Choose how you'll describe the motor, pick phase and starting method, then click Calculate.
Pump presets use typical small single-phase motor PF/efficiency estimates (rising from ~0.60 PF / 56% efficiency at 0.5 HP to ~0.70 PF / 68% efficiency at 2 HP) — edit Power Factor / Efficiency below if you have nameplate values.
A simplified single-impedance estimate: (motor starting kVA ÷ source available fault kVA) × 100. Not a substitute for a power-quality or motor-starting study.
Estimated starting current across all five methods for this motor. Your selected method is highlighted in gold.
Watch the current settle from starting inrush to running full-load amps. Purely illustrative — a smoothed animation over your chosen duration, not a captured waveform.
Enter the FLA of every motor on this feeder or service. Combined current = sum of all motors' FLA + 25% of the single largest motor's FLA.
Results are engineering estimates. Full-load current, when not entered directly, uses the standard three-phase/single-phase motor power formula with adjustable power factor and efficiency — not a NEC Table 430.250/430.248 nameplate lookup. NEMA code letter kVA/HP values are range midpoints per NEMA MG 1 Table 12.35.1. Starting-method multipliers (DOL default 6×, Soft Starter default 3.0×, VFD default 1.1×) are typical adjustable defaults per NEMA MG 1 / IEC 60034-12 / IEC 60947-4-2 / IEC 61800-2 — always verify against the motor nameplate and starter/drive documentation. Voltage-sag estimates use a simplified single-impedance approximation per IEEE Std 141 and are a first-pass screening figure only. NEC 430.24 combined feeder current does not include continuous-load, conductor-ampacity, or overcurrent-device sizing — those require separate calculation. Motor-load calculations should be independently verified by a licensed electrician or electrical engineer before any starter, drive, protective device, or conductor is selected or installed. Actual starting current varies with motor design, NEMA/IEC design class, load inertia, and starter/drive settings.
Locked-rotor kVA per horsepower by NEMA code letter, per NEMA MG 1 Table 12.35.1. The value used in calculations is the midpoint of each letter's published range — check the motor nameplate for the exact letter.
| Code Letter | kVA / HP Used | Published Range |
|---|
Three real-world scenarios, one per input method. Click "Load This Example" to run it in the calculator above.
Why starting current spikes
At the instant of energization, an induction motor's rotor is stationary and generates no back-EMF, so the only thing limiting current is the winding's resistance and reactance — much like a near short circuit. As the rotor accelerates, back-EMF builds and opposes the applied voltage, bringing current down to its normal running value. The whole process typically takes a fraction of a second to a few seconds depending on motor size and connected load inertia.
Choosing an input method
Full-Load Amps is most accurate when you have the nameplate FLA in hand. HP + Voltage estimates FLA from the standard motor power formula when only horsepower is known — useful at the planning stage before a specific motor is selected. NEMA Code Letter uses the nameplate's locked-rotor code letter for a starting-current figure grounded in that specific motor's design, rather than a generic multiplier.
DOL, Star-Delta, Soft Starter, VFD, Autotransformer
DOL (across-the-line) applies full voltage immediately — maximum torque, maximum current, the simplest and cheapest method. Star-Delta starts in star and switches to delta at speed, cutting line current to 1/3 of DOL, but needs a 6-lead motor. Soft Starters use SCRs to ramp voltage smoothly with an adjustable current limit. VFDs ramp both voltage and frequency, giving the lowest starting current and full speed/torque control, at higher cost. Autotransformers supply a reduced-voltage tap (50/65/80%) during start — line current drops by the tap squared, a stronger reduction than star-delta at the higher taps, without needing a 6-lead motor.
NEMA code letters (NEMA MG 1 Table 12.35.1)
A nameplate code letter from A to V indicates the motor's locked-rotor kVA per horsepower band — A is the lowest (softest starting), V the highest. Multiplying that kVA/HP figure by horsepower gives locked-rotor kVA, which converts to locked-rotor current with the standard three-phase (÷ √3 × V) or single-phase (÷ V) kVA-to-current relationship. This calculator uses the midpoint of each letter's published range as a representative value.
Voltage sag at the motor bus (advanced/optional)
Starting current flows through the same source impedance serving every other load on that bus, so a large sudden current draw produces a proportional sudden voltage drop. This calculator estimates that sag as motor starting kVA divided by the source's available fault kVA, expressed as a percentage — a simplified single-impedance approximation (IEEE Std 141-style first pass) that ignores the motor's own X/R split and any intervening cable impedance. Source fault kVA can be entered directly from a fault current or fault study, or derived from a source/transformer's kVA rating and %Z as kVA ÷ (%Z ÷ 100) — the same methodology used for fault-current estimates on the Transformer Sizing Calculator.
NEC 430.24 — combining multiple motors for feeder sizing
A feeder or service supplying more than one motor is sized at the sum of every motor's full-load current, plus 25% of the single largest motor's full-load current — the extra 25% covers that motor's starting current on top of the others already running. This does not by itself determine conductor ampacity or overcurrent device rating, which require separate calculation per the applicable NEC articles.