Star Connection Diagram
This is a free printable star connection diagram: download the diagram as SVG or open it and print to paper or PDF.
A star connection diagram shows the three-phase Y configuration where one end of each phase winding meets at a common neutral point, with V_line = √3 × V_phase and I_line = I_phase.
A star connection — also written as Y connection — is one of the two fundamental methods of connecting a three-phase electrical system, the other being the delta (Δ) connection. In the star configuration, one terminal of each of the three phase windings is connected together to form a single common point called the neutral point or star point. The remaining free terminals form the three line terminals (L1, L2, L3) that connect to the three-phase supply or load.
The key voltage relationship in a star connection is: V_line = √3 × V_phase, where V_line is the line-to-line voltage (between any two line terminals) and V_phase is the phase voltage (between any line terminal and the neutral point). In a standard 400 V (line-to-line) three-phase supply, the phase voltage is 400 ÷ √3 ≈ 231 V — which is why single-phase 230 V equipment can be connected between any one line and the neutral in a three-phase star system.
The key current relationship in a star connection is: I_line = I_phase. The line current (flowing in each supply cable) is identical to the phase current (flowing through each winding). This contrasts with the delta connection, where I_line = √3 × I_phase.
The neutral point in a balanced three-phase star system carries no current, because the three phase currents are equal in magnitude and 120° apart in phase, summing to zero. In an unbalanced system (unequal loads on the three phases), the neutral conductor carries the residual imbalance current. This is why a neutral conductor is required in four-wire three-phase systems supplying unbalanced loads such as single-phase circuits.
Star connections are used in alternator stator windings, three-phase motor stator windings, transformer secondary windings (star-grounded neutral for distribution), and star-delta motor starters (which start the motor in star to reduce inrush current, then switch to delta for running). All three-phase wiring must comply with IEC 60364 or the applicable national standard and must be installed by a qualified electrician.
How to wire star connection diagram
- Identify the three phase winding terminals In a three-phase motor or transformer, each winding has two terminals. For a star connection, the windings are conventionally designated U1/U2, V1/V2, W1/W2 (IEC notation) or T1/T4, T2/T5, T3/T6 (NEMA notation). One terminal of each winding (U2, V2, W2) will form the star point; the other terminals (U1, V1, W1) become the line connections L1, L2, L3.
- Connect the star point by joining the neutral-end terminals Connect U2, V2, and W2 together using a suitable conductor or busbar. This junction is the neutral point (N). On a motor or transformer terminal block, these connections are usually made with short jumper links supplied with the machine. Ensure all three connections are mechanically secure and make reliable electrical contact.
- Connect the line terminals to the three-phase supply Connect L1 to U1, L2 to V1, and L3 to W1. The line conductors must be rated for the full line current (which equals the phase current in a star connection). For motors, the line cables must also be rated for the motor's starting current, which can be 5–7 times the rated running current.
- Connect the neutral conductor if required For a four-wire three-phase system supplying unbalanced or single-phase loads, connect a neutral conductor from the star point (N) to the supply neutral. This conductor must be rated for the maximum expected neutral current. In a perfectly balanced three-phase load, the neutral conductor carries no current, but it must still be present for unbalanced load conditions.
- Verify the connection with a multimeter before applying three-phase supply With supply isolated, measure resistance between each line terminal and the neutral point — the readings should be equal and match the expected winding resistance. Measure between any two line terminals — this should read the series resistance of two phase windings. Unequal readings indicate a wiring error or a fault in a winding.
- Apply three-phase supply and measure line and phase voltages With the circuit energised, measure the phase voltage between each line terminal and neutral — all three should be equal and approximately V_line / √3. Measure line voltage between each pair of line terminals — all three should be equal and approximately √3 × V_phase. Asymmetric voltages indicate a supply imbalance or a wiring fault.
Specifications
| Line-to-phase voltage relationship | V_line = √3 × V_phase (≈ 1.732 × V_phase) |
|---|---|
| Line-to-phase current relationship | I_line = I_phase (current is equal in line and phase) |
| Phase voltage (400 V system) | 400 ÷ √3 ≈ 231 V |
| Neutral current (balanced load) | 0 A (vector sum of three equal 120°-displaced currents) |
| Phase angle between phases | 120° (electrical degrees) |
| IEC terminal designation (line ends) | U1, V1, W1 (connect to L1, L2, L3) |
| IEC terminal designation (neutral-point ends) | U2, V2, W2 (connected together at star point) |
| Applicable standards | IEC 60364, BS 7671, NEC/NFPA 70, AS/NZS 3000, IEC 60034 (motors) |
Safety warnings
- Three-phase systems operate at line voltages of 380–415 V or higher, which are immediately lethal on contact. All three-phase wiring installation, connection, and maintenance must be carried out by a qualified and licensed electrician and must comply with IEC 60364, BS 7671, NEC/NFPA 70, or AS/NZS 3000.
- Before working on any three-phase circuit, isolate the supply at the three-phase circuit breaker or isolator, lock out the isolator, and verify all three phases are dead using a calibrated three-phase voltage tester. Verify phase-to-phase and phase-to-neutral are all dead.
- In a star system with a neutral, removing the neutral conductor while unbalanced loads are connected causes the star point to shift, potentially applying significantly higher than rated voltage to loads on lightly loaded phases and causing immediate equipment damage.
- Three-phase motors continue to coast and can regenerate voltage briefly after supply isolation. Do not touch terminals until the motor has fully stopped and you have re-verified the circuit is dead.
- Protective earth (PE) connections are mandatory on all exposed conductive parts of three-phase equipment. Never omit the earth connection.
Tools needed
- Digital multimeter (AC voltage, resistance)
- Three-phase voltage tester / phase rotation meter
- Clamp meter (three-phase current measurement)
- Lockout/tagout padlock and hasps
- Insulated screwdrivers and spanners
- Wire stripper and crimper
- Personal protective equipment (insulating gloves, safety glasses)
Common mistakes
- Confusing the neutral-end terminals with the line terminals, resulting in incorrect star point formation and asymmetric winding connections.
- Omitting the neutral conductor in an unbalanced four-wire star system, causing neutral point shift and overvoltage on some phases.
- Connecting the star point to the protective earth (PE) incorrectly — in TN-S systems the star point of the supply transformer is earthed, but on the load side the star point is typically only connected to the neutral conductor, not directly to earth.
- Not verifying phase rotation before commissioning a motor — incorrect phase sequence reverses motor rotation, which can damage driven machinery or cause safety incidents.
- Leaving the star point connection loose — a high-resistance star point causes unequal phase loading and asymmetric voltages across the three windings.
Troubleshooting
- One or more phase voltages are unequal when measured at the load
- Cause: Unbalanced loading across the three phases, a loose or high-resistance connection at the star point, or an open neutral conductor in a four-wire system Fix: Measure all three phase-to-neutral voltages with a multimeter. If one phase reads high and another reads low, suspect a lost or high-resistance neutral. Inspect the star point busbar connections for looseness or corrosion. Check the neutral conductor continuity back to the supply. Balance loads across phases where possible.
- Motor runs in the wrong direction of rotation
- Cause: Phase sequence of L1, L2, L3 is reversed relative to the motor's designed rotation direction Fix: Swap any two of the three line conductors at the motor terminals (e.g., swap L1 and L2 at U1 and V1). This reverses the rotating magnetic field direction and therefore the motor rotation. Use a phase rotation meter to confirm the correct sequence before initial commissioning of new installations.
- Motor overheats without obvious overload
- Cause: Single-phasing condition — loss of one phase supply causes the motor to run on two phases, drawing excessive current in the remaining two windings Fix: Check all three supply voltages at the motor terminals. If one phase reads significantly lower than the others or reads zero, trace the supply back to the three-phase circuit breaker. Check for a blown fuse (on fused installations), a tripped single-pole protective device, or a broken connection on one phase. Single-phasing causes rapid overheating and can destroy the motor if sustained.
Frequently asked questions
What is a star connection in a three-phase system?
A star (Y) connection joins one terminal of each three-phase winding to a common neutral point. The three remaining terminals form the three line outputs. The arrangement creates two voltage levels from one three-phase source: line voltage (between any two phase lines) and phase voltage (between any phase line and neutral). Line voltage is always √3 times the phase voltage in a balanced star system.
What is the difference between line voltage and phase voltage in a star connection?
Phase voltage (V_phase) is measured between any single line terminal and the neutral point — for example, from L1 to N. Line voltage (V_line) is measured between any two line terminals — for example, from L1 to L2. In a star connection, V_line = √3 × V_phase ≈ 1.732 × V_phase. For a 400 V three-phase supply, V_phase = 400 ÷ 1.732 ≈ 231 V.
Does the neutral wire carry current in a star connection?
In a perfectly balanced star system (equal load on all three phases), the neutral current is zero — the three phase currents cancel at the neutral point. In an unbalanced system, the neutral carries the vector sum of the three phase currents. Removing or omitting the neutral conductor in an unbalanced system causes the neutral point to shift, resulting in unequal phase voltages and potential overvoltage damage to loads on lightly loaded phases.
Why is star connection used for motor starting (star-delta)?
When a motor starts in star connection, each winding receives only V_line / √3 (the phase voltage, approximately 58% of line voltage). The starting torque and inrush current are both reduced to one-third of their direct-on-line (DOL) values. After the motor accelerates to near full speed, a contactor set switches the windings from star to delta, connecting them across full line voltage for running. This reduces the starting inrush that would otherwise stress the supply network.
What is the neutral point and why does it matter?
The neutral point (star point) is the junction of the three phase windings in a star connection. In a grounded (earthed) star system, the neutral point is connected to the earth/ground potential, establishing a stable reference voltage for each phase. This allows safe single-phase loads to be connected between line and neutral. An ungrounded star point (isolated neutral) provides no fixed reference and requires special earth fault protection for safety.
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