Single waves vs. three waves
Ordinary household power is single-phase: one alternating voltage that rises to a peak, falls through zero, reverses, and repeats 60 times a second in the US. Each time it passes through zero, it briefly delivers no power.
Three-phase adds two more waves, timed so one is always near its peak. The combined delivery is continuous.
How it's generated and delivered
- Generation: a power-plant generator has three coil sets spaced evenly around its stator, producing three phases at once.
- Transmission: step-up transformers raise the voltage, often to hundreds of kilovolts, so power can travel long distances with lower losses.
- Distribution: substations step the voltage down to distribution levels, commonly 4–35 kV, and carry it along overhead or underground lines. The construction side is covered in our line construction guide.
- Service: a final transformer near the building supplies a usable voltage such as 208Y/120 V or 480Y/277 V.
Who uses three-phase power?
Factories, warehouses, hospitals, office buildings, retail centers, farms with irrigation pumps, and any site with large motors or HVAC equipment. Inside these buildings, three-phase panels feed big loads directly and also supply ordinary single-phase circuits for lights and outlets.
Learn more
For formulas, voltages and the engineering detail behind all this, continue to Understanding Three-Phase Power or compare the two systems side by side in Single-Phase vs. Three-Phase Power.