Motor Starting Current Calculator
Pick the motor, voltage and nameplate code letter to get locked-rotor amps, inrush multiple and expected voltage dip.
Step-by-step
- Enter valid values to begin.
FLC for the ratio comes from NEC Table 430.250 when the horsepower matches a table row. Voltage dip is the simple source-impedance approximation, kVAstart / (kVAsc + kVAstart), neglecting cable impedance and the motor power factor; run a motor-starting study for critical loads.
Motor Starting Current: Code Letters, LRA and What the Source Sees
Core Engineering Principles
When you close the contactor on a stopped induction motor, the rotor is standing still, so the rotating field cuts the rotor bars at full line frequency and induces maximum current. The motor is, for that moment, just a transformer with a shorted secondary. The impedance is low, the current is high, usually 5 to 8 times full-load amps, and it falls as the rotor speeds up and the slip drops. That initial current is called locked-rotor amps (LRA). It lasts a second or two on a fan or pump, and longer on a loaded crusher or compressor.
The NEC code letter on the nameplate tells you how much kVA per horsepower the motor draws at standstill. Code A is under 3.15 kVA/HP, code G is 5.6 to 6.29, and code V is 22.4 and up. Multiply by the horsepower and you have the starting kVA, then divide by √3 times the voltage for amps. That’s the number that hits the source, and it matters because the transformer, generator or the long cable feeding it responds by sagging in voltage. A deep sag can drop out contactors, trip drives and make the motor itself fail to accelerate.
Voltage dip ≈ kVAstart / (kVAsc,source + kVAstart) × 100
NEC & Standard References
NEC Table 430.7(B) lists the code letters: A is 0–3.14, B 3.15–3.54, C 3.55–3.99, D 4.0–4.49, E 4.5–4.99, F 5.0–5.59, G 5.6–6.29, H 6.3–7.09, J 7.1–7.99, K 8.0–8.99, L 9.0–9.99, M 10.0–11.19, N 11.2–12.49, P 12.5–13.99, R 14.0–15.99, S 16.0–17.99, T 18.0–19.99, U 20.0–22.39, V 22.4 and up. NEC 430.52 lets the short-circuit device be larger than full-load current precisely so it rides through that inrush. NEMA MG-1 12.35 sets locked-rotor current limits for design letters A, B, C and D. For the voltage dip, IEEE 141 (Red Book) and IEEE 399 give the method, and 10 to 15% is the usual limit at the motor terminals for starting.1. Starting kVA = 50 × 6.29 = 314.5 kVA.
2. LRA = 314,500 / (1.732 × 460) = 395 A, about 6.1 times the 65 A table FLC.
3. Source short-circuit capacity = 750 / 0.0575 = 13,043 kVA.
4. Voltage dip = 314.5 / (13,043 + 314.5) = 2.4%. Fine.
Put the same motor on a 150 kVA unit and the dip jumps to about 10.8%, which can drop out control relays.
- Use the nameplate letter. When there’s no letter, look for the LRA on the nameplate or use 6 times FLA as a rough value.
- Breaker trip curves. A magnetic-only breaker has to be set above the peak asymmetrical inrush, which can be 1.7 times LRA in the first half cycle.
- Generator sets feel it more. A genset has a much higher source impedance than a utility transformer, so check the dip carefully.
- Don’t restart hot motors in a row. Two starts in a row without a cool-down can overheat the rotor, even if the stator looks fine.
- Reduced-voltage starters cut the current but also the torque by the square of the voltage ratio.