Calculator
Formula Used
Input from kW: Base kVA = kW ÷ power factor.
Input from kVA: Base kW = kVA × power factor.
Input from HP: Base kW = HP × 0.746 ÷ efficiency.
Running load: Running kW = base kW × quantity × demand factor.
Running apparent load: Running kVA = base kVA × quantity × demand factor.
Starting kVA: Starting kVA = base kVA × quantity × starting multiplier.
Peak start check: Peak kVA = other running kVA + selected starting kVA.
Adjusted generator size: Required size = load × reserve factor ÷ derating factor.
Three phase current: Amps = kVA × 1000 ÷ (1.732 × voltage).
Single phase current: Amps = kVA × 1000 ÷ voltage.
How to Use This Calculator
Enter the project name, site voltage, phase, generator power factor, reserve percentage, and derating percentage.
Add each construction load in the sheet. Use kW, kVA, or HP. Enter quantity, demand percentage, power factor, efficiency, and starting multiplier.
Press Calculate. The result appears above the form and below the header. Review running load, starting load, required kVA, suggested generator size, and current.
Use CSV for spreadsheet records. Use PDF for a simple printable report.
Example Data Table
| Load | Qty | Input | PF | Demand | Start Factor |
|---|---|---|---|---|---|
| Tower crane | 1 | 30 HP | 0.85 | 100% | 3.5 |
| Site lighting | 1 | 12 kW | 0.95 | 100% | 1 |
| Concrete mixer | 1 | 18 kVA | 0.82 | 80% | 3 |
| Office cabins | 1 | 8 kW | 0.90 | 70% | 1.2 |
Generator Load Planning for Construction Sites
Why Load Planning Matters
Generator load planning supports safe site power. It helps a contractor choose capacity before renting or buying equipment. A good sheet separates running demand from starting surge. This matters because motors, pumps, compressors, and lifts can need high current for a short time.
What This Sheet Calculates
This calculator builds a practical load schedule. Each row can hold quantity, load type, power factor, efficiency, demand factor, and starting multiplier. You may enter kilowatts, kilovolt amps, or horsepower. The sheet uses the best available value. It then estimates running kW, running kVA, and possible starting kVA for each item.
Reserve and Derating
Construction sites rarely use every device at full load. Demand factor reduces a load when normal use is lower than nameplate rating. Reserve adds capacity for future tools, temporary lighting, and small changes. Derating covers hard site conditions. High altitude, high heat, dirty air, and long duty cycles can reduce useful generator output.
Starting Load Check
The starting check is important. The sheet assumes one main load starts while other selected loads are already running. That gives a conservative peak kVA. It is useful for tower cranes, hoists, concrete mixers, welders, saws, sump pumps, and air compressors. When exact motor data is available, use manufacturer locked rotor or starting current values.
Current and Site Distribution
Voltage and phase also affect cable and breaker planning. Three phase current is calculated from kVA, voltage, and the square root of three. Single phase current uses kVA and voltage only. These current values help with feeder selection, transfer switches, and temporary distribution boards. They do not replace electrical code review.
Using the Output
Use the output as a planning sheet. Compare the recommended kW and kVA with common generator ratings. Choose the larger requirement. Add professional judgment when loads are sensitive, nonlinear, or motor heavy. For final site work, confirm fuel type, duty rating, grounding, protection, harmonics, and local rules with a qualified electrician.
Record Keeping
Good records also help during inspections and handover. Save the calculated sheet with project notes, fuel assumptions, and equipment lists. Review it whenever the site layout changes. Recheck loads after adding heaters, dewatering pumps, chargers, or welding sets. A small update can prevent nuisance trips, low voltage, overheating, and costly downtime during critical work. It also improves rental comparisons because each option uses the same assumptions.
FAQs
1. What is a generator load calculation sheet?
It is a structured worksheet that lists site loads. It estimates running demand, starting surge, reserve, derating, current, and generator size.
2. Should I size by kW or kVA?
Check both values. The generator must handle real power in kW and apparent power in kVA. Use the larger final requirement.
3. Why is starting load higher than running load?
Motors often need extra current during startup. Pumps, cranes, compressors, and mixers can create a short surge before normal running speed.
4. What reserve percentage should I use?
Many planning sheets use 10% to 25%. Use more reserve when future tools, uncertain loads, or harsh conditions are expected.
5. What does derating mean?
Derating reduces available generator output. Heat, altitude, poor ventilation, fuel limits, and long duty cycles can reduce practical capacity.
6. Can this replace an electrician?
No. It supports planning only. Final cable size, protection, grounding, transfer switching, and code compliance need qualified review.
7. What is a starting multiplier?
It estimates motor startup demand compared with running kVA. Soft starters and drives usually reduce this multiplier.
8. Why add demand factor?
Demand factor reflects normal usage. It prevents oversizing when equipment is not expected to run at full nameplate load continuously.