Motor starting current — Quick answer
Motor starting current (also called inrush or locked-rotor amps, LRA) is the surge current drawn when an induction motor energises before it spins up. Typically 5-8× the full-load current.
Istart = Kstart × IFLA
Where Kstart = 6 (DOL), 2 (star-delta), 2–3 (soft-start), 1–1.5 (VFD)
- IFLA — motor full-load amps (nameplate)
- Kstart — starting method multiplier
- LRA — locked-rotor amps (~7× FLA for NEMA Design B)
Worked example: 22 kW three-phase 415 V motor, FLA = 40 A. Direct-on-line start: Istart = 6 × 40 = 240 A. Star-delta starter: 2 × 40 = 80 A. VFD: roughly 40–50 A (rated-current limited).
Motor starting current by method
| Starting method | Multiplier | Torque | Use case |
|---|
| Direct-on-line (DOL) | 6–8 × FLA | 100% rated | Small motors < 7.5 kW |
| Star-delta (Y/Δ) | 2 × FLA | 33% rated | Light-load starts, fans, pumps |
| Auto-transformer | 1.5–4 × FLA | 50–80% | Medium motors with reduced-torque start |
| Soft-starter | 2–3 × FLA | Adjustable | Conveyors, mills, controlled ramp |
| VFD / inverter | 1.0–1.5 × FLA | Full at any speed | Variable-speed drives, modern installs |
Standard / source: NEMA MG 1 (motor design classes); IEC 60034-12 (starting performance); NEC NFPA 70 Article 430 (motor branch circuits).
Used for: Switchgear breaking-capacity selection, transformer voltage-dip checks, generator sizing, cable sizing for motor branch circuits, soft-starter / VFD selection.
Standards & method
✓ Formula independently verified 12 July 2026- Governing standard
- NEC Table 430.7(B)
- Clauses applied
- Locked-rotor kVA/HP by NEMA code letter
- Upper bound of each band — the design case
- Core formula
I_LR = code letter kVA/hp × hp × 1000 / (√3 × V) · DOL ≈ 6–8× FLC- Why this matters
- Locked-rotor current is 6–8× full-load for DOL starting, at a power factor of only 0.2–0.35. That inrush is what collapses terminal voltage and drops out contactors — and it is why the starting case, not the running case, often governs the cable.
- Independently verified
- 12 July 2026 — Formula re-derived from the standard and checked numerically against worked reference cases from the code book, not merely tested for “returns a number”.
Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.
Motor Inrush Current Principles
When an AC induction motor is energized, it draws a significant surge of current, known as starting current or inrush current, before settling down to its normal run current (Full Load Amps).
The multiplier depends on the starting method:
- Direct On Line (DOL): 500% to 800% of FLA
- Star-Delta: Approx. 33% of DOL current (~200% FLA)
- Soft Starter: 200% to 400% of FLA
- VFD: 100% to 200% of FLA
Frequently Asked Questions
How many times is starting current compared to full load current?
⌄
For a Direct-On-Line (DOL) starter, the starting current is typically 6 times (or 600% of) the motor's Full Load Amps (FLA).
What is motor starting current?
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Motor starting current (inrush or locked-rotor current) is the high current drawn the moment a motor starts, when the rotor is stationary. It typically equals 6–8× the motor's full-load current (FLC) for direct-on-line (DOL) starting. This surge typically lasts 2–10 seconds while the rotor accelerates to rated speed, then current drops to the normal running level.
Why does a motor draw so much current at startup?
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At startup, the stationary rotor generates zero back-EMF (counter-electromotive force). Without back-EMF to oppose the supply voltage, the stator windings act as near-short-circuit load. As the rotor accelerates, back-EMF builds up and limits current to the normal operating value. The starting current is limited primarily by stator impedance, which is low at standstill.
What motor starting methods reduce inrush current?
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Methods to reduce motor starting current: Star-Delta starter — reduces starting voltage to 1/√3, limiting starting current to ~33% of DOL (but starting torque also reduces to 33%); Autotransformer starter — selectable 50/65/80% voltage taps; Soft starter — electronic gradual voltage ramp, 2–4× FLC starting current; VFD (Variable Frequency Drive) — smoothest start, typically 1.5× FLC, highest cost and most flexible.
How does motor starting current affect transformer sizing?
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When a motor starts, the inrush current (typically 600–800% FLC) causes a voltage dip at the transformer secondary proportional to: ΔV% ≈ (Istart × %Ztransformer) / Irated_transformer × 100. A large motor starting on a small transformer can cause a 10–20% voltage dip, tripping sensitive equipment. Rule of thumb: motor kW should not exceed 25–33% of transformer kVA for DOL starting.
What is a NEMA locked-rotor code letter?
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NEMA assigns code letters (A–V) to motors indicating locked-rotor kVA per horsepower. Common codes: Code F — 5.0–5.6 kVA/hp; Code G — 5.6–6.3 kVA/hp; Code H — 6.3–7.1 kVA/hp; Code J — 7.1–8.0 kVA/hp; Code K — 8.0–9.0 kVA/hp. Higher letters mean higher starting current. Most standard NEMA squirrel-cage motors are Code G or H. The code letter is on the motor nameplate.