3-Phase Power Calculator
Use this balanced three-phase calculator for quick electrical estimates. Enter known values and calculate power or current for motors, panels, and distribution loads.
A) Calculate Power from Voltage and Current
B) Calculate Current from Real Power
If the kW value is electrical input power (not motor output), set efficiency to 100.
How 3-Phase Power Is Calculated
Three-phase systems are the backbone of commercial and industrial electrical design because they deliver smoother and more efficient power than single-phase systems. The key relationship for balanced loads is based on line voltage, line current, and power factor.
P (kW) = √3 × V(L-L) × I × PF / 1000
Q (kVAR) = √(S² − P²)
Where:
- S = apparent power in kVA
- P = real (active) power in kW
- Q = reactive power in kVAR
- PF = power factor (0 to 1)
Why the √3 Factor Appears
The square-root-of-three term comes from the 120° phase displacement in a three-phase system. When using line-to-line voltage and line current in a balanced load, that geometry introduces √3 into the formula. If you use phase quantities instead, the equation form changes.
Common Use Cases for a 3-Phase Calculator
- Sizing feeders and breakers for motor loads
- Estimating current draw from a known kW nameplate
- Checking transformer loading and spare capacity
- Comparing high-PF vs low-PF operation impacts
- Preliminary electrical budget calculations for projects
Worked Example
Given:
400 V line-to-line, 30 A line current, 0.88 power factor.
Step 1: Apparent Power
S = 1.732 × 400 × 30 / 1000 = 20.78 kVA
Step 2: Real Power
P = 20.78 × 0.88 = 18.29 kW
Step 3: Reactive Power
Q = √(20.78² − 18.29²) = 9.86 kVAR (approx.)
Power Factor and Efficiency: Don’t Mix Them Up
Power factor tells you how effectively current is converted into useful real power from the grid perspective. Efficiency tells you how much of electrical real power is converted into mechanical output (for motors) or useful work (for other devices).
That means a machine can have a good efficiency but still cause high current if PF is poor. In design and troubleshooting, both values matter.
Practical Design Notes
- Use nameplate PF and efficiency when available; avoid guessing.
- For variable loads, calculate at both normal and worst-case conditions.
- Account for harmonics and non-linear loads separately in detailed design.
- Follow local code requirements for overcurrent protection and derating.
- For long cable runs, include voltage drop calculations in addition to ampacity.
Typical Mistakes to Avoid
- Entering phase voltage instead of line-to-line voltage
- Using PF as a percentage (85) instead of decimal (0.85)
- Forgetting efficiency when converting motor output kW to electrical input
- Assuming all systems are perfectly balanced
Final Thoughts
A reliable three-phase power calculator saves time and reduces design errors. Use it for quick checks, planning, and communication across teams. For final engineering decisions, always validate with equipment datasheets, site measurements, and applicable electrical standards.