Example Data Table
| Load Type | Running kW | Power Factor | Demand Factor | Starting Note |
|---|---|---|---|---|
| Lighting and panels | 18 | 0.95 | 100% | No large surge |
| Workshop tools | 35 | 0.85 | 70% | Intermittent use |
| Water pump motor | 22 | 0.82 | 100% | Soft starter |
| Cooling equipment | 25 | 0.80 | 90% | Compressor surge |
Formula Used
Running kVA = Running kW ÷ Power Factor. If known kVA is entered, the calculator uses that value.
Demand kVA = Running kVA × Demand Factor.
Motor Running kVA = Motor HP × 0.746 ÷ Motor Efficiency ÷ Motor Power Factor.
Motor Starting kVA = Motor Running kVA × Starting Multiplier.
Extra Starting kVA = Motor Starting kVA − Motor Running kVA.
Pre-Derate kVA = Demand kVA + nonlinear allowance + imbalance allowance + future growth + reserve + extra starting kVA.
Required kVA = Pre-Derate kVA ÷ Derate Factor.
Line Current = kVA × 1000 ÷ √3 ÷ Line Voltage.
Fuel Use = Engine kW × Fuel Rate × Operating Time.
How To Use This Calculator
- Enter the total running load in kilowatts.
- Add known kVA if your load schedule already provides it.
- Enter voltage, power factor, and demand factor.
- Add growth, reserve, imbalance, nonlinear load, and derating values.
- Enter the largest motor size and select the starting method.
- Press the calculate button.
- Review the required kVA and recommended standard size.
- Download the CSV or PDF report for records.
About 3 Phase Generator Sizing
Why Proper Sizing Matters
A three phase generator must match real electrical demand, not only nameplate totals. Many sites add motor loads, compressors, pumps, lighting, tools, chargers, and control panels. Each load can behave differently when it starts. This calculator helps convert those details into a practical generator size.
Running Load And Demand
The first step is the running load. Enter connected kilowatts and a demand factor. The demand factor reflects how much of the equipment runs together. A workshop may not use every tool at the same time. A pumping station may run near full demand for long periods.
Power Factor And kVA
Power factor is also important. Generators are normally rated in kVA. Loads are often described in kW. The link between them is power factor. A lower power factor needs more kVA for the same useful power.
Motor Starting Demand
Motor starting is the largest hidden issue. A motor can draw several times its running kVA during starting. Across the line starting can be severe. Soft starters and drives can reduce the surge. The calculator adds only the extra starting demand above running load.
Derating And Reserve
Derating protects the estimate. High altitude reduces engine output. High temperature also reduces capacity. Nonlinear loads and phase imbalance can add heat and stress. Reserve margin gives space for future equipment and stable voltage.
Practical Planning
The recommended size is not a final engineering approval. It is a planning estimate. Real projects need site measurements, utility rules, local codes, and manufacturer data. Fuel quality, transient response, voltage dip limits, and enclosure ventilation also matter.
Choosing A Final Unit
Use the result to compare generator classes. Check running kVA, starting kVA, estimated current, and fuel usage. Then choose the next standard size above the requirement. A slightly larger unit often runs cooler and starts motors better. An oversized unit can waste fuel at light load. Balance is the goal. This tool gives a clear starting point for that decision.
Where It Helps
It is useful for farms, shops, events, clinics, small factories, and backup systems. Enter conservative values when data is uncertain. Review every large motor separately. Check whether critical equipment must start together or in sequence. Sequenced starting can reduce required capacity and cost. Always confirm the final selection with a qualified electrician, generator supplier, or design engineer before purchase or installation. It also improves maintenance planning.
FAQs
What is a 3 phase generator sizing calculator?
It estimates the kVA rating needed for a three phase generator. It uses running load, power factor, motor starting, reserve margin, and derating values.
Why is generator size shown in kVA?
Generator alternators are commonly rated in kVA. kVA includes real power and reactive power. It gives a better view of electrical capacity.
Can I use only total kW?
Yes. Enter total kW and power factor. The calculator converts kW into kVA. Known kVA can also be entered when available.
Why is motor starting important?
Motors can need several times their running current at startup. This temporary surge may control the required generator size.
What does derating mean?
Derating reduces available generator output for harsh conditions. High altitude, high temperature, and site limits can reduce safe capacity.
Should I add reserve margin?
Yes. Reserve margin supports future loads and steadier voltage. Many users add 10% to 25%, depending on the project.
Does this replace an engineer?
No. It is a planning tool. Final generator selection should follow local codes, site measurements, and manufacturer guidance.
How do I reduce generator size?
Use sequenced starting, soft starters, or variable frequency drives. Reducing simultaneous motor surge can lower required capacity.