Enter Site Loads and System Assumptions
Example Data Table
| Load | Qty | Running W | Surge mult. | Duty % |
|---|---|---|---|---|
| LED work lights | 20 | 50 | 1.0 | 100 |
| Concrete mixer | 1 | 1500 | 3.5 | 60 |
| Angle grinders | 4 | 900 | 1.3 | 50 |
| Submersible pump | 1 | 1100 | 4.0 | 40 |
| Air compressor | 1 | 2200 | 5.0 | 30 |
Formula Used
- Running load (W): Σ(Qty × Running W × Duty).
- Surge load (W): Σ(Qty × Running W × Surge multiplier).
- Demand load (W): (Running × Diversity) ÷ Efficiency.
- Growth and surge allowance (W): Demand × (1 + Growth) + (Surge − Running) × Simultaneous ÷ Efficiency.
- Peak load (W): (Growth and surge allowance) × (1 + Safety margin).
- Apparent power (VA): Peak W ÷ Power factor; kVA = VA ÷ 1000.
- Current (A): Single-phase: VA ÷ V; Three-phase: VA ÷ (√3 × V).
- Breaker estimate: apply a continuous factor and 80% utilization, then round up to a standard size.
How to Use This Calculator
- Enter your site voltage and choose single-phase or three-phase.
- Add each tool or machine with quantity and running watts.
- Set a surge multiplier for motors, pumps, compressors, and mixers.
- Adjust diversity, efficiency, growth, and safety margins to match conditions.
- Click Calculate to see kVA, amps, and breaker guidance.
- Use Download CSV or Download PDF for reporting.
Load inventory with realistic running watts
Start with a site load list that uses nameplate watts or measured values. Typical LED task lighting is 30–70 W per fixture, while handheld grinders often range 800–1,200 W. Enter quantities and apply duty percent to represent how long each load runs during a shift.
Account for motor starting and surge behavior
Motors and compressors draw higher power during start. Direct‑on‑line induction motors commonly need 3× to 6× running watts for a few seconds, while soft starters or VFDs can reduce that spike. Use the surge multiplier and the simultaneous start percentage to model how many motors start together.
Use diversity and duty cycle to reflect operations
Construction sites rarely run every tool at once. Diversity factors of 60–90% are common for mixed trades, and duty cycles can range from 20% for intermittent pumps to 100% for lighting. These inputs reduce oversizing while still maintaining dependable capacity.
Convert to kVA and current for distribution planning
Supplies and generators are rated in kVA, so the calculator converts peak watts using power factor. Mixed sites often operate at PF 0.85–0.95. Current is then computed from voltage and phase, helping you choose feeders, distribution boards, and connector ratings.
Validate margins, cables, and protection settings
After diversity, add growth and safety margins—10–25% is a practical starting band for evolving sites. Check cable length, ambient temperature, and voltage drop, and confirm breaker selection with local codes. Treat results as a sizing baseline, then verify with a qualified electrician. For temporary generators, derate for altitude and heat, and keep operating load near 70–80% for best fuel efficiency. If you expect nonlinear loads like welders or chargers, consider harmonic effects and neutral loading. Document assumptions in the notes so exported reports remain auditable. Recheck whenever equipment changes or phases are added.
FAQs
1) What if my equipment list is in kW instead of watts?
Multiply kW by 1,000 to convert to watts, then enter Running W (each). If a tool is rated in amps, estimate watts as V × A × PF for single‑phase, or √3 × V × A × PF for three‑phase.
2) How do I choose a surge multiplier?
Use 1.0 for resistive loads and electronics. For induction motors without soft‑start, start with 3.0–6.0. For compressors and pumps, 4.0–6.0 is common. If you have soft‑start or VFD control, 1.5–3.0 may fit.
3) What does the diversity factor change?
Diversity reduces running load to reflect that not all tools operate together. A mixed trade area may use 60–90%. Set 100% when every load is truly simultaneous, such as lighting or a dedicated process line.
4) Why does power factor affect kVA and current?
Supplies and generators deliver apparent power. At lower PF, the same watts require more kVA and more current, increasing cable and breaker needs. If PF is unknown, 0.90 is a practical starting assumption for mixed site loads.
5) Does this work for generators and utility feeders?
Yes. The kVA and current outputs can guide generator selection, transformer sizing, and feeder planning. For generators, also check manufacturer derating for temperature, altitude, and transient motor starting capability.
6) How is the recommended breaker determined?
The calculator applies a continuous‑load factor and an 80% utilization rule-of-thumb, then rounds up to a standard breaker size. Always confirm conductor ampacity, temperature correction, and local code requirements before final selection.
7) What should I enter for efficiency and safety margin?
Use 90–95% efficiency for typical supply and distribution losses. Start with a 10–25% safety margin when loads may change or measurements are uncertain. Increase margins for long runs, high heat, or heavy motor starting.