Line Voltage, Phase Voltage, Line Current and Phase Current: Core Differences & Calculation Rules

In all three-phase generator and motor systems, line voltage, phase voltage, line current and phase current are four basic electrical indicators, which must be distinguished correctly for parameter calculation, cable selection and fault diagnosis.

1. Phase voltage refers to the voltage across a single winding of a generator or motor. It only exists inside the winding and between one live wire and neutral wire in Y-connected circuits.

2. Line voltage stands for the voltage between any two live phase wires, which is the rated voltage marked on most three-phase equipment.

3. Phase current is the current flowing inside each independent winding. It reflects the actual load of a single coil.

4. Line current means the current transmitted through external power cables. It is the main basis for selecting circuit breakers and cable cross-sections. Basic Terminology

Conversion Formula Under Two Wiring Modes

Wye (Y) Connection 

Voltage relation: Line Voltage = √3 × Phase Voltage (UL=√3 Up)

Current relation: Line Current = Phase Current (IL=Ip) 

In Y connection, the winding current directly flows out to the cable, so line current equals phase current. Neutral wire can detect phase voltage for single-phase load.

Delta (Δ) Connection 

Voltage relation: Line Voltage = Phase Voltage (UL=Up)

Current relation: Line Current = √3 × Phase Current (IL=√3 Ip) 

In Delta connection, there is no neutral point. The cable current is the vector superposition of two winding currents, so line current is √3 times phase current. Star-delta reduced voltage starting applies this feature to cut starting current.

Practical Application for Generators & Motors

• Parameter Calibration: When sorting generator technical documents, we need to convert the rated power into corresponding line current and phase current according to wiring type, to match cable and switch specifications

• Star-Delta Starting: For 380V low-power motors, Y startup cuts winding voltage to 1/√3 rated line voltage to reduce startup impact. After acceleration, switch to Delta mode for full-load operation.

• Fault Inspection: Unbalanced phase voltage usually indicates neutral wire breakage or single-phase grounding fault; unbalanced line current reflects phase loss, poor cable contact or winding inter-turn short circuit.

Distinguishing line voltage, phase voltage, line current and phase current is the foundation of three-phase electrical design, product document sorting and equipment maintenance for diesel generator sets. Mastering their conversion rules can accurately complete parameter calculation, equipment selection and rapid fault troubleshooting.

Dingbo Power is a professional manufacturer specializing in the design, production, installation and commissioning of diesel generators, providing you with comprehensive and considerate one-stop solutions for diesel generator sets. Please visit the official website of Dingbo Electric Power to learn more about diesel generators.