Basic Difference
A single-phase AC source has one alternating voltage waveform. A three-phase source has three waveforms separated by 120 electrical degrees.
Three-phase power transfers energy more evenly through the cycle. This supports efficient motors and allows more power to be delivered for a given conductor current and voltage.
Power Formulas
Single-phase kW = V × I × PF ÷ 1000
Three-phase kW = √3 × VLL × IL × PF ÷ 1000
The three-phase formula above uses line-to-line voltage and line current for a balanced load. The square-root-of-three factor comes from the phase relationship between line and phase quantities.
Line and Phase Voltage
In a wye-connected system:
VLL = √3 × VLN
For example, a nominal 208Y/120 V system has approximately 208 V line-to-line and 120 V line-to-neutral. A 480Y/277 V system has approximately 480 V line-to-line and 277 V line-to-neutral.
Delta systems have different phase relationships and may not provide a neutral. Always identify the actual system and measurement points.
Balanced and Unbalanced Loads
The common three-phase power equation assumes the three phase currents and power factors are balanced. When loads are significantly unbalanced, calculate each phase separately or use measured total power.
Nonlinear single-phase loads can also create neutral harmonic current that is not predicted by a simple balanced fundamental-frequency calculation.
Typical Applications
| Single-phase | Three-phase |
|---|---|
| Residential receptacles and lighting | Industrial motors and machinery |
| Small appliances and equipment | Large HVAC and pumping systems |
| Lower-power branch loads | Commercial and industrial distribution |
Typical use does not determine the actual supply. Verify nameplates, diagrams, and measured system voltage.