The safest purchasing path is to size from the motor nameplate and operating sequence, then verify the selected generator against the allowable voltage dip. A fixed HP-to-kVA chart cannot account for every motor or starting system.
- Send the nameplateHP or kW, voltage, phase, frequency and full-load current.
- Confirm how it startsDOL, star-delta, soft starter or VFD, plus starts per hour.
- List simultaneous loadsPumps, compressors, lighting and equipment that remain online.
10HP, 20HP and 50HP motor sizing starting point
Horsepower conversion shows shaft output only. It does not include motor efficiency, starting current, voltage dip or the other loads already connected to the generator.
| Motor rating | Approx. shaft output | What must be checked before choosing generator kVA |
|---|---|---|
| 10HP motor | 7.46kW | Nameplate full-load current, DOL or reduced-current start, driven load and simultaneous load. |
| 20HP motor | 14.92kW | Motor efficiency, starting method, acceleration time, starts per hour and allowable voltage dip. |
| 50HP motor | 37.30kW | Complete load schedule, starter or VFD data, alternator transient response and site duty. |
These figures are not generator recommendations. Use the motor-load RFQ below so the proposed size can be checked from actual electrical data.
Details needed before sizing a generator for a three-phase motor
Start with the motor nameplate, not an estimated horsepower. The key values are rated HP or kW, voltage, phase, frequency, full-load current and efficiency where shown. Then describe what the motor drives and how it starts.
| Input to send | Why it changes the recommendation |
|---|---|
| Motor HP/kW and nameplate photo | Confirms the actual rated machine and electrical data. |
| DOL, star-delta, soft starter or VFD | Each method creates a different starting-current and torque profile. |
| Pump, compressor, fan or conveyor | The driven load affects starting torque and acceleration time. |
| Starts per hour and starting sequence | Frequent or simultaneous starts can be more demanding than one isolated start. |
| Other simultaneous loads | The generator must carry the existing running load while the motor starts. |
| Allowed voltage dip | Sensitive controls may require a tighter transient-voltage limit. |
Why a water-pump motor can need more generator capacity at startup
An induction motor can draw substantially more current while it accelerates than it uses in normal operation. The actual starting demand depends on the motor, driven load and starting method. That temporary demand can cause voltage dip or prevent the pump from accelerating if the generator is selected only from steady running kW.
DOL starting usually places a larger immediate demand on the generator than a correctly applied reduced-voltage starter. Star-delta, soft starters and VFDs can change the starting profile, but they do not remove the need to check torque, harmonics, control settings and the generator alternator response.
Example: what size generator for a 10HP motor?
The electrical input is higher than shaft output because the motor is not 100% efficient. The generator must also handle starting kVA, voltage dip and every load that remains online. Two 10HP pumps can require different generator selections when one is DOL, the other uses a VFD, or the site starts both pumps together.
For a 20HP or 50HP pump, follow the same process: collect the nameplate and operating sequence, model the largest load step, then validate the proposed generator and alternator. Critical fire-pump, process-water and production applications should also be reviewed by the responsible electrical engineer and equipment manufacturers.
How to request a checked motor-load generator quote
Send one clear nameplate photo for each large motor, a list of the smaller loads, the starting method, the sequence in which equipment starts, required voltage and frequency, daily runtime, ATS requirement and delivery city. Tell us whether the generator is emergency standby or a daily prime-power source.
We can then compare suitable open, silent or containerized generator options. Nigeria Generator Supply is a China-based factory-direct supply website. A Nigerian warehouse, installation team or local service office is included only when specifically written in the quotation.
Technical basis and further reading
For engineering background, see Caterpillar's guidance on generator-set sizing and motor-load sizing, plus the Cummins generator application manual. Final selection should use the proposed equipment data rather than a general online rule.
Build a three-phase motor generator RFQ
Complete the known motor and site details. Attach the nameplate photo after WhatsApp opens.
FAQ
What size generator is needed for a 10HP three-phase motor?
A 10HP motor equals about 7.46kW of mechanical output, but that is not the required generator size. Motor efficiency, full-load current, starting method, voltage dip and simultaneous loads must be checked.
Why does a water pump need extra generator starting capacity?
An induction motor can demand much more current while starting than while running. The exact effect depends on the motor, pump load, starting method and acceptable voltage dip.
Does star-delta, soft starter or VFD change generator sizing?
Yes. The starting method changes current, torque and acceleration behavior, so it must be included in the sizing review.
Can I send a motor nameplate photo on WhatsApp?
Yes. A clear nameplate photo plus the starting method and list of simultaneous loads is the best starting point for a useful quotation.
