Delta Wiring Diagram
This is a free printable delta wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A technical reference for three-phase delta (Δ) winding connections, covering phase-to-line voltage relationships, line current calculation, and delta versus star comparison.
In a three-phase electrical system, the delta (Δ) connection is one of two fundamental ways to interconnect the three windings of a transformer, motor, or generator. The other is the star (Y or wye) connection. Understanding delta wiring is essential for engineers and electricians working with three-phase power systems, transformer banks, motor connections, and power distribution.
In a delta connection, the three windings are connected end-to-end in a triangular (closed loop) configuration. Each winding connects from one line terminal to another — winding 1 connects L1 to L2, winding 2 connects L2 to L3, and winding 3 connects L3 to L1. There is no neutral point — the three line conductors are the only external connections. The junction of each pair of windings is a line terminal.
The voltage across each winding in a delta connection is equal to the line-to-line voltage. For a 400V three-phase supply, each delta winding sees 400V. This contrasts with a star connection, where each winding sees the phase voltage: 400V ÷ √3 ≈ 231V.
The current relationship in a delta connection is governed by Kirchhoff's current law at the junction nodes. The line current (the current flowing in each of the three external supply conductors) equals √3 times the phase current (the current through each individual winding): I_line = √3 × I_phase. This is the inverse of the star connection, where the line current equals the phase current.
This current relationship has significant practical implications. In a delta-connected motor on 400V three-phase: if each winding draws 10 A phase current, the line conductors carry 10 × √3 ≈ 17.3 A. Supply cables and switchgear must be rated for the line current, not the winding current.
Delta connections are used when a supply system has no neutral, for three-phase loads that do not require neutral access, for transformer secondary windings where a neutral is deliberately excluded to prevent zero-sequence current circulation, and in motor windings designed for delta operation at the given supply voltage.
Dual-voltage motors (e.g., 230/400V) use delta connection at the lower voltage and star connection at the higher voltage — the key insight is that the winding voltage is equal to the supply voltage in delta, but only 1/√3 of supply voltage in star.
A delta wiring diagram illustrates how three-phase windings are connected end-to-end to form a closed triangle (delta, Δ), in contrast to the star (Y/wye) connection where all windings share a common neutral point. Delta connections are common on the high-voltage primary of distribution transformers and on motors where full line voltage is required across each winding. Understanding whether a motor nameplate specifies delta or star connection is essential before energising — incorrect connection can overvoltage windings and cause failure. You can map out any delta or star-delta configuration visually in the free online diagram editor.
How to wire delta wiring diagram
- Identify the winding terminals to be delta-connected For a three-phase motor with IEC terminal marking, the six terminals are U1, U2, V1, V2, W1, W2. For delta connection: U1 connects to W2, V1 connects to U2, and W1 connects to V2. The three line supply conductors then connect to the three junction points: L1 to the U1/W2 junction, L2 to the V1/U2 junction, L3 to the W1/V2 junction. This arrangement applies each line-to-line voltage directly across one winding.
- Verify the delta connection matches the supply voltage Confirm that the motor's delta winding voltage rating matches the three-phase supply line-to-line voltage. The nameplate on the motor will show the rated voltage alongside a delta symbol for delta connection. If the motor is dual-voltage (e.g., 230/400V), delta is correct for the lower voltage supply (230V line-to-line) and star for the higher (400V line-to-line). Connecting in the wrong configuration will destroy the motor windings.
- Apply the connection links at the motor terminal block Inside the motor terminal box, fit the three shorting links as follows for delta connection: one link from U1 to W2, one from V1 to U2, one from W1 to V2. The terminal block is typically arranged in two rows of three to facilitate both star and delta connections. The links are copper bars or solid links supplied with the motor. Tighten all terminal screws to the specified torque.
- Connect the three supply conductors Connect L1 to the U1/W2 junction terminal, L2 to the V1/U2 junction terminal, and L3 to the W1/V2 junction terminal. Tighten all supply conductors to the motor's terminal torque specification. Connect the equipment earth to the dedicated earth terminal on the motor frame.
- Calculate line current and verify protection settings Calculate the line current from the motor nameplate: I_line = motor kW × 1000 / (√3 × V_line × power factor × efficiency). Alternatively, read the FLC directly from the nameplate. Set the overload relay to this FLC value. The supply cables, fuses, and contactor must all be rated for the line current — not the individual winding (phase) current. Line current is always √3 × phase current in delta.
- Verify phase rotation and rotation direction Apply power momentarily and verify correct rotation direction before coupling to the mechanical load. If rotation is incorrect, isolate and swap any two of the three supply conductors (L1 and L2, for example). This reverses the phase sequence and thus the rotor direction without changing the delta connection links inside the terminal box.
Specifications
| Phase voltage (delta) equals | Line voltage (V_phase = V_line) |
|---|---|
| Line current (delta) equals | √3 × phase current (I_line = 1.732 × I_phase) |
| Total three-phase power (delta) | P = √3 × V_line × I_line × power factor |
| Standard three-phase supply voltages (line-to-line) | 400V AC (IEC/Europe/Africa/Aus), 480V AC (USA industrial), 690V AC (high-voltage industrial) |
| Delta terminal connections (IEC motor) | U1-W2, V1-U2, W1-V2 (three links required) |
| Neutral availability | None — delta connection produces no neutral terminal |
| Phase sequence for forward rotation | L1-L2-L3 in correct phase sequence; swap any two lines to reverse rotation |
Safety warnings
- Verify the delta winding connection is correct for the supply voltage before energising. Connecting a delta-wound motor to a voltage higher than its winding rating (e.g., connecting a 230V-winding motor in delta on a 400V supply) will immediately destroy the motor windings.
- All work on motor terminal connections must be done with the motor isolated, locked out, and tagged out per IEC 60204-1 or the applicable national Lockout/Tagout standard. Verify dead at the motor terminal box before opening.
- Delta connections carry line voltage across each winding — in a 400V system, each winding terminal operates at 400V potential difference. Treat all terminals as live until isolation is confirmed and verified.
- Motor wiring must comply with IEC 60034 (rotating machines), IEC 60204-1 (machine electrical safety), and the applicable installation standard — NEC/NFPA 70, BS 7671, AS/NZS 3000, SANS 10142, or the applicable national standard.
- Never operate a three-phase motor in delta on a supply that is missing one phase (single-phasing). Each winding carries the full line voltage and can experience severe overload when one phase is absent. Install phase-loss protection on all delta-connected motors.
Tools needed
- Non-contact voltage tester
- Digital multimeter (voltage, resistance, continuity)
- Torque screwdriver or torque wrench (motor terminal torque compliance)
- Phase sequence tester
- Clamp meter (for measuring line and phase currents)
- Lockout/Tagout kit
- Insulated screwdrivers and hex keys
Common mistakes
- Connecting the motor in delta on a supply voltage equal to the star voltage rating — for a 400/690V dual-voltage motor, delta is correct on 400V; connecting in delta on 690V places 690V across 400V-rated windings and destroys the motor.
- Confusing winding (phase) current with line current when selecting supply cables and switchgear — in delta, line current is √3 times winding current. Undersizing based on winding current leads to overloaded supply infrastructure.
- Omitting one or more delta links — leaving a winding open-circuited creates an open delta condition that allows the motor to start but reduces available torque and causes unbalanced phase currents.
- Connecting all three supply conductors to the same junction point — this connects all three phases together (short circuit) rather than distributing them to three separate junction points.
- Failing to check rotation direction before connecting to the mechanical load — incorrect rotation in delta connection is corrected by swapping any two supply conductors, not by changing the terminal links.
Troubleshooting
- Motor starts but runs at reduced power with unbalanced phase currents
- Cause: One delta link is missing or making poor contact — the circuit is in open-delta configuration Fix: Isolate and lock out. Open the motor terminal box and verify all three delta links are correctly installed and tightly fastened. Measure resistance between each pair of line terminals with supply disconnected — all three readings should be equal. An open-delta configuration shows one reading approximately double the others.
- Windings fail immediately on energisation
- Cause: Motor is connected in delta on a supply voltage higher than the winding's rated voltage Fix: Verify the motor nameplate voltage rating for delta connection. If the supply voltage exceeds the delta winding voltage rating, reconnect in star for the higher voltage or replace with a motor rated for the supply voltage. Do not re-energise until the correct configuration is established.
- Measured line current is significantly lower than the nameplate FLC but the motor appears to be running at full load
- Cause: The overload relay or ammeter is measuring winding (phase) current rather than line current, or the motor is lightly loaded Fix: Confirm clamp meter is placed on one of the three supply line conductors feeding the motor (not inside the terminal box on a winding link). Line current in a delta motor is always √3 × phase current. If the load is correct, recalculate FLC from the nameplate and verify overload relay set point.
Frequently asked questions
What is the relationship between line voltage and phase voltage in a delta connection?
In a delta connection, the phase voltage (voltage across each winding) is equal to the line voltage (voltage between any two line conductors). There is no voltage step-down across the connection topology. For a 400V three-phase supply, each delta winding sees exactly 400V. This contrasts with star connection, where phase voltage = line voltage ÷ √3 (≈ 231V for a 400V supply).
What is the line current formula for a delta-connected load?
For a delta-connected load, line current (I_line) = √3 × phase current (I_phase), where I_phase is the current through each individual winding or load element. For example, if each winding carries 10 A, the line conductors carry 10 × 1.732 = 17.32 A. This factor of √3 is the defining mathematical relationship of the delta connection and must be used when sizing cables and switchgear.
Why does a delta connection have no neutral?
The delta topology forms a closed triangular loop with no common star point — therefore there is no neutral terminal available. Only three line conductors emerge from a delta winding. If a neutral is required by the connected loads, a star-connected winding must be used on that portion of the system, or a zig-zag transformer can derive a neutral from a delta source.
When should a motor be connected in delta rather than star?
A motor is connected in delta when the supply line voltage equals the motor winding's rated voltage. On a 400V three-phase supply, a motor with 400V-rated windings connects in delta. The same motor's windings on a 690V supply would connect in star, where each winding then sees 690V ÷ √3 = 400V — matching the winding rating. The motor's nameplate specifies the correct connection for each voltage.
What is a delta-star (Δ/Y) transformer and why is it used?
A delta-star transformer has its primary winding connected in delta and its secondary in star. This is the standard distribution transformer configuration. The delta primary provides a circulating path for third-harmonic currents (preventing them from entering the supply system), while the star secondary provides a neutral point for single-phase load distribution. The voltage ratio between primary and secondary includes both the turns ratio and the √3 factor.
What is the difference between a Y (star) delta motor wiring diagram and a plain delta connection?
In a star (Y) connection, one end of each winding connects to a common neutral point and line voltage appears across two windings in series, so each winding sees line voltage ÷ √3 (approximately 58 % of line voltage). In a delta (Δ) connection, the windings form a closed loop with no neutral, and each winding is directly across a full line-to-line voltage. A star-delta starter uses both: it starts the motor in star (reduced voltage, lower starting current) then switches to delta (full voltage, running torque) once the motor has accelerated.
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