Home / Calculators / Motors & Industrial / Motor Current & Starting
    Interactive calculator

    Motor Current & Starting

    Rated and delta winding current, and four starting-method figures.

    EstimatePreliminary result, not a design verification.
    Before you calculate

    Rated current, delta winding current and starting-method figures

    Enter the motor power and voltage, or the nameplate current in Advanced. The built-in reference is a two-series IE3 four-pole estimate, not your motor's data. The four starting-method figures are different quantities: none is a protection or sizing value.

    Learn more — complete guide →
    Step 1

    Motor and starting method

    Defaults: a three-phase 15 kW motor on 400 V, 50 Hz, with the built-in IE3 four-pole reference. Enter the nameplate values in Advanced when you have them.

    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.
    Step 2

    Results

    Rated line current (3φ)28.5 AReference assumption (efficiency and power factor from the reference model)
    Delta winding current (if Δ-connected)16.4 AConditional: the motor connection is not confirmed. This is I_line/√3 only if the motor is delta-connected; it is not a verified nameplate winding current.
    Estimated locked-rotor current (DOL)216.3 AIs/In = 7.60 · Current: Reference assumption · Multiplier: Reference assumption
    What the starting-method figure means: Standstill RMS line-current estimate at rated voltage: I_n × Is/In. It excludes the first-cycle asymmetrical peak, the duration of the start, any voltage dip and whether the load accelerates. The rated line current and the delta winding current above do not change with the starting method.
    Input power P_in16.27 kW
    Apparent power S19.72 kVA
    Reactive power Q11.14 kvar
    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: a preliminary estimate. The four starting-method figures are different quantities and are not interchangeable. None of them is a protection, breaker, cable, generator or voltage-dip sizing value. Use the motor nameplate and the equipment manufacturer's data for final selection.

    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. Estimated locked-rotor current (DOL)

    Standstill RMS line-current estimate at rated voltage: I_n × Is/In. It excludes the first-cycle asymmetrical peak, the duration of the start, any voltage dip and whether the load accelerates.

    Istart≈In×kLRI_{start}\approx I_n\times k_{LR}
    Istart≈28.4633×7.6=216.3213 AI_{start}\approx 28.4633\times 7.6=216.3213\ \text{A}

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

    Related: Soft Starter Sizing · VFD Sizing · AC Power · Voltage Drop · LV Cable Sizing