In short. Starting kVA is the apparent power a motor draws at the instant of starting. For three-phase, starting kVA = 1.732 x V x Istart / 1000, where Istart is the locked-rotor current. A 415 V motor drawing 600 A at start demands about 431 kVA. The figure sizes the starting event; whether a generator survives it depends on the alternator’s motor-starting capability at the acceptable voltage dip and on the load already connected.
What starting kVA measures
When a squirrel-cage motor is switched on, the rotor is stationary and the stator draws locked-rotor current, limited only by the machine’s leakage reactance. That current, multiplied by the supply voltage, is the starting kVA. It is almost entirely reactive, with a power factor typically between 0.2 and 0.4, which is why it stresses the alternator’s voltage far more than it stresses the engine. The engine sees the real power only as the motor accelerates and begins to drive its load.
The quantity is useful because it puts the starting event on the same scale as the generator rating. A 45 kW motor is not a 45 kW problem to the generator; at start it may be a 300 kVA problem. Expressing it in kVA allows a direct comparison with the alternator’s published capability, which manufacturers also state in kVA at a given dip.
The formula and where the current comes from
Starting kVA = (1.732 x V x Istart) / 1000 for three-phase; Starting kVA = (V x Istart) / 1000 for single-phase
Here V is the line-to-line voltage in volts for three-phase or line-to-neutral for single-phase, and Istart is the starting current in amperes. The best source of Istart is the motor data sheet, which gives locked-rotor current either directly or as a ratio to full-load current. IEC motors carry an “Ia/In” figure for this purpose, and NEMA motors carry a code letter that gives locked-rotor kVA per kW of output. Where only the nameplate full-load current is known, a multiple of 6 to 7 is a reasonable screening assumption for standard motors, but high-efficiency IE3 and IE4 motors frequently draw 7 to 9 times full-load current, and that must be checked.
| NEMA code letter | Locked-rotor kVA per kW of output (approximate) | Typical application |
|---|---|---|
| F | 6.7 to 7.5 | Older standard motors |
| G | 7.5 to 8.4 | Common general-purpose motors |
| H | 8.4 to 9.5 | Many 2-pole and high-efficiency motors |
| J | 9.5 to 10.7 | High-efficiency motors |
| K | 10.7 to 12.0 | Small high-efficiency motors |
The code letter table is stated per horsepower in the NEMA standard; the figures above are converted to kW for Indian use and are approximate. For a code G motor of 37 kW, starting kVA is about 37 x 8 = 296 kVA, which agrees well with the locked-rotor current method for a typical machine. Use the manufacturer’s data when the two methods disagree.
Adjusting for the starting method
The formula gives the direct-on-line value. Reduced-voltage starting lowers the current the generator sees, and therefore the effective starting kVA, by a factor that depends on the method. The motor starting guide compares the methods in detail; the multipliers below are the usual screening values.
- Star-delta: about 33 percent of the DOL starting kVA, provided the motor reaches near full speed before the changeover to delta.
- Autotransformer at 65 percent tap: about 42 percent of DOL; at 80 percent tap, about 64 percent.
- Soft starter with current limit set at 3.5 times full-load current: starting kVA is 3.5 x full-load kVA, typically 50 to 60 percent of DOL.
- Variable-frequency drive: starting kVA is close to the running kVA, often 100 to 150 percent of full-load kVA, with harmonic consequences for the alternator.
Comparing with the generator
Alternator manufacturers publish motor-starting capability as the kVA that can be applied in one step from no load for a stated transient voltage dip, usually 20 or 30 percent, sometimes with a curve for intermediate values. This figure, not the alternator’s continuous rating, is what the starting kVA must be compared against. For a standard alternator the capability at 30 percent dip is often around 2.5 to 3 times the continuous rating, which means a DOL motor with starting kVA of about 25 percent of the set rating can usually be started from no load at a dip near 10 percent, and a motor at 60 to 70 percent of set rating will produce a dip near 30 percent. These are typical relationships and the actual alternator data must be used.
Two corrections matter. If the set already carries load when the motor starts, the available capability is reduced and the dip increases, so the starting kVA must be assessed on top of the existing load. And the dip at the alternator terminals is not the dip at the motor; cable impedance adds more. The voltage dip guide covers the full calculation, and the alternator motor-starting performance page explains why alternators of equal rating differ in capability.
A worked example (illustrative)
A water treatment plant near Vijayawada has a 37 kW borewell pump motor with a nameplate full-load current of 68 A and a data sheet locked-rotor ratio of 6.8. Starting current is 68 x 6.8 = 462 A, and starting kVA at 415 V is 1.732 x 415 x 462 / 1000 = 332 kVA. The plant’s other load when the pump starts is about 80 kVA of lighting, dosing pumps and controls. The total apparent demand during the start is therefore roughly 80 + 332 = 412 kVA, although the two components do not add exactly because their power factors differ; the arithmetic sum is a conservative screening figure.
The plant has been offered a 160 kVA set. On typical data such a set might start about 400 kVA from no load at a 30 percent dip, and considerably less with 80 kVA already connected, so the 412 kVA event would produce a dip well beyond 30 percent and most likely an under-voltage trip. The choices are a soft starter at 3.5 times full-load current, which brings the pump event down to about 171 kVA and a total near 250 kVA, well within the 160 kVA set’s capability at a modest dip, or a set in the 250 kVA class if the pump must remain DOL. The final decision needs the alternator’s actual starting curve and the acceptable dip for the plant’s controls.
Common mistakes
- Using six times full-load current for a high-efficiency motor whose locked-rotor ratio is eight or nine, which understates the starting event by a third.
- Comparing starting kVA with the generator’s continuous rating rather than its published motor-starting capability, which produces either a grossly oversized set or an unexplained trip.
- Ignoring the load already on the set, so a motor that starts easily during a test on an empty generator stalls during normal operation.
- Treating the starting kVA as a complete answer, when acceleration time, load torque, engine response and cable drop also decide whether the motor reaches speed.
What to send us
Send the motor nameplate and, if available, the data sheet showing locked-rotor current or code letter, the starting method and any current-limit setting, the load on the generator at the moment the motor starts, and the generator make, model and alternator type if a set already exists. A photograph of the starter panel and of the generator nameplate is usually enough to begin. Send your application engineering enquiry and we will calculate the starting kVA and compare it with the alternator’s published capability.
Questions engineers ask
Is starting kVA the same as locked-rotor kVA?
For a DOL start, yes; both describe the apparent power at standstill. For reduced-voltage or soft starting, the starting kVA the generator sees is lower than the motor’s locked-rotor kVA by the factor the starting method imposes.
What multiple of full-load current should I assume if I have no data?
Six to seven for older standard motors, and seven to nine for IE3 and IE4 high-efficiency motors. Label the result as an assumption and replace it with data sheet values before selecting a generator.
Can I simply add starting kVA to the running load?
Adding them arithmetically gives a conservative screening figure because the starting current is at a much lower power factor than the running load. Manufacturer sizing software combines them vectorially and will give a slightly lower total.
How much starting kVA can a generator handle?
It depends on the alternator, not the kVA rating alone. Typical alternators accept a single step of about 2.5 to 3 times their continuous rating at a 30 percent dip from no load, and much less if the set is already loaded. Read the manufacturer’s motor-starting curve.
Does starting kVA affect the engine?
Only indirectly. Starting kVA is mostly reactive and loads the alternator and its excitation. The engine responds to the real power as the motor accelerates, which is covered by load acceptance and frequency dip rather than starting kVA.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.