Transformer Fundamentals
Wye and Delta Transformer Connections
Compare the voltage and current relationships and the system's neutral requirement before specifying the transformer connection.
Wye and delta describe how three phase windings are connected. Compare the voltage and current relationships and the system's neutral requirement before specifying the transformer connection.
Read the line and phase relationships
In a balanced wye connection, line voltage is √3 times phase voltage and line current equals phase current. In a balanced delta connection, line voltage equals phase voltage and line current is √3 times phase current. Apply each relationship to the winding side being evaluated.
The illustration shows balanced three-phase relationships. Confirm phase sequence, terminal designations and the applicable line or phase voltage basis on the project drawings.
| Item or condition | What to compare | Input or result |
|---|---|---|
| Wye | VLL = √3 × Vphase | IL = Iphase |
| Delta | VLL = Vphase | IL = √3 × Iphase |
| Project example | 480 V wye line-to-line | About 277 V phase-to-neutral |
Identify the neutral arrangement
A wye winding has a common star point that can be brought out when specified. Its grounding arrangement follows the system design. A delta winding connects the three phases in a closed loop. Record terminal labels and the required grounding system on the drawing.
Wye star point
The three phase windings meet at a common point. A neutral terminal is available only when the winding and terminals are specified to bring that point out. Grounding is a separate system decision.
Delta closed loop
Each phase winding connects between two line terminals to form a closed loop. There is no wye star point; document the grounding and protection approach required by the network.
Coordinate both winding sides
Specify primary and secondary connections, phase displacement and the utility requirements together. A wye-delta motor starter is a different application from a transformer's winding connection. For paralleling, compare the complete vector group and equipment data.
Check the connection alongside transformer capacity, voltage and winding requirements, and use the approved terminal diagram when preparing the specification.
Frequently Asked Questions
Does wye always mean grounded wye?
Grounding is specified separately; use the project drawing and transformer terminal arrangement.
Can the connection be changed on site?
Use only the reconnection arrangements supported by the specific design and manufacturer documentation.
Prepare the connection specification
Share primary and secondary winding connections, line and phase voltages, neutral and grounding requirements, phase displacement and the current project drawing.