How three-phase power works
A generator at a power plant has three sets of windings spaced 120° apart around its stator. As the rotor turns, each winding produces its own sine wave of voltage. Each wave is offset by one-third of a cycle from the others: on a 60 Hz US system, one cycle lasts about 16.7 milliseconds, so each phase peaks about 5.6 ms after the one before.
In a balanced system the three currents add up to zero at every instant. That property is why a three-phase circuit can deliver far more power than a single-phase circuit for the amount of conductor used, and why the neutral of a balanced wye system carries little or no current. For a gentler introduction, see What Is Three-Phase Electricity?.
Line voltage vs. phase voltage
Two voltages describe a three-phase system. Line-to-line voltage is measured between any two phase conductors. Line-to-neutral (phase) voltage is measured from one phase to the neutral in a wye system. For a wye connection, line voltage equals phase voltage × √3 (about 1.732).
| System | Line-to-line | Line-to-neutral | Typical use |
|---|---|---|---|
| 208Y/120 V, 4-wire wye | 208 V | 120 V | Offices, retail, schools, light commercial |
| 480Y/277 V, 4-wire wye | 480 V | 277 V | Industrial plants, large commercial, lighting at 277 V |
| 240 V delta (incl. high-leg) | 240 V | 120 V on two legs (high-leg delta) | Older commercial and light industrial buildings |
| 600 V / 347 V | 600 V | 347 V | Some heavy industrial sites; more common in Canada |
How the windings are connected — wye or delta — determines which voltages are available. Our Wye vs. Delta guide covers the trade-offs.
Why industry runs on three-phase
- Constant power delivery. Power in a balanced three-phase circuit is steady rather than pulsing at twice line frequency, so motors run smoother with less vibration.
- Less copper for the same power. For a given power and line voltage, three-phase needs less conductor material than an equivalent single-phase circuit.
- Self-starting motors. Three-phase induction motors produce a rotating magnetic field naturally and need no start capacitor or auxiliary winding.
- Scalability. Utilities generate, transmit and distribute power as three-phase, so large facilities connect directly to that system.
We cover each advantage in more depth in Benefits of Three-Phase Power.
Calculating three-phase power
Use these formulas for a balanced three-phase load, with V as line-to-line voltage and I as line current:
- Apparent power: kVA = √3 × V × I ÷ 1,000
- Real power: kW = √3 × V × I × PF ÷ 1,000
- Current from kW: I = kW × 1,000 ÷ (√3 × V × PF)
For example, a 480 V load drawing 100 A at 0.9 power factor uses about 74.8 kW. Try your own numbers in our Three-Phase Power Calculator.
Where three-phase power is used
Most large electrical loads in the US run on three-phase: HVAC chillers and rooftop units, pumps, compressors, conveyors, machine tools, elevators, data center UPS systems, commercial kitchens and EV fast chargers. Most homes get split-phase 120/240 V service instead. A house can be fed with three-phase, but only where the utility offers it, usually at extra cost.
Not sure which service a building needs? Compare the options in Single-Phase vs. Three-Phase Power, or check the quick answers in our FAQs.