Three-Phase Motor Wiring: Star vs Delta, Leads, and Rotation
Three-phase motor wiring explained — star (wye) vs delta connections, the 6 lead terminals, dual-voltage motors, and how to reverse rotation safely.
Three-phase motor wiring is one of those fundamental skills that every maintenance technician and controls engineer needs to get right — not just once, but reliably, under pressure, often in a panel that hasn't been touched in a decade. Wire it wrong and the motor either refuses to start, runs on two phases, or draws current until the overload trips. Get the connection right and you have a machine that will run for years.
This guide walks through everything you need to make the correct connection: the terminal naming conventions, the geometry of star and delta, the dual-voltage wiring tables, how to read the nameplate diagram, and how to reverse rotation when the shaft is spinning the wrong direction. It also covers how the connection ties into the starter circuit — because in practice you never wire a motor in isolation.
Three-Phase Motor Basics: Three Windings, Six Terminals
A standard three-phase induction motor contains three separate stator windings, one for each phase. Each winding has two ends, giving six physical terminals in total. Every wiring decision — star, delta, low voltage, high voltage — is just about which of those six terminals you connect together and which ones you feed line voltage to.
Terminal Naming Conventions
Two conventions appear on motor nameplates worldwide:
| IEC (International) | NEMA (North America) |
|---|---|
| U1 – U2 | T1 – T4 |
| V1 – V2 | T2 – T5 |
| W1 – W2 | T3 – T6 |
- IEC motors label the supply end of each winding U1, V1, W1 and the opposite end U2, V2, W2.
- NEMA motors number all six leads T1 through T6 (and T7–T9 for 9-lead dual-voltage motors).
In either system, each pair belongs to one winding:
- Winding A: U1 / U2 (or T1 / T4)
- Winding B: V1 / V2 (or T2 / T5)
- Winding C: W1 / W2 (or T3 / T6)
When you open the terminal box, you will find these leads waiting. What you do with them next determines the connection type.
Star (Wye) Connection
In a star connection (also called wye, written Y), one end of each winding is joined to a common neutral point, and the other end receives a phase of the supply voltage.
How to Make the Star Connection
IEC 6-lead star:
- Connect U2, V2, and W2 together — this forms the neutral star point.
- Connect L1 → U1, L2 → V1, L3 → W1.
NEMA 6-lead star (low-voltage terminals only):
- Join T4, T5, and T6 together.
- Connect L1 → T1, L2 → T2, L3 → T3.
The joined ends (the neutral point) are not connected to the supply neutral in a three-phase motor — they simply float internally.
Voltage Across Each Winding in Star
Each winding sees the phase voltage (line voltage ÷ √3). On a 400 V line:
- Phase voltage = 400 ÷ 1.732 ≈ 231 V
This reduced voltage across each winding means lower starting torque and lower running torque compared to delta at the same line voltage. For that reason, star connection is often used for starting (see the star-delta starter section below) and for motors designed for higher-voltage networks.
Delta Connection
In a delta connection (written Δ), the windings are connected end-to-end in a loop. No neutral point exists. The supply phases feed the three junction points of the loop.
How to Make the Delta Connection
IEC 6-lead delta:
- Connect U1 to W2.
- Connect V1 to U2.
- Connect W1 to V2.
- Connect L1 to the U1/W2 junction, L2 to the V1/U2 junction, L3 to the W1/V2 junction.
NEMA 6-lead delta:
- Connect T1 to T6.
- Connect T2 to T4.
- Connect T3 to T5.
- Connect L1 to the T1/T6 junction, L2 to the T2/T4 junction, L3 to the T3/T5 junction.
Voltage Across Each Winding in Delta
Each winding sits directly across two line conductors, so it sees the full line voltage. On a 400 V system, each winding receives 400 V.
Star vs Delta: Voltage, Current, and Torque Compared
Understanding the electrical differences between star and delta is essential for choosing the right connection and for understanding reduced-voltage starters.
| Parameter | Star (Y) | Delta (Δ) | Ratio |
|---|---|---|---|
| Voltage across winding | V_line ÷ √3 | V_line | 1 : √3 |
| Line current | I_line = I_winding | I_line = I_winding × √3 | √3 : 1 |
| Starting torque | 33% of delta torque | Full torque | 1 : 3 |
| Starting current | 33% of delta starting current | Full starting current | 1 : 3 |
| Power | ⅓ of delta power | Full rated power | 1 : 3 |
Key insight: Star connection reduces both starting current and starting torque to one-third of the delta values. This is why a star-delta starter uses the star connection during run-up — the reduced current protects the supply, and the motor accelerates to near synchronous speed before switching to delta for full-load running.
For continuous running, a motor is connected in the configuration shown on its nameplate for the available supply voltage.
Dual-Voltage Motors: Low and High Voltage Wiring
Many industrial motors are wound for two voltage ratings — for example, 220/440 V or 230/460 V (NEMA) or 230/400 V (IEC). These motors have nine leads (NEMA: T1–T9) and can be connected for either voltage using the same physical windings.
Each full winding is split into two halves. For a 9-lead NEMA motor:
- Winding A halves: T1 (first half) and T7 (second half), with T4 as the midpoint.
- Winding B halves: T2 and T8, with T5 as midpoint.
- Winding C halves: T3 and T9, with T6 as midpoint.
Low-Voltage (230 V) Delta Connection
All winding halves are connected in parallel:
| Together | Feed |
|---|---|
| T1, T7, T2 → | L1 |
| T2, T8, T4 → wait — use table below | — |
The correct NEMA low-voltage delta connection:
- L1 → T1, and join T7 + T4 together at L1
- L2 → T2, and join T8 + T5 together at L2
- L3 → T3, and join T9 + T6 together at L3
In compact notation: connect T4–T7 to L1, T5–T8 to L2, T6–T9 to L3.
High-Voltage (460 V) Delta Connection
All winding halves are connected in series:
- Connect T4 to T7, T5 to T8, T6 to T9 (series the halves).
- Connect L1 → T1, L2 → T2, L3 → T3.
Low-Voltage (230 V) Wye Connection
- Connect T4, T5, T6, T7, T8, T9 all together (this is the neutral star point with all halves in parallel).
- Connect L1 → T1, L2 → T2, L3 → T3.
High-Voltage (460 V) Wye Connection
- Connect T4 to T7, T5 to T8, T6 to T9 (series the halves).
- Connect T7, T8, T9 together as the star point.
- Connect L1 → T1, L2 → T2, L3 → T3.
Always verify the supply voltage before connecting. Running a 460 V-connected motor on 230 V causes it to draw excessive current and overheat within minutes.
Reading the Connection Diagram on the Nameplate
Every motor nameplate carries a wiring diagram — a small schematic showing exactly how the leads must be joined for each available voltage. Never guess the connection from the voltage rating alone. Always find and follow the diagram.
Things to confirm on the nameplate before wiring:
- Voltage rating — matches available supply (e.g., 400 V / 50 Hz or 460 V / 60 Hz). See motor nameplate explained for a full breakdown of every field.
- Connection type shown — Y or Δ symbol next to each voltage.
- Lead numbers — confirm the diagram uses the same numbering as the leads in the terminal box.
- Frame and enclosure — confirms the motor is rated for the environment.
If the nameplate diagram is illegible or missing, consult the motor manufacturer's data sheet using the frame size and model number stamped on the nameplate. Do not proceed without the correct wiring information.
Reversing Rotation: Swap Any Two Phases
To reverse the direction of rotation of a three-phase induction motor, swap any two of the three line conductors. The motor does not need to be rewired at the terminal box — the swap is made at the supply side, typically at the contactor or terminal strip feeding the motor.
Why It Works
The direction of rotation is determined by the phase sequence of the supply. If the supply delivers phases in the order L1 → L2 → L3 (A-B-C sequence), the rotating magnetic field sweeps clockwise (from a given viewpoint). Swapping any two phases reverses the sequence to A-C-B, which sweeps the field in the opposite direction.
Practical Procedure
- Isolate the motor (see Safety section below).
- At the motor starter or terminal strip, swap L1 and L2 (or any other pair).
- Re-energize and confirm direction with a phase rotation meter or by briefly jogging the motor and observing the driven load.
Never swap leads at the motor terminal box if the motor is connected for a specific voltage. Swapping at the box risks incorrect winding connections.
Reversing with a Reversing Starter
For applications that require regular direction changes (conveyors, hoists, machine tools), a reversing starter uses two contactors — forward and reverse — wired so that energising the reverse contactor automatically swaps two phases. The wiring of the motor itself never changes. For more on this, see motor start-stop ladder logic.
Safety: LOTO Before Wiring
No wiring task on a three-phase motor is safe without a full lockout/tagout (LOTO) procedure. Three-phase voltages (230 V to 690 V line-to-line) are immediately lethal, and stored energy in capacitor banks, VFD bus bars, and residual magnetic fields can cause injury even after the breaker is open.
Minimum LOTO Steps for Motor Wiring
- Identify all energy sources — main breaker, control transformer, any UPS or backup supply feeding the panel.
- Open and lock the isolation device — the motor circuit breaker or disconnect switch. Apply a personal lock; keep the key on your person.
- Test for absence of voltage at the motor terminals with a calibrated, CAT III/IV meter before touching any conductor.
- Tag the isolation device so no other person can re-energize the circuit.
- Verify mechanical energy — confirm the driven load cannot rotate due to gravity or stored mechanical energy.
- Complete wiring, verify connections against the nameplate diagram, then restore in reverse order.
A motor nameplate also shows the full-load amps (FLA) and service factor — values you need when sizing the overload relay before re-energizing.
The Controls View: How Motor Wiring Ties to the Starter
Understanding the terminal box connection is only half the picture. In a working panel, the motor is downstream of a starter circuit that controls when and how it starts.
Direct-on-Line (DOL) Starter
The simplest arrangement: a single contactor connects all three phases directly to the motor terminals. The motor starts in whatever connection is shown on the nameplate (star or delta for the available voltage). DOL starters are used where the supply and driven load can tolerate the full starting current — typically smaller motors up to about 7.5 kW on a stiff supply. Learn more about the difference between a contactor and a complete motor starter in our guide to motor starter vs contactor.
Star-Delta Starter
For larger motors, the star-delta starter uses three contactors and a timer:
- Main contactor (KM1): always closed when the motor runs, connects L1/L2/L3 to U1/V1/W1.
- Star contactor (KM3): closes at start, joins U2/V2/W2 together (star point).
- Delta contactor (KM2): closes after the timer, connects the winding ends in delta.
The motor terminal box must be a 6-lead configuration for a star-delta starter. A motor internally connected in delta at the factory (3-lead terminal box) cannot be used with a star-delta starter without rewiring inside.
Transition sequence:
- KM1 + KM3 close → motor starts in star.
- Timer counts down (typically 5–15 seconds).
- KM3 opens → brief pause → KM2 closes → motor runs in delta.
The pause between KM3 opening and KM2 closing is critical: it prevents both contactors from closing simultaneously (dead short across the windings). Electrical and mechanical interlocking between KM2 and KM3 is mandatory.
Checking Rotation After Wiring
After any new wiring or reconnection, always check rotation direction before coupling the motor to the load. Procedure:
- Disconnect the mechanical coupling or confirm the load can safely spin in either direction.
- Jog the motor (brief pulse from the control circuit) and observe shaft rotation.
- Compare to the required direction marked on the driven machine.
- If incorrect, isolate and swap two phases at the incoming supply side of the starter.
On motors driving pumps or fans, running in reverse even briefly can cause cavitation, reverse flow, or bearing damage. Confirm rotation before running at speed.
Frequently Asked Questions
How do you wire a three-phase motor?
Identify whether the motor requires a star (Y) or delta (Δ) connection for your supply voltage from the nameplate diagram. For a 6-lead motor, star connection joins U2, V2, W2 (or T4, T5, T6) together and feeds L1, L2, L3 to U1, V1, W1 (or T1, T2, T3). Delta connection joins U1-W2, V1-U2, W1-V2 in a loop and feeds the three junctions. Always verify supply voltage matches the nameplate rating before energizing.
What is the difference between star and delta connection?
In star, each winding sees the phase voltage (line voltage ÷ √3), which lowers starting torque and current to one-third of delta values. In delta, each winding sees full line voltage, giving full rated torque and current. Star is used for starting (in star-delta starters) or for motors designed for higher voltages. Delta is the normal running connection for motors rated at the line voltage.
How do you reverse a three-phase motor?
Swap any two of the three line conductors feeding the motor — typically at the contactor or terminal strip, not at the motor terminal box. Swapping two phases reverses the phase sequence, which reverses the direction of the rotating magnetic field and therefore the shaft rotation.
What is a dual-voltage motor?
A dual-voltage motor contains winding halves that can be connected in series (high-voltage, wye or delta) or in parallel (low-voltage, wye or delta). A 9-lead NEMA motor rated 230/460 V can run on either voltage by changing how T4–T9 are connected. The nameplate shows both wiring diagrams. Always confirm the supply voltage and use the correct diagram before connecting.
Can you run a star-connected motor in delta?
Only if the motor's winding insulation is rated for the higher voltage it will see in delta. A motor nameplate rated "400 V Y only" must not be connected in delta on a 400 V supply — each winding would see 400 V instead of 231 V, causing immediate overheating and winding failure. If the nameplate shows both connections (e.g., 230 V Δ / 400 V Y), then the correct connection depends solely on the available line voltage.
