Three-Phase Power Calculator
Calculate three-phase power, current and line values for star or delta.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Three-Phase Power Calculator does
Three-phase power uses a √3 factor because the three phases peak 120° apart. Star and delta connections give the same total power in line terms, but distribute voltage and current differently between the phases.
Formula
Power = √3 × Line voltage × Line current × PFStar: Phase voltage = Line ÷ √3, Phase current = Line currentDelta: Phase voltage = Line voltage, Phase current = Line ÷ √3
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Connection | one of 2 options | Yes | — |
| Solve for | one of 2 options | Yes | — |
| Line voltage | V | Yes | Accepts more than 0. |
| Line current | A | In some modes | Accepts more than 0. Shown Solve for is Power from voltage and current. |
| Power | W | In some modes | Accepts more than 0. Shown Solve for is Current from power and voltage. |
| Power factor | number | Yes | Accepts 0.01 or more, up to 1. |
How to use it
- Choose Connection and Solve for.
- Enter Line voltage and Power factor.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
A 400 V three-phase star supply drawing 20 A at 0.9 power factor.
- Connection
- Star
- Voltage
- 400 V
- Current
- 20 A
- PF
- 0.9
P = √3 × 400 × 20 × 0.9 = 12,470 W. Phase voltage 230.9 V, phase current 20 A.
Frequently asked questions
Why is three-phase used for large loads?
It delivers constant power rather than pulsing, uses less conductor material for the same power, and lets motors self-start without extra circuitry.
What is star-delta starting?
Starting a motor in star reduces the voltage across each winding, cutting starting current to a third. Switching to delta at speed restores full power.
Method and sources
Method. Balanced three-phase power, P = √3 × V_line × I_line × power factor, with the line-to-phase relationships for star and delta connections.
Applies to. None for the arithmetic — but any installation work is governed by national wiring regulations
Assumptions
- The load is balanced across all three phases, and the supply is sinusoidal.
- The power factor entered is the true (displacement) power factor of the load.
Limitations
- An unbalanced load breaks the √3 relationship, and the per-phase currents must then be calculated individually.
- Harmonic distortion from electronic loads makes the apparent power larger than this model predicts, so a measured figure can exceed the calculated one.
Sources
- The national wiring regulations applying where the installation is located — Varies by country. Everything about how the result may be used in practice; the calculation itself is standard AC circuit theory.