Delta Transformer Diagram
This is a free printable delta transformer diagram: download the diagram as SVG or open it and print to paper or PDF.
A delta transformer diagram shows how three single-phase windings are connected end-to-end in a closed triangle, used in 3-wire and high-leg 4-wire distribution systems.
A delta-connected transformer uses three windings arranged in a triangle (delta) configuration. Each winding connects between two phases, forming the sides of the triangle. The terminals at each corner of the triangle become the phase conductors of the supply or load.
In a 3-wire delta system, no neutral conductor exists. All three phases carry equal voltage (for example, 240 V phase-to-phase), and loads must be connected phase-to-phase. This configuration is common for three-phase motor loads where a neutral is not required.
The high-leg (wild-leg) 4-wire delta is a North American distribution configuration that adds a centre-tap neutral on one winding — typically the B-phase winding. This provides 120 V from both A and C phases to neutral for single-phase loads, but the B-phase (high leg) sits at approximately 208 V to neutral (exactly 120 × √3). The high leg must be identified with an orange conductor per NEC 230.56 and 408.3(F) and must never be used to supply 120 V single-phase loads.
Delta-connected primaries offer the advantage that if one transformer fails in a delta bank, the remaining two can continue operating in an open-delta (V-V) configuration at roughly 57.7 % of the original three-phase kVA capacity. This characteristic makes delta transformers well suited to industrial facilities where supply continuity is important.
The delta configuration circulates third-harmonic currents within the closed loop, preventing them from appearing on the line and reducing harmonic distortion seen by the rest of the distribution system. However, the absence of a grounded neutral on both sides of a delta-delta bank means ground-fault detection can be more complex and usually requires a zig-zag grounding transformer or corner grounding.
Transformer nameplate vector-group notation for a delta-connected winding uses the letter D (primary) or d (secondary). A delta-delta bank is written Dd0, where 0 indicates zero degrees of phase displacement between primary and secondary.
How to wire delta transformer diagram
- Identify winding terminals Label each of the three transformer windings with start and finish terminals (H1/H2 for primary, X1/X2 for secondary on each unit). Confirm polarity markings on the nameplate before making any connections.
- Connect the delta loop Join the finish terminal of winding 1 to the start terminal of winding 2, the finish of winding 2 to the start of winding 3, and the finish of winding 3 back to the start of winding 1. This closes the triangle. Each junction becomes a phase terminal (A, B, C).
- Identify the high-leg tap (4-wire systems only) For a high-leg delta, locate the centre-tap terminal on the designated winding (typically the B-phase winding). Connect the neutral conductor to this tap. Mark this neutral connection point clearly on the diagram.
- Label the high leg with orange The phase conductor connected to the untapped end of the centre-tapped winding is the high leg. Label it with orange on the diagram and specify orange conductor insulation or orange tape identification on the physical installation per NEC 408.3(F).
- Verify phase voltages before energising With a calibrated voltmeter, confirm phase-to-phase voltages are equal and phase-to-neutral voltages on the two standard legs match expectations (e.g. 120 V). Measure the high-leg voltage to neutral and confirm it is approximately 1.732 times the standard phase-to-neutral voltage.
- Document the vector group Record the vector group (e.g. Dd0) on the diagram, along with rated primary voltage, secondary voltage, kVA rating, and impedance percentage from the nameplate. This information is essential for paralleling transformers and for fault-level calculations.
Specifications
| Configuration | Delta (closed triangle) — 3-wire or 4-wire high-leg |
|---|---|
| Vector group (delta-delta) | Dd0 (zero degrees phase displacement, per IEC 60076-1) |
| High-leg voltage to neutral (120/240 V system) | Approximately 208 V (= 120 × √3) |
| Open-delta capacity | 57.7 % of original closed-delta three-phase kVA |
| High-leg conductor identification colour | Orange (NEC 230.56 and 408.3(F)) |
| Typical primary voltage (North American distribution) | 4 160 V, 7 200 V, 12 470 V, or 13 800 V (utility-dependent) |
| Applicable standards | NEC/NFPA 70, IEC 60076-1, AS/NZS 3000, BS 7671 |
Safety warnings
- Always isolate and lock out / tag out (LOTO) the transformer on both the primary and secondary sides before inspecting or working on any connections. Verify the circuit is dead with a calibrated voltage tester before touching conductors.
- High-leg (wild-leg) conductors carry approximately 208 V to neutral in a 120/240 V 4-wire delta system. Never connect 120 V rated equipment to the high leg. Clearly identify the high leg with orange markings throughout the installation.
- Transformer work involving primary voltages above 600 V must be performed only by qualified electricians in accordance with applicable codes (NEC/NFPA 70, BS 7671, AS/NZS 3000, IEC 60364). Local authority permits may be required.
- Energised transformer enclosures may carry induced voltages even when the primary supply appears isolated. Treat all exposed conductors as live until confirmed dead with a properly rated test instrument.
- This diagram is provided for illustrative and reference purposes only. Always engage a licensed electrical engineer or electrician for design, installation, and commissioning.
Tools needed
- Calibrated true-RMS voltmeter (CAT III or CAT IV rated for the system voltage)
- Clamp-type ammeter
- Insulation resistance (megohm) tester
- Phase-rotation indicator
- Torque wrench for terminal bolts
- Lockout/tagout devices and locks
- Permanent marker and orange conductor tape for high-leg identification
Common mistakes
- Reversing the polarity of one winding during delta closure, creating a short-circuit loop when energised — always verify terminal polarity before closing the delta.
- Connecting 120 V loads to the high leg in a 4-wire delta system, exposing equipment to approximately 208 V and causing immediate damage.
- Omitting overcurrent protection on the secondary side, relying solely on the primary fuse — each secondary circuit requires its own protection per NEC 450.3.
- Attempting open-delta (V-V) operation at full original kVA, not recognising the 57.7 % capacity limitation and overloading the two remaining transformers.
- Failing to document or label the vector group after commissioning, leading to incorrect paralleling of transformers with different phase displacement groups.
Troubleshooting
- Unequal secondary phase voltages after energising
- Cause: One winding is connected with reversed polarity or a winding has an internal fault, disrupting the balanced delta loop Fix: De-energise and lock out. Check polarity marks on each winding and trace all terminal connections against the diagram. Use a megohm tester to check winding insulation before re-energising.
- High circulating current within delta loop at no load
- Cause: Transformers with mismatched turns ratios or impedances connected in parallel delta, causing circulating current even without external load Fix: Verify nameplate voltage ratios and impedance percentages match on all units. Mis-matched impedances require load-sharing calculations; consult a qualified engineer before paralleling.
- Tripping of primary overcurrent protection immediately on energisation
- Cause: Transformer inrush current exceeding protective device rating, or a polarity reversal creating a fault current path within the closed delta Fix: Confirm polarity connections first. If correct, check that the overcurrent device is rated to withstand inrush (typically 8–12 times full-load current for 0.1 s). Consider time-delay fuses or circuit breakers with appropriate instantaneous settings.
Frequently asked questions
What is the difference between a 3-wire delta and a 4-wire high-leg delta?
A 3-wire delta has no neutral conductor — all loads connect phase-to-phase. A 4-wire high-leg delta adds a centre-tap neutral to one winding, providing 120 V from two phases to neutral but leaving the third phase (high leg) at approximately 208 V to neutral, which must not feed 120 V loads.
Why is the high leg identified with orange colour?
NEC 230.56 and 408.3(F) require the conductor with the higher voltage to ground in a 4-wire delta system to be identified by orange colouring so electricians immediately recognise that it cannot supply standard 120 V single-phase loads and avoid dangerous miswiring.
Can a delta transformer bank continue operating with one transformer removed?
Yes. Two remaining transformers form an open-delta (V-V) connection that still supplies three-phase voltage. However, the available three-phase kVA capacity drops to approximately 57.7 % of the original closed-delta rating, so loads must be reduced accordingly.
What does the vector group notation Dd0 mean?
D indicates a delta-connected primary, d indicates a delta-connected secondary, and 0 indicates zero degrees of phase displacement between primary and secondary voltages. This notation follows IEC 60076-1 and appears on transformer nameplates.
Why does a delta winding suppress third-harmonic distortion?
Third-harmonic currents (and multiples of 3) are in phase with each other in a three-phase system. A closed delta winding provides a circulating path for these currents within the triangle, preventing them from flowing onto the line conductors and polluting the distribution network.
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