These three names describe the same event, and the confusion around them is why you will find near-identical calculators for each. Starting current is the general term. Inrush current is the same surge, viewed from the supply side. Locked-rotor current (LRA) is the specific, nameplate-rated value you read off the nameplate or derive from the code letter.
The general term for the elevated current a motor draws while it accelerates from standstill to running speed. It is the event itself — the surge that occurs during startup.
The same surge, viewed from the supply side — it is what the generator, transformer, and cables momentarily have to deliver. Inrush is the demand on the supply system.
The specific, nameplate-rated value of that surge — the current the motor would draw if the rotor were held stationary and full voltage applied. LRA is the rated value you read off the nameplate.
All three are typically 5 to 8 times the full-load current, and the number is dominated by the motor's winding impedance. The starting power factor is low — around 0.15 to 0.30 — and the resulting kVA is even higher than the current alone suggests. This is why a motor's starting kW can look modest while its starting kVA is enormous. You size the generator on the kVA, not the kW.
Starting current, inrush, and locked rotor describe the same surge from different perspectives. The terms are often used interchangeably — but each has a specific meaning on a nameplate.
The starting surge is typically 5 to 8 times the motor's full-load current. This is the number that stresses the generator, cables, and protection devices.
At startup, the power factor drops to 0.15–0.30, making the starting kVA much larger than the starting kW. This is why generators are sized on kVA, not kW.
The locked-rotor current is the rated value on the motor nameplate — either direct or derived from the NEMA code letter. Use it for accurate generator sizing.
The calculation runs in two steps, each with a different source of truth. Step 1 finds the full-load current (FLA) from rated power, voltage, efficiency, and power factor. Step 2 applies a starting-current multiplier based on the starting method — DOL, Star-Delta, Soft Starter, or VFD.
Enter the motor's horsepower or kilowatts, voltage, efficiency, and power factor. The tool returns the full-load current (FLA) in amps. Then choose a starting method to see the inrush surge.
Enter the motor's horsepower and NEMA code letter from the nameplate. The tool returns the locked-rotor current (LRA) and starting kVA directly from the code letter value.
The starting method is a decision, not a detail — and it changes the current, the torque, and the generator size all at once. The table below uses a 30 kW, 380 V motor drawing 58.3 A full‑load as the reference.
| Starting Method | Current Multiplier | Starting Current | Approx. Starting Torque | Effect on Generator Size | Best For |
|---|---|---|---|---|---|
| DOL Direct-on-Line |
6–8× | ~350–466 A | 100% | Largest generator | High breakaway torque |
| Star-Delta Wye-Delta |
2–3× (≈⅓ of DOL) | ~117–175 A | ~33% | 1–2 sizes smaller | Centrifugal loads |
| Autotransformer 65% Tap |
~2.5–3.5× | ~146–204 A | ~42% | Reduced by tap² | Better torque than star-delta |
| Soft Starter Ramped Voltage |
2–4× (adjustable) | ~117–233 A | 25–100% (adjustable) | Smallest practical set | Retrofit, marginal existing sets |
| VFD Variable Frequency Drive |
1–1.5× | ~58–87 A | 100% (any speed) | Smallest generator | Lowest inrush, variable speed |
For North American motors, the nameplate's code letter (A through V) encodes the locked‑rotor kVA per horsepower, per NEC Table 430.7(B). From it you get the starting current without guessing the multiplier. Most standard three‑phase induction motors are Code F, G, or H (5.0–7.1 kVA/hp).
A: 0–3.14 |
B: 3.15–3.54 |
C: 3.55–3.99 |
D: 4.0–4.49 |
E: 4.5–4.99
These code letters indicate lower locked‑rotor kVA per horsepower. Found on
high‑efficiency or specialty motors with reduced inrush.
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
Most standard three‑phase induction motors fall in F, G, or H.
This is the range you will see on most industrial motors.
L: 9.0–9.99 |
M: 10.0–11.19 |
N: 11.2–12.49 |
P: 12.5–13.99
Higher kVA per horsepower means higher starting current
for the same motor size. Common on motors with higher locked‑rotor torque
or lower impedance.
R: 14.0–15.99 |
S: 16.0–17.99 |
T: 18.0–19.99 |
U: 20.0–22.39 |
V: 22.4 and above
These are specialty motors with very high inrush — often encountered on
high‑torque, high‑inertia loads like crushers and large compressors.
Starting kVA = kVA/hp × hp
ILR = SkVA × 1000 ÷ (√3 × V)
A 50 hp, 460 V, Code G motor (up to 6.29 kVA/hp) draws
314.5 kVA and 395 A locked‑rotor.
Always read the letter — never assume the multiplier.
5.0–7.1 kVA/hp — the range for most standard three‑phase induction motors. A 50 hp Code G motor starts at 314.5 kVA. The same motor at Code H would draw more — which is why you read the letter rather than assume the multiplier.
The NEMA code letter gives you the locked‑rotor kVA per horsepower. From that single nameplate value, you can calculate both the starting kVA and the locked‑rotor current for any motor — without guessing the starting‑current multiplier.
Locate the motor's code letter on the nameplate. Use the NEC Table 430.7(B) to find the corresponding kVA per horsepower range. For Code G, the range is 5.6–6.29 kVA/hp.
SkVA = kVA/hp × hp
For a 50 hp motor with Code G (6.29 kVA/hp):
SkVA = 6.29 × 50 = 314.5 kVA.
This is the apparent power the generator must deliver during startup.
ILR = SkVA × 1000 ÷ (√3 × V)
At 460 V: ILR = 314,500 ÷ (1.732 × 460) = 395 A.
This is the starting current the generator, cables, and breakers must handle.
Most standard three‑phase induction motors are Code F, G, or H (5.0–7.1 kVA/hp). Code G is the most common mid‑band estimate for Design B motors. Always confirm the actual letter on the nameplate before calculating.
A 50 hp Code G motor draws 314.5 kVA at start. The same motor at Code H (7.09 kVA/hp) draws 354.5 kVA — 13% more. This is why you must read the letter, not assume a generic multiplier.
The starting kVA is what sizes the generator, not the amps alone. A 50 hp Code G motor requires 315 kVA of generator transient capability. The amp value (395 A) is used for cables and breakers — but the generator is sized on kVA.
Four examples, calculated with the formulas above, to show the shape of the answer. Each demonstrates the starting current and SkVA for a different motor scenario.
FLA = 30,000 ÷ (1.732 × 380 × 0.92 × 0.85) = 58.3 A
DOL (6×): 350 A |
Star-Delta (2×): 117 A
Soft Starter (3×): 175 A |
VFD (1.2×): 70 A
The method-comparison reference.
SkVA = 7.10 × 100 = 710 kVA
ILR = 710,000 ÷ (1.732 × 460) = 891 A
A large industrial motor — the locked‑rotor current is near 900 A.
SkVA = 6.29 × 50 = 314.5 kVA
ILR = 314,500 ÷ (1.732 × 460) = 395 A
The code-letter reference — 395 A locked‑rotor.
At 5.6 kVA/hp: SkVA = 840, ILR = 1,054 A
At 6.29 kVA/hp: SkVA = 944, ILR = 1,184 A
A big pump or crusher motor — starting kVA of roughly 840–945 kVA.
The starting current is an order of magnitude larger than the running current, and it is the kVA — not the amps — that sizes the generator. The exact generator for that SkVA, and the voltage dip it produces, is covered on the companion motor starting generator sizing calculator.
Final motor starting verification should be checked against the motor's nameplate code letter, the generator's transient capability, and the acceptable voltage dip. The exact generator for the SkVA you calculate here is covered on our companion motor starting generator sizing calculator.
Use the motor starting current calculator above, then feed the starting kVA into our motor starting generator sizing calculator to find the generator and the voltage dip, and convert the running load with our generator hp to kVA calculator.
A: Multiply the motor’s full-load current by a starting-current multiplier. Full-load current is P ÷ (√3 × V × η × cos φ) for three-phase; the multiplier is 6–8 for direct-on-line, 2–3 for star-delta, 2–4 for soft start, and 1–1.5 for VFD.
A: Locked-rotor current (LRA) is the current a motor draws at standstill with full voltage applied — the rated value of its starting surge. It is typically 5 to 8 times the full-load current and can be derived from the NEMA code letter.
A: A direct-on-line motor draws roughly 6 to 8 times its full-load amps at startup. A 30 kW, 380 V motor that runs at 58.3 A therefore draws about 350 A on a DOL start, and less if a star-delta, soft starter, or VFD is used.
A: They describe the same surge. Starting current is the elevated current during acceleration; inrush current is the same surge viewed from the supply side — what the generator, transformer, and cables must deliver. Locked-rotor current is the nameplate-rated value of that surge.
A: Find the code letter (A–V) on the motor nameplate, read its locked-rotor kVA per horsepower, then multiply by the horsepower to get starting kVA. Convert to current with I = SkVA × 1000 ÷ (√3 × V).
A: A VFD reduces starting current the most, to roughly 1 to 1.5 times full-load current, because it controls the current directly. Soft starters and star-delta reduce it to about 2 to 4 times, with a corresponding loss of starting torque.
A: A 100 hp, 460 V motor at NEMA Code H (7.10 kVA/hp) draws about 891 A locked-rotor. On a direct-on-line start a similar 100 hp motor draws roughly 6 to 8 times its full-load current, around 700 to 900 A.
A: Starting kVA (SkVA) is the apparent power a motor draws at startup, equal to the locked-rotor current times the voltage times 1.732 divided by 1000. Because the starting power factor is low, SkVA is much higher than the starting kW — and it is the figure the generator must be sized for.
