Wye (star) connection
- Line voltage = √3 × phase voltage; line current = phase current.
- Neutral available for single-phase loads (e.g., 120 V from 208Y/120 V, 277 V from 480Y/277 V).
- Neutral is usually solidly grounded, so faults are easy to detect and clear.
- Standard choice for transformer secondaries serving buildings.
Delta connection
- Line voltage = phase voltage; line current = √3 × phase current.
- No inherent neutral. A high-leg (wild-leg) delta center-taps one winding to provide 120/240 V, leaving one leg at about 208 V to neutral. NEC requires that leg to be marked orange.
- Circulating currents in the closed loop trap triplen harmonics, so they don't pass to the supply.
- Common on transformer primaries and on many motor windings.
Comparison table
| Characteristic | Wye | Delta |
|---|---|---|
| Neutral | Yes | No (except high-leg) |
| Voltages available | Line-line and line-neutral | Line-line (plus 120/240 on high-leg) |
| Grounding | Typically solidly grounded neutral | Ungrounded, corner-grounded or high-leg |
| Harmonics | Passes triplen harmonics in neutral | Traps triplen harmonics in loop |
| Typical use | Building distribution secondaries | Transformer primaries, motors, legacy services |
Transformer combinations
The most common US distribution transformer is delta primary, wye secondary (Δ–Y), for example 480 V delta to 208Y/120 V. It gives a grounded neutral on the load side and isolates triplen harmonics from the primary. Note that Δ–Y introduces a 30° phase shift, which matters when paralleling transformers. Learn more in Three-Phase Transformers Explained.
Wye-delta motor starting
Some motors start in wye and switch to delta once running. Starting in wye cuts each winding's voltage by about 42%, which reduces starting current and torque to about one-third of their delta values. The motor needs six leads and the load must tolerate the reduced starting torque. Soft starters and VFDs are common alternatives.