3-Phase Motor Wiring Diagram: Star and Delta Connections
Opening the terminal box of a three-phase induction motor reveals six terminals arranged in two rows, plus a ground stud. Which way you link those terminals -- star or delta -- determines the voltage each winding sees and whether the motor runs on a 230V supply or a 400V supply. This guide covers the terminal box layout, how to read the nameplate voltage designations, and how to make the physical star and delta connections correctly.
This is distinct from star-delta starting (which uses contactors and a timer to switch between configurations during startup). This guide is about how to wire the motor terminal box for a given supply voltage.
The Terminal Box Layout
A standard three-phase induction motor has six winding terminals labeled according to IEC 60034-8:
Top row (winding starts): U1 V1 W1
Bottom row (winding ends): W2 U2 V2
This specific arrangement -- W2 directly below U1, U2 below V1, V2 below W1 -- is the IEC standard layout. It is designed so that placing a link bar across adjacent top-bottom pairs naturally creates a delta connection.
Older motors (and some US motors following NEMA MG-1 standard) use different terminal designations: T1, T2, T3, T4, T5, T6 or (for 9-lead motors) T7, T8, T9 in addition. This guide uses the IEC U1-V1-W1 / U2-V2-W2 designations.
Locating the Ground Terminal
The terminal box also includes an earth (ground) terminal, usually marked with the ground symbol or PE (Protective Earth). This must always be connected to the equipment grounding conductor, regardless of whether the motor is wired star or delta.
Reading the Nameplate Voltage Designation
The motor nameplate carries the voltage rating in the format:
230/400V or 400/690V
This dual-voltage designation tells you exactly which connection to use at which supply voltage.
| Nameplate | Star (Y) voltage | Delta (Δ) voltage |
|---|---|---|
| 230/400V | 400V | 230V |
| 400/690V | 690V | 400V |
The rule: the higher voltage in the pair requires star connection; the lower voltage requires delta connection.
A 230/400V motor on a 400V three-phase supply runs in star. The same motor on a 230V three-phase supply runs in delta. In European industrial installations, where 400V (line-to-line) is the standard supply, most 230/400V motors run in star.
A 400/690V motor on a 400V supply runs in delta. On a 690V supply, it runs in star.
If the nameplate shows a single voltage (e.g., "400V Δ" or "400V Y"), the motor is wound for that voltage only in that configuration -- there is no dual-voltage option.
Star (Wye) Connection
In a star connection, the three winding ends (U2, V2, W2) are joined together to form a neutral point, and the three supply phases are connected to the winding starts (U1, V1, W1).
Each winding receives the phase voltage -- line voltage divided by √3 (1.732). On a 400V supply: 400 / 1.732 = 231V per winding.
How to Make the Star Connection
Link bar method (most common for terminal box connections):
- Place a single link bar (shorting bar) connecting U2, V2, and W2 together. This joins all three bottom terminals.
- Connect supply phase L1 to U1.
- Connect supply phase L2 to V1.
- Connect supply phase L3 to W1.
- Connect the equipment grounding conductor to the earth terminal.
The link bar is usually a copper or brass bridge supplied with the motor. For permanent connections, some installers use a wire jumper loop connecting all three bottom terminals to a single point.
Star connection (terminal box view):
L1 ----- U1 V1 ----- L2
| |
[winding 1] [winding 2]
| |
U2 ---- V2 ---- W2 ----- [joined / star point]
|
[winding 3]
|
W1 ----- L3
Delta Connection
In a delta connection, the end of each winding connects to the start of the next, forming a closed triangle. Each winding sees the full line voltage.
On a 230V three-phase supply, each winding receives 230V -- the correct voltage for a 230/400V motor in delta.
How to Make the Delta Connection
Link bar method (using the IEC standard terminal layout):
The IEC layout makes this systematic. Adjacent terminal pairs in the standard layout are the correct pairs to link:
- Connect (link) U1 and W2 together -- they share the same position in the delta loop.
- Connect (link) V1 and U2 together.
- Connect (link) W1 and V2 together.
- Connect supply phase L1 to the U1/W2 junction.
- Connect supply phase L2 to the V1/U2 junction.
- Connect supply phase L3 to the W1/V2 junction.
In most terminal boxes, this is achieved with three short link bars positioned diagonally, each bridging one top terminal to the adjacent bottom terminal.
Delta connection (terminal box view):
U1 -- W2
| |
[winding 1] [winding 3]
| |
L1 ----- U1/W2 link V1/U2 link ----- L2
|
[winding 2]
|
W1/V2 link ----- L3
Phase Rotation and Motor Direction
Connecting L1, L2, L3 to U1, V1, W1 in sequence gives the motor a defined direction of rotation (usually clockwise when viewed from the drive end). To reverse the motor, swap any two of the three supply phases at the motor terminals -- for example, swap L1 and L2 at U1 and V1.
Do this at the supply side contactor or at the terminal box (not in the motor itself). NEC 430 and IEC 60204-1 both require the means of reversing to be accessible and lockable.
Current Ratings: Star vs Delta
On the nameplate, the current rating may show two values:
- Star current (lower) -- for the higher voltage connection
- Delta current (higher) -- for the lower voltage connection
The power output is the same in both cases (assuming the rated winding voltage). For a 5.5 kW motor at unity power factor:
- Star (400V): I = 5500 / (√3 × 400 × 0.85 pf) ≈ 9.3A
- Delta (230V): I = 5500 / (√3 × 230 × 0.85 pf) ≈ 16.2A
The delta current is approximately √3 times the star current for the same power output. This affects cable sizing and overload relay settings.
Common Wiring Mistakes
- Wrong connection for supply voltage: Wiring a 230/400V motor in delta on a 400V supply applies 400V to windings rated for 231V -- rapid winding insulation failure.
- Incomplete star point: Forgetting to connect one of the three winding ends to the star point creates a single-phase condition. The motor may start under no load but overheat.
- Phase sequencing with links left over from factory shipping: Motors sometimes arrive with temporary star links from factory testing. Remove and reinstall correctly for your supply voltage.
- Not connecting the earth terminal: The motor case must be bonded to earth (PE). NEC 430.12 and IEC 60204-1 both require this.
IEC vs NEMA Terminal Marking Standards
Which naming convention you're looking at depends on where the motor was built and which standard it follows.
IEC-built motors, common outside North America, typically use the U1/V1/W1 (winding starts) and U2/V2/W2 (winding ends) markings covered above. NEMA-built motors, common in North America, more often use T-numbering instead: a basic single-voltage motor may have just three leads marked T1, T2, T3, while a dual-voltage motor with more reconfiguration options can have nine leads marked T1 through T9. On a 9-lead NEMA motor, the extra leads let you wire the windings in different series and parallel combinations to match a lower or higher supply voltage -- similar in purpose to the IEC star/delta connections described above, just with a different numbering scheme and different link patterns. Always work from the wiring diagram glued inside the terminal box lid rather than assuming a numbering convention, since a 6-lead IEC motor and a 9-lead NEMA motor can both show up in the same facility.
Testing Motor Windings with a Multimeter
Before wiring or troubleshooting a three-phase motor, two quick multimeter checks catch common problems early.
Winding-to-winding resistance: Measure resistance across each pair of winding leads (U1-V1, V1-W1, W1-U1, or the equivalent T-numbered pairs). All three readings should be close to each other. A winding reading noticeably higher, lower, or open compared to the other two indicates a damaged or shorted winding.
Winding-to-frame (insulation) check: Measure resistance from each winding lead to the motor frame or ground. This should read very high resistance, effectively open, not continuity. A low-resistance or continuity reading between a winding and the frame points to an insulation fault, and the motor shouldn't be energized until that's investigated further. A standard multimeter only gives a rough pass/fail check here -- proper insulation resistance testing uses a dedicated megohmmeter (megger), which applies a much higher test voltage and gives an actual resistance value in megohms.
Troubleshooting 3-Phase Motor Wiring Problems
| Symptom | Likely Cause | Fix |
|---|---|---|
| Motor hums but doesn't start | Single-phasing -- a blown fuse or open winding leaves the motor running on two phases | Check all three supply phases and terminal connections; check fuses and continuity |
| Motor runs backward | Two supply phases swapped at the terminals or contactor | Swap any two of the three supply leads at the terminal box or contactor |
| Motor overheats under load despite correct star/delta wiring | Unbalanced supply voltage across the three phases, or unbalanced winding resistance | Measure supply voltage phase-to-phase and winding resistance; correct the source imbalance |
| Overload relay trips immediately at startup | Relay set for the wrong current value, or high inrush current from a direct-on-line start | Verify the relay setting matches the actual connection (star or delta); consider a star-delta or soft starter for high-inertia loads |
| Motor trips randomly under normal load | Loose or corroded terminal connection adding resistance and heat at that joint | Retorque terminals to spec and clean the contact surfaces |
Create Your Own 3-Phase Motor Wiring Diagram
Documenting the motor's terminal connections, cable sizes, and overload relay settings before installation prevents the wrong-connection mistakes above. With CircuitDiagramMaker, you can:
- Draw the terminal box in both star and delta configurations with labeled terminals
- Show the supply connection from the contactor to each terminal
- Add nameplate data (voltage, FLC, power factor) as annotations
- Draw the ground connection from the terminal box to the equipment grounding conductor
- Export the diagram for inclusion in the motor's maintenance record
Create your own 3-phase motor wiring diagram -- free
Key Takeaways
- A three-phase induction motor has six terminals: U1, V1, W1 (winding starts) and U2, V2, W2 (winding ends) in the IEC standard layout.
- The dual-voltage nameplate designation (e.g., 230/400V) tells you which connection to use: star for the higher voltage, delta for the lower.
- Star connection: join U2, V2, W2 together; supply to U1, V1, W1. Each winding sees line voltage / √3.
- Delta connection: link U1-W2, V1-U2, W1-V2; supply connects to each linked pair. Each winding sees full line voltage.
- Applying the wrong connection for the supply voltage (star on delta voltage or vice versa) causes immediate or rapid winding damage.
- Reversing any two supply phases at the terminal box reverses motor rotation.
- Delta connection draws √3 times more line current than star for the same power -- cable and overload settings must account for this.
Frequently asked questions
Can you run a 3-phase motor on single-phase power?
Not directly. A three-phase motor needs three-phase voltage to start and run correctly under load. Running it from single-phase power requires a phase converter or a variable frequency drive (VFD) with single-phase input, and even then the motor is typically derated below its full three-phase nameplate output.
Which wire is L1 on a 3-phase motor?
There's no universal color code for L1, L2, and L3 -- these labels refer to the supply phase conductors coming from the panel or contactor, not fixed motor lead colors. Connect them in sequence to the motor's U1/V1/W1 (or T1/T2/T3) terminals; the order you connect them determines the motor's direction of rotation.
Can I use a VFD instead of a star-delta starter?
Yes, on many motors. A VFD ramps voltage and frequency electronically, which removes the need for star-delta starting to limit inrush current. The motor still needs to be wired in the connection that matches the VFD's rated output voltage -- usually delta for a 230/400V motor on a 400V drive, since a VFD doesn't switch between star and delta during operation.
What size cable do I need for a 3-phase motor?
Cable size depends on the motor's full-load current for the actual connection in use (star or delta), the cable run length, and ambient conditions -- consult your local electrical code ampacity tables. If a motor could ever be reconnected from star to delta, size the cable for the higher delta current to stay safe.
Is it safe to touch the motor terminal box while it's running?
No. The terminal box carries full line voltage during operation and should never be opened while energized. Lock out and tag out the supply, then verify zero voltage with a meter before opening the terminal box or touching any winding leads.
Can a 3-phase motor run with one winding disconnected?
It can attempt to, but this causes single-phasing at the motor itself -- the motor may hum, fail to start under load, draw excessive current on the remaining windings, and overheat rapidly. This is a common cause of burned-out motors, so check winding continuity if you suspect a missing phase.