, before any ambient, altitude or enclosure derating.
    Assumed soft-starter current-limit setting99.6 A350 % of the rated current: a requested SETTING, not a prediction of the start current or of acceleration.
    The delta winding current is not the starter rating: for this motor it would be 16.4 A per winding only if the motor is delta-connected. A standard in-line starter carries the LINE current (28.5 A). An inside-delta starter needs a manufacturer-approved topology and its own ratings.
    NOT ASSESSED by this tool:
    • Starting torque and whether the load accelerates (needs the motor and load torque curves and the inertia)
    • Starting time and repetitive duty, unless the candidate duty envelope and your requirements are entered
    • Bypass arrangement, motor thermal protection and short-circuit coordination
    • Exact starter model selection
    Reference model in use (not a nameplate): IE3, 4 poles, 50 Hz, 400 V. A plain arithmetic mean of two catalog series per rating (ABB General Performance, edition 10-2021, and WEG W22 Rev. 34 of 04/2024), taken at the listed rating. It is a two-series reference estimate, not a market average and not the data of your motor; locked-rotor ratios in particular differ widely between motors of the same size. ABB later editions print different power factors and Is/In for some small ratings. Both series were compared value by value with the original catalog pages (ABB 10-2021 page 15, WEG 50025712 Rev. 34 page 34).
    Engineering scope: preliminary screening, not a product selection or an IEC 60947-4-2 compliance statement. A soft starter controls the motor voltage or current while starting; it does not control the running speed like a VFD.

    1. Efficiency, power factor and locked-rotor ratio in use

    Entered values replace the reference. The reference (IE3, 4 poles, 50 Hz, 400 V family) is the plain mean of two catalog series. Efficiency: two-series reference; power factor: two-series reference; Is/In: two-series reference.

    η,cos⁡φ,kLR=Is/In\eta,\quad \cos\varphi,\quad k_{LR}=I_s/I_n
    η=0.922,cos⁡φ=0.825,kLR=7.6\eta=0.922,\quad \cos\varphi=0.825,\quad k_{LR}=7.6

    2. Rated line current (three-phase)

    Mechanical output power divided by the electrical quantities at the rated operating point.

    In=1000 P3 ULL ηcos⁡φI_n=\frac{1000\,P}{\sqrt{3}\,U_{LL}\,\eta\cos\varphi}
    In=1000×153×400×0.922×0.825=28.4633 AI_n=\frac{1000\times 15}{\sqrt{3}\times 400\times 0.922\times 0.825}=28.4633\ \text{A}

    3. Delta winding current (rated running, only if the motor runs in delta)

    In a balanced delta connection each winding carries the line current divided by √3. This is the rated RUNNING winding current; it is not a starting current and does not depend on the starting method. In-line starters and drives are sized from the LINE current, never from this value.

    Iw,Δ=Iline3I_{w,\Delta}=\frac{I_{line}}{\sqrt{3}}
    Iw,Δ=28.46333=16.4333 AI_{w,\Delta}=\frac{28.4633}{\sqrt{3}}=16.4333\ \text{A}

    4. Input, apparent and reactive power

    Sinusoidal rated operating point.

    Pin=Pη,S=Pincos⁡φ,Q=Pintan⁡(arccos⁡φ)P_{in}=\frac{P}{\eta},\quad S=\frac{P_{in}}{\cos\varphi},\quad Q=P_{in}\tan(\arccos\varphi)
    Pin=16.269 kW,S=19.72 kVA,Q=11.1444 kvarP_{in}=16.269\ \text{kW},\quad S=19.72\ \text{kVA},\quad Q=11.1444\ \text{kvar}

    5. Assumed soft-starter current-limit setting

    A requested current-limit SETTING (350% of I_n by default), not a prediction of the actual motor current and not a statement that the motor will accelerate.

    Ilimit=setting100 InI_{limit}=\frac{\text{setting}}{100}\,I_n
    Ilimit=350 %×28.4633=99.6217 AI_{limit}=350\,\%\times 28.4633=99.6217\ \text{A}

    Calculations use full precision internally; the displayed numbers are rounded for readability.

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