Inputs
Example Data Table
| Scenario | Total kW | Demand | PF | Eff (%) | Growth (%) | Margin (%) | Phase | V | Typical Recommended kVA |
|---|---|---|---|---|---|---|---|---|---|
| Small site office + lighting | 12 | 0.85 | 0.95 | 98 | 15 | 20 | 1 | 230 | 25 |
| Rebar + cutting tools | 35 | 0.80 | 0.90 | 98 | 20 | 25 | 3 | 400 | 100 |
| Concrete batching support loads | 70 | 0.75 | 0.88 | 98 | 25 | 25 | 3 | 400 | 160 |
| Workshops + compressors | 110 | 0.80 | 0.85 | 97 | 30 | 30 | 3 | 415 | 315 |
| Large temporary power distribution | 180 | 0.85 | 0.90 | 98 | 35 | 25 | 3 | 400 | 500 |
Example values are illustrative; always verify against equipment nameplates and starting conditions.
Formula Used
kWdem = kWtotal × Demand
kVAbase = kWdem ÷ (PF × Eff) (Eff as per-unit, e.g., 98% → 0.98)
kVAreq = kVAbase × (1 + Growth) × (1 + Margin) (Growth/Margin in per-unit, e.g., 25% → 0.25)
kVAadj = kVAreq ÷ (1 − Derating) (simple ambient/altitude allowance; verify with manufacturer data)
Current: I = (kVA × 1000) ÷ (√3 × V) for three-phase, and I = (kVA × 1000) ÷ V for single-phase.
How to Use This Calculator
- Sum equipment nameplate real power to get total kW.
- Apply a demand factor if loads will not run together.
- Enter realistic power factor and efficiency assumptions.
- Add growth and continuous margins to match your project needs.
- Choose phase and enter the correct supply voltage.
- Optionally set ambient temperature and altitude for derating.
- Press Calculate and review kVA, current, and notes.
- Use the CSV/PDF buttons to save the calculation record.
Professional Article
1) Why sizing matters on active sites
Temporary power systems face changing crews, rotating equipment, and shifting duty cycles. Undersized transformers cause nuisance trips, voltage drop, and overheating. Oversized units add cost, increase no‑load losses, and can reduce efficiency at light loading. A documented sizing method supports safer installations and predictable commissioning.
2) Start with connected real power
Build a load schedule from nameplates: lighting, welders, rebar cutters, compressors, hoists, pumps, and site offices. Convert any horsepower ratings to kW and apply realistic operating assumptions. The calculator begins with total real power in kW, which reflects the useful work demand before electrical conversion effects.
3) Apply demand factor to reflect diversity
Demand factor accounts for non‑coincident operation. For example, a batching plant and a welding bay may not peak at the same time. Typical site demand ranges from 0.6 to 0.9 depending on supervision and sequencing. Using demand reduces oversizing while still honoring likely peak conditions.
4) Convert kW to kVA using power factor
Transformers are rated in kVA, not kW. Power factor captures reactive current from motors and magnetic devices. A PF of 0.85 means you need about 18% more apparent power than real power. Improving PF with correction banks can reduce kVA, feeder current, and voltage drop.
5) Include efficiency and continuous margins
Efficiency allowance converts delivered kW to required input kVA. Continuous margin adds headroom for long shifts and warmer enclosures. Many projects target 15% to 30% margin to keep temperatures stable, limit insulation aging, and support continuous duty without operating at the edge of rating.
6) Plan for growth and starting currents
Future growth margin covers added cabins, extra tools, and later trade packages. Motor starting, welder inrush, and harmonic distortion can create short peaks that exceed steady‑state calculations. Where large motors or VFDs exist, review manufacturer data and consider additional kVA headroom and harmonic mitigation. When estimating peaks, review cable lengths, temporary distribution boards, and allowable voltage drop. A modest buffer can prevent nuisance shutdowns during simultaneous starts, especially when generators or long feeders supply the transformer on busy pours and night shifts.
7) Account for ambient temperature and altitude
Cooling capability drops at high ambient temperature and reduced air density. This calculator applies a simple derating approach above 40 C and above 1000 m to illustrate risk. Always confirm the final selection with transformer datasheets, cooling class, ventilation, and enclosure conditions.
8) Select a standard rating and document the decision
Procurement and logistics typically require standard kVA sizes. After calculating adjusted kVA, round up to the next standard rating, then review expected utilization to avoid excessive oversizing. Exporting CSV or PDF provides an auditable record for project files, inspections, and future site expansions.
FAQs
1) Should I enter lighting and small power together?
Yes. Include all steady loads, then apply a demand factor to reflect real usage patterns. Keep separate notes for intermittent tools or seasonal loads so you can revise assumptions easily.
2) What power factor should I use if I do not know it?
For mixed motor and lighting site loads, 0.85 to 0.95 is common. If you have many motors, start nearer 0.85. If most loads are LED lighting and electronics, use 0.95.
3) Why does low power factor increase transformer size?
Low power factor means more current is needed for the same real power. Transformers are limited by current and heating, so apparent power (kVA) rises as power factor falls.
4) How do I handle large motor starting or welding peaks?
Add extra margin or model the highest starting scenario separately. Consider soft starters, VFDs, or sequencing. For welders, check duty cycle and peak kVA from the nameplate.
5) Is the ambient and altitude derating exact?
No. It is a simplified, conservative illustration. Final derating depends on transformer design, cooling method, and manufacturer curves. Use the results as a screening tool, then verify specifications.
6) Which standard set should I select?
Select the list that matches your procurement market and utility practices. If you are unsure, use the set most common for your region and confirm availability with suppliers before ordering.
7) What does utilization percentage tell me?
Utilization compares calculated adjusted kVA to the selected standard rating. Very low utilization may indicate oversizing, while very high utilization suggests limited headroom for peaks, harmonics, or future expansion.
Size the transformer right, then build with confidence daily.