Starting kVA depends on the voltage and starting current for the stated supply arrangement. The starting-current input should come from the relevant motor or starting-system data, or an identified measurement. A generic multiplier should not be treated as a verified value for every motor and starter combination.
Keep apparent power separate from generator selection
The calculation describes one aspect of the starting event. It does not account by itself for acceleration duration, voltage recovery, engine response or the other connected loads. The result should therefore accompany the wider motor-starting information when a generating set is assessed.
What this number does not tell you
- voltage dip
- frequency dip
- motor acceleration
- alternator recovery
Better input
Where available, use motor/manufacturer data rather than a generic multiple of full-load current.
Starting kVA describes the size of the electrical event. Generator transient data determines how the machine handles it.
Three-phase starting kVA
Starting kVA = (√3 × VLL × Istart) ÷ 1000. VLL is line-to-line voltage in volts and Istart is line starting current in amperes. At 415 V and 600 A, starting apparent power is approximately 431 kVA. Starting duration, acceptable voltage dip, other connected loads and the generating-set response still require assessment; 431 kVA is not a generator recommendation.
State the conditions behind the example
The worked example on this page illustrates the relationship between line voltage, starting current and three-phase apparent power. It is not a recommended generator rating. Retain the units and supply assumptions when reusing the calculation, and confirm the actual equipment data before making a selection.
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Related guidance
Engineering intent
Starting kVA should be evaluated as part of the complete power system rather than as an isolated catalogue item. Operating duty, connected load, interfaces, protection, controls, installation conditions, testing requirements and lifecycle consequences determine whether the arrangement is appropriate.
Engineering considerations
But motor-starting risk also depends on when the motor starts. Does it start while other major loads are already running? A smaller motor starting at the wrong time can create a larger system problem than a bigger motor starting under controlled conditions. Starting current is the current demanded during motor acceleration. It should not be assumed from a generic rule unless no better data exists and the assumption is clearly marked. Where only estimates are available, mark the result as preliminary. Starting kVA expresses the apparent-power demand during motor starting. This number helps quantify the size of the starting event. But it does not by itself prove the generator will start the motor successfully.
Selection and design considerations
Motor terminal voltage during starting may be lower than generator terminal voltage. Motor starting should be tested or verified before relying on backup power.
| High preliminary | nameplate current and starting current known |
|---|---|
| Engineering required | critical load, high starting kVA, long cable, UPS load, public/healthcare/data-centre/pharma/cold-storage |
| running current and motor size both blank | request motor data |
| generator kVA blank | calculate starting kVA but cannot compare |
| Engineering review required | do not finalise from calculator |
| Need kW/kVA conversion | kW to kVA Calculator |
This result is preliminary and does not replace manufacturer motor data, generator transient-performance data, voltage-dip study, cable voltage-drop review, Battery Backup Is a Time Window — Not a Complete Power Strategy.
Integration and protection considerations
Total apparent starting event ≈ 80 + 216 = 296 kVA The generator may need to withstand approximately 296 kVA during the starting event, This is a simplified screening result, not a final design. Generator acceptance depends on alternator, engine transient response, voltage dip, frequency recovery, cable voltage drop, starting method and load sequence. A motor may have a moderate running load but a severe starting event. But the root cause may be sizing, starting method, voltage drop, load sequence, cable length or protection setting. Mechanical and process loads need additional engineering analysis because of starting characteristics, duty cycles and criticality.
| Healthcare | pumps, HVAC, ventilation, lifts, medical facility support |
|---|---|
| Data centres | cooling pumps, CRAC/CRAH units, chillers |
Testing, commissioning and verification
Therefore voltage dip is not only a generator number. “X kVA generator will produce Y% voltage dip for Z motor.” Voltage response is primarily connected to the electrical machine and excitation system. Frequency response is strongly connected to engine torque and speed. The frequency event depends on the generator and load step. Sensitive equipment may have frequency limits or acceptance windows.
Operating and lifecycle considerations
Those depend on the complete generator–motor–cable system. Generator transient data and system design determine how the event is handled. Motor-starting discussions often use generic multiples of full-load current. That can be useful for preliminary screening, but it is not a final engineering basis. For production-grade sizing, use actual motor/starter data wherever possible. Direct-on-line starting often creates the highest direct inrush among common starting arrangements. Do not assume a generator can support DOL starting merely because the motor runs normally afterward. Star-delta starting changes the starting current and torque characteristics. It may reduce the starting demand seen by the source compared with direct-on-line starting.
Direct answer
Starting kVA depends on the voltage and starting current for the stated supply arrangement. The starting-current input should come from the relevant motor or starting-system data, or an identified measurement.
The practical decision for Starting kVA should be based on verified load, equipment, interface and site information. Where equipment-specific data, regulatory applicability or a project study is required, this guidance does not replace that verification.
Related Reddy Generators pathways
- Power Engineering Centre
- Generator Finder
- Integrated Power Systems
- Industry Power Solutions
- Application Engineering Enquiry
Starting-system reliability chain
Generator starting depends on the complete DC chain: battery → terminals and cables → control logic → starter relay/solenoid → starter motor → engine cranking. A healthy open-circuit battery voltage does not prove starting capability; cranking voltage, voltage drop, starter current, grounding and charger performance must be considered together.