What is a Electrical Calculator?
This calculator applies Ohm's Law, and computes three-phase power, power factor, and voltage drop for electrical wiring.
How it's calculated
V = I × R (Ohm's Law)
Single-phase power P = V × I × cosθ
Three-phase power P = √3 × V × I × cosθ
Variable definitions
| Variable | Meaning | Unit |
|---|---|---|
| V | Voltage | V |
| I | Current | A |
| R | Resistance | Ω |
| cosθ | Power factor | 0–1 (unitless) |
Assumptions
This assumes the load is linear and operating in steady state. Transient conditions, like a motor's inrush current at startup, draw far more current momentarily and aren't captured by this formula — breaker and wiring sizing must account for that separately.
Worked example
A 380V three-phase motor drawing 20A at 0.85 power factor
- P = √3 × 380 × 20 × 0.85
- P ≈ 11,186 W (about 11.2 kW)
What is power factor?
Motors and transformers use part of their current to build a magnetic field, and that current flows back and forth without doing useful work — but it still flows through the wires. Real power (kW) is the power that actually does work; apparent power (kVA) is what the wiring actually has to handle; and power factor = real power ÷ apparent power. A low power factor means thicker wiring and larger equipment are needed to deliver the same useful work, which is exactly why many utility companies charge a penalty for power factor below about 90% and offer a discount above it — the reason factories install correction capacitors.
Good to know
- The √3 in three-phase formulas comes from the three voltage phases being offset by 120 degrees — not a simple 3x multiple, but a vector sum that works out to √3 (about 1.732) times.
- Three-phase power is used for efficiency — it needs less copper to deliver the same power compared to single-phase, and three-phase motors can self-start their rotation direction.
- Voltage drop comes from wire resistance and increases with distance, which is why long wiring runs can leave equipment at the far end under-voltaged. Electrical codes typically cap this at around 3%.
- The reason transmission uses extremely high voltage follows the same logic: for the same power, higher voltage means lower current, and since losses scale with the square of current, losses drop dramatically.
- What determines shock danger is current, not voltage. Static electricity can be thousands of volts but carries negligible current, making it harmless, while household voltage carries enough current to be dangerous.
Frequently asked questions
Q. What's the difference between kW and kVA?
kW is real power actually doing work; kVA is apparent power the equipment must handle. Multiplying kVA by power factor gives kW.
Q. What are the benefits of improving power factor?
Lower current reduces strain on wiring and transformers, and avoids utility penalty charges for low power factor.
Q. How much voltage drop is acceptable?
It varies by application, but electrical design typically targets around 2–3% of rated voltage.