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Courses/Three-Phase Systems/Star (Wye) Connection

lesson · 9min · Lesson 6 of 28

Star connection topology

Course syllabusCourse overview
01Three-Phase Fundamentals
  1. ReadWhy three-phase?: advantages over single-phase
  2. ReadGeneration of three-phase voltages
  3. ReadPhase sequence: L1, L2, L3
  4. ReadPhasor representation of three-phase
  5. quizPhase fundamentals quiz
02Star (Wye) Connection
  1. ReadStar connection topology
  2. ReadLine voltage = √3 × phase voltage: proof
  3. ReadNeutral current in star systems
  4. exerciseStar circuit analysis problems
03Delta Connection
  1. ReadDelta connection topology
  2. ReadLine current = √3 × phase current: proof
  3. ReadCirculating currents in delta
  4. exerciseDelta circuit analysis problems
04Three-Phase Power
  1. ReadActive, reactive, and apparent power
  2. ReadPower factor in three-phase
  3. ReadTwo-wattmeter method
  4. ReadPower correction capacitor sizing
  5. quizThree-phase power quiz
05Three-Phase Induction Motors
  1. ReadHow induction motors work
  2. ReadMotor nameplate data and efficiency classes
  3. ReadStarting currents and starting methods
  4. ReadDOL starters: design and wiring
  5. ReadStar-delta starters: wiring and timing
06Transformers
  1. ReadTransformer construction and principles
  2. ReadTurns ratio and voltage/current transformation
  3. ReadThree-phase transformer connections
  4. quizTransformer quiz
  5. quizFinal assessment
Lesson · 9min
THREE-PHASE · 120° APARTL1 · L2 · L3STARY: Vʟ=√3·VₚΔ: Iʟ=√3·IₚDELTA
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Star connection topology

Three Phase Systems · Lesson 6 · Intermediate

IntermediateReview: professional review pending

Purpose

Trace current and voltage paths in a three-phase star connection with and without an accessible neutral.

Before you beginThree-phase phasors · Kirchhoff current law

Learning objectives

  • Identify the star point
  • Distinguish line and phase quantities
  • Trace neutral current
  • Explain a floating star point

In a star connection, one end of each phase impedance joins at a common star point; the other ends connect to the three line conductors. The star point may be connected to a neutral conductor or left isolated.

STAR VOLTAGE AND CURRENT PATHSSTAR VOLTAGE AND CURRENT PATHSL1 · 0°L2 · −120°L3 · +120°VL = √3 VphIL = Iph · IN = phasor sum

Core theory

Each phase impedance is connected between one line and the star point, so phase voltage is line-to-neutral voltage when the star point is held at source neutral potential.

Line current equals the current in its corresponding phase branch because each line feeds one branch without a junction between them.

A neutral stabilises phase voltages for unbalanced line-to-neutral loads and carries their phasor-sum current. With no neutral, an unbalanced star point shifts and phase voltages redistribute.

Terms, symbols, and units
TermMeaningSymbolUnit
Star pointCommon junction of the three phase branchesNot applicableNot applicable
Line currentCurrent in a supply line conductorILA
Phase voltageVoltage across one star-connected phase impedanceVphV
Worked exampleA balanced star load has 230 V per phase and 23 Ω resistive phase impedance. Find line current.

Assumptions: Balanced ideal supply; Purely resistive branches.

  1. Phase current: Iph = Vph/Zph = 230/23 = 10 A.
  2. Topology: In star, each line conductor is in series with one phase branch.
  3. Line current: IL = Iph = 10 A.

Each line current is 10 A.

Reasonableness check: A 23 Ω branch across 230 V draws 10 A, and no current split occurs before that branch.

Common mistakes
  • Multiplying star line current by √3
  • Assuming the star point is always earthed
  • Ignoring neutral loss on unbalanced loads

Where this appears in practice

Star connections appear in distribution systems, machine windings, heaters, transformers, and mixed single-phase loads.

SafetyAn isolated or open neutral can produce damaging line-to-neutral overvoltage. Do not disconnect a loaded neutral or assume a star point is at Earth potential.
Local code checkConfirm the nominal system voltage/frequency, earthing and neutral arrangement, phase-sequence convention, conductor and protective-device data, harmonic assessment, isolation method, and current local installation standard. The 230/400 V examples are instructional values, not site measurements or universal supply values.

Knowledge check

What is the line-to-phase current relationship in an ideal star load?

IL = Iph. Each line feeds its phase branch directly.

Answer: IL = Iph. Each line feeds its phase branch directly.

Practical exercise

Trace all four conductor currents for a paper three-phase four-wire star load.

Summary

  • Star branches share a common point
  • Line current equals phase current
  • Neutral condition controls unbalanced voltages

Sources and review

  • IEC 60038: IEC standard voltages: IEC; 2009+A1:2021; International
  • Broken PEN: IET Wiring Matters; Issue 84, 2021; United Kingdom
  • Minimizing unnecessary live testing for initial verification: IET Wiring Matters; Issue 105, 2025; United Kingdom

Editorial review date: 2026-08-22. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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