Star-Delta Control Wiring: Panel Layout, Terminal Numbers, and Connection Sequence
This is a free printable star delta control wiring: download the diagram as SVG or open it and print to paper or PDF.
Star-delta control wiring translates the control circuit diagram into physical conductor runs between terminal blocks, contactors, and the timer relay. Correct terminal numbering and connection sequence prevent the phase-to-phase short that destroyed more than one starter panel.
The star-delta control circuit diagram tells you what connects to what electrically. The star-delta control wiring is the physical execution of that diagram — conductor colours, terminal numbers, ferrule labels, and the physical layout of components inside the starter enclosure. These are two distinct documents and both are needed for a correct installation.
In IEC practice, contactor terminals are standardised: main power terminals are numbered 1/2, 3/4, 5/6 (1, 3, 5 = line side; 2, 4, 6 = load side). Auxiliary contact terminals are numbered in pairs: 13/14 (normally open), 21/22 (normally closed), 23/24 (normally closed second pair if available), 33/34, 43/44, and so on. Understanding this numbering system is critical because the interlocking contacts — the safety-critical elements of the starter — must be wired to specific auxiliary terminals.
The six motor winding terminals are labelled U1, V1, W1 (start ends of the three windings) and U2, V2, W2 (finish ends). In star-delta starting: - KM1 (main) connects supply L1, L2, L3 to motor terminals U1, V1, W1 respectively. - KM2 (star) connects U2, V2, W2 together (short-circuited star point), forming the neutral point. - KM3 (delta) cross-connects: L1 (via KM1) to W2, L2 to U2, L3 to V2 — which is the delta connection.
The control wiring runs in a separate zone of the panel from the power wiring, typically with smaller-gauge conductors (1.0–1.5 mm² is common for 24 V DC or 110 V AC control circuits). Each conductor is fitted with a printed ferrule at both ends showing the wire number, which corresponds to the numbers on the circuit diagram. This allows fault-finding by wire number without needing to trace the physical conductor.
The sequence in which components are wired inside the panel matters. Power wiring is typically completed first — supply incomer through isolator to contactors, then contactors to motor terminal block. Control wiring follows. The interlock contacts (normally-closed auxiliary contacts wired in series with the opposing contactor coil) are the last control wires added and the first to be verified before commissioning.
Cable management inside the panel uses cable ducts (trunking) to route conductors neatly from component to component. All cables entering the panel from outside pass through cable glands sized to the cable diameter to maintain the panel's IP rating.
How to wire star delta control wiring
- Plan the panel layout before drilling Position components on a DIN rail arrangement before fixing anything. Group power components (main isolator, fuses, contactors) on the upper rail and control components (timer, auxiliary relays, terminal blocks) on the lower rail. Leave adequate clearance for heat dissipation around the contactors and ensure cable trunking routes do not force conductor bends tighter than the minimum bend radius.
- Mount the DIN rail, trunking, and components Secure the DIN rail to the back plate. Snap contactors, the timer relay, overload relay, and terminal blocks onto the rail in the planned layout. Fit cable trunking between component rows. Label each component position on the back plate with the designation (KM1, KM2, KM3, TR, OL) for reference during wiring.
- Wire the main power circuit Using cable sized for the motor full-load current, wire the supply (L1, L2, L3) through the main isolator to terminals 1, 3, 5 of KM1. From KM1 terminals 2, 4, 6, run to the motor terminal block (U1, V1, W1) and simultaneously to terminals 1, 3, 5 of KM3. From KM3 terminals 2, 4, 6, run to motor terminal block points U2, V2, W2 in the cross-connected delta pattern. From U2, V2, W2, also run to KM2 terminals 1, 3, 5. Short KM2 terminals 2, 4, 6 together for the star point.
- Fit ferrules and run control cable trunking Cut all control wires to length and fit printed ferrule sleeves at both ends before inserting into terminals. Assign consecutive wire numbers starting from 1 for the control positive rail (or phase) and incrementing at each junction. Insert wires into the cable trunking and route to their destinations.
- Wire the control circuit — start/stop and KM1 From the control supply positive (or line), wire through the stop button (NC), through the overload relay NC contact (OL 95/96), to the start button (NO). From the start button output, wire to KM1 coil terminal A1. Wire KM1 auxiliary NO contact (13/14) in parallel with the start button for sealing. Connect the control circuit neutral (or return) to KM1 coil terminal A2.
- Wire KM2 (star) and timer relay with interlocks From a KM1 auxiliary NO contact (33/34), wire to the NC auxiliary contact of KM3 (31/32 of KM3 auxiliary block). From there, wire to coil A1 of KM2. Wire KM2 coil A2 to the control neutral. Wire the timer relay coil in parallel with KM2 coil (A1 to A1, A2 to A2) so the timer starts simultaneously with the star connection.
- Wire KM3 (delta) with interlock and verify From the timer relay timed-close contact, wire through the NC auxiliary contact of KM2 (21/22 of KM2 auxiliary block) to KM3 coil A1. Wire KM3 coil A2 to the control neutral. Also wire an NC instantaneous contact of KM3 (31/32) into the KM2 coil circuit (in series before the KM2 A1 terminal) to drop KM2 before KM3 picks up. Before energising, verify with a multimeter that KM2 A1 and KM3 A1 are not simultaneously fed by any single control path.
Specifications
| IEC power terminal numbering (line side) | 1, 3, 5 |
|---|---|
| IEC power terminal numbering (load side) | 2, 4, 6 |
| IEC auxiliary contact designation (NO) | 13/14, 23/24, 33/34, 43/44 |
| IEC auxiliary contact designation (NC) | 21/22, 31/32 |
| IEC coil terminal designation | A1 (+), A2 (−) |
| Motor winding start terminals | U1, V1, W1 |
| Motor winding finish terminals (star point or delta connections) | U2, V2, W2 |
Safety warnings
- Never energise the panel without first verifying that the interlock contacts are correctly wired and functional. Use a multimeter to confirm that KM2 coil A1 and KM3 coil A1 cannot be simultaneously energised by any control circuit path.
- All electrical panel work must be carried out by a licensed electrician to IEC 60204-1, IEC 60364, NEC/NFPA 70, BS 7671, or the applicable local standard. Do not attempt this work without appropriate qualifications and authorisation.
- Isolate the supply, lock out the isolator, and verify dead before working inside the panel. Use a calibrated voltage tester — not an LED test light alone.
- The motor terminal box must be correctly wired for delta operation at the supply voltage. Wiring the motor terminal box in star while the starter is set for star-delta operation will result in the motor running in star permanently, overloading and damaging the motor.
Tools needed
- Digital multimeter
- Insulated screwdrivers (flat and Pozidriv)
- Wire stripper
- Ferrule crimping tool and printed ferrule sleeves
- Cable ties and trunking clips
- Label printer or pre-printed ferrule stock
- Clamp meter (for commissioning current checks)
Common mistakes
- Wiring the overload relay NC contact (95/96) outside the stop button series chain — the overload must disconnect all power to all coils when it trips; placing it only in one coil circuit does not guarantee a complete stop.
- Using the main power terminal on a contactor as a junction point for control wiring — control conductors must connect only to auxiliary contact terminals and coil terminals, never to power terminals.
- Omitting the second interlock contact — wiring only the KM2 NC contact in KM3's circuit (or vice versa) without the reverse interlock leaves the starter with asymmetric protection that fails on a specific sequence of events.
- Not fitting a hold-on (seal-in) auxiliary contact across the start button — without the seal-in contact from KM1 (13/14) in parallel with the start button, releasing the start button will drop out KM1 and stop the motor.
- Running control wires and power wires in the same cable trunking — at power levels typical of star-delta starters, induced noise in control wiring can cause spurious contactor operation. Use separate trunking.
Troubleshooting
- Pressing start causes KM1 to pick up briefly then drop out
- Cause: The seal-in auxiliary contact (KM1 NO auxiliary 13/14 in parallel with the start button) is not wired or its contact is open-circuit Fix: Measure across the start button with KM1 energised — if the voltage does not collapse when the button is released, the seal-in is working. If it does collapse, trace the auxiliary contact wiring from terminal 13 to 14 on KM1 and verify continuity when KM1 is manually held in.
- KM2 and KM3 both energise simultaneously, blowing the fuse
- Cause: One or both electrical interlock contacts are missing, wired in the wrong terminal, or have a short across them Fix: Isolate and do not re-energise. Carefully trace both interlock paths: KM3 NC (31/32) in series with KM2 coil A1; KM2 NC (21/22) in series with KM3 coil A1. Verify each NC contact is open-circuit when the respective contactor is manually pressed closed. Replace any contactor with a faulty auxiliary contact block.
- Motor starts in star but trips the overload relay before transitioning to delta
- Cause: Overload relay is set to the line current value rather than the winding current value, or the mechanical load is too high in star and the motor is drawing excessive current Fix: Verify the overload relay setting is correct for its position (line-side: set to FLC; winding-side: set to FLC / 1.732). If the overload is correctly set and still tripping, investigate the mechanical load — an excessive load in star starting is a sign the motor or star-delta method is not suited to this application.
Frequently asked questions
What terminal numbers do I use for the interlock contacts on a standard IEC contactor?
On a standard IEC contactor, normally-closed auxiliary contacts are typically designated 21/22 or 31/32. The normally-closed contact of KM3 wired in series with KM2's coil, and the normally-closed contact of KM2 in series with KM3's coil, must both be from the respective contactor's auxiliary contact block. Always verify the specific contactor's datasheet for terminal assignments.
What wire size should I use for the control circuit wiring in a star-delta starter?
For control circuits operating at 24 V DC or 110 V AC, 1.0 mm² or 1.5 mm² stranded copper conductor is typical. For 230 V AC control circuits, 1.5 mm² is generally used. The exact size depends on the current drawn by the contactor coils and timer relay. Power wiring must be sized for the full motor current.
What do the motor terminal labels U1, V1, W1, U2, V2, W2 mean?
U1, V1, W1 are the start (or line) ends of the three stator windings. U2, V2, W2 are the finish (or neutral) ends. In star configuration, U2, V2, and W2 are shorted together. In delta configuration, U2 connects to V1, V2 connects to W1, and W2 connects to U1 — creating the delta loop. The exact connection pattern should always be verified against the motor's terminal diagram.
How should I label wires in a star-delta control panel?
Each wire should carry a printed ferrule at both ends bearing the same unique wire number. Wire numbers are taken from the circuit diagram. This one-to-one correspondence between diagram wire numbers and physical ferrules allows any wire to be identified and traced in seconds during fault-finding, without needing to follow the conductor physically through cable trunking.
Why is the overload relay sometimes placed differently in a star-delta starter compared to a DOL starter?
In a DOL starter, the overload relay is placed in the main supply line and set to the full-load current. In a star-delta starter, if the overload is placed in the delta contactor circuit (winding side), it sees winding current, not line current — which is lower by a factor of 1.732 in delta running. If placed in the line side, it must be set to the full line current but will not protect against all winding fault conditions equally.
Related diagrams
- automatic star delta starter control circuit diagram
- control circuit of star delta starter
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