Wye (star) vs delta
| Wye (Y) | Delta (Δ) | |
|---|---|---|
| Neutral | Present | No neutral |
| V_line | = √3 · V_phase | = V_phase |
| I_line | = I_phase | = √3 · I_phase |
| Typical use | Utility distribution, 4-wire | Motor windings, industrial |
| Ground fault | Neutral gives a return path | Ungrounded — harder to detect |
Common system voltages
| System | V_line | V_phase |
|---|---|---|
| US 120/208 V (wye) | 208 V | 120 V |
| US 277/480 V (wye) | 480 V | 277 V |
| US 240 V delta | 240 V | 240 V |
| EU 230/400 V (wye) | 400 V | 230 V |
| Japan 100/200 V | 200 V | 100 V |
Power formulas
| Real power P | = √3 · V_line · I_line · cos φ |
|---|---|
| Apparent power S | = √3 · V_line · I_line |
| Reactive power Q | = √3 · V_line · I_line · sin φ |
| Phase sequence | ABC (positive) or ACB (negative) — motors rotate opposite direction |
Notes
- Three-phase delivers constant instantaneous power — ideal for motors (no torque ripple).
- For the same power, three-phase needs less copper than three single-phase lines combined.
- Unbalanced loads on a 3-phase system create neutral current (wye) or circulating current (delta).
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