When to Specify a Delta to Delta Connected Transformer for Industrial Loads

A delta to delta connected transformer is a three-phase transformer configuration where both the primary and secondary windings are wired in a closed triangular (Δ) loop, providing line-to-line voltage without a neutral point. Unlike the ubiquitous Delta-Wye setup found in commercial buildings, this configuration fundamentally changes how zero-sequence harmonics behave and eliminates the ability to easily derive a standard line-to-neutral voltage for 120V/277V lighting. You will almost exclusively encounter this topology in heavy industrial facilities, utility substations, and renewable energy farms where the load is purely three-phase and harmonic mitigation is critical.

How the Math Works: Line vs. Phase in a Delta-Delta Setup

To properly size conductors and overcurrent protection for a delta to delta connected transformer, you must understand the mathematical split between line values (what you measure at the busbar) and phase values (what the transformer windings actually experience). In a delta configuration, line voltage equals phase voltage ($V_L = V_P$), but line current is $\sqrt{3}$ (1.732) times the phase current ($I_L = 1.732 \times I_P$).

Let’s run a concrete numeric example using a standard industrial step-down transformer: a 150 kVA, 480V Primary to 240V Secondary, 3-Phase unit.

• Secondary Line Voltage: 240V (This is what you measure between any two phases: L1-L2, L2-L3, L3-L1).
• Secondary Line Current: $150,000 \text{ VA} / (240\text{V} \times 1.732) = \mathbf{360.8\text{A}}$. This is the current flowing through your feeder cables to the motor control center.
• Secondary Phase Current: $360.8\text{A} / 1.732 = \mathbf{208.3\text{A}}$. This is the current flowing inside the actual copper windings of the transformer.

Because there is no neutral point in a pure delta setup, the vector sum of the three phase currents must always equal zero under balanced conditions. If the load becomes severely unbalanced, the delta windings circulate the imbalance current internally rather than pushing it to a ground or neutral reference.

Where You Meet This in Practice (and Why It Matters)

You won't find a pure delta to delta connected transformer powering a strip mall or an office building. Its specific electrical characteristics make it the mandatory choice for three distinct real-world environments:

1. Heavy Industrial Motor Control Centers (MCCs): Large induction motors, arc furnaces, and heavy compressors only require three-phase line-to-line voltage. They do not need a neutral. Feeding them from a delta secondary eliminates the cost of running a massive neutral conductor and prevents neutral-to-ground voltage rise issues caused by heavy starting currents.
2. Solar Farms and Inverter Step-Up: Grid-tied solar inverters often output 480V delta. The step-up transformer connecting the solar array to the utility grid is frequently Delta-Delta (or Delta-Wye with a delta primary). The delta winding acts as a trap for triplen harmonics (3rd, 9th, 15th). Think of the delta winding as a continuous roundabout for 3rd-harmonic currents; they circulate endlessly inside the closed loop and are prevented from injecting back into the utility grid or the sensitive inverter electronics.
3. Open-Delta Fallback Scenarios: If one transformer in a three-transformer delta bank fails, you can physically disconnect and remove it. The remaining two transformers will continue to supply three-phase power in an "open-delta" (or V-V) configuration. Note: The bank's capacity derates to exactly 57.7% of its original kVA rating. This is a critical redundancy feature for remote mining operations or water treatment plants where a total shutdown is unacceptable while waiting for a replacement coil.

The Common Confusion: Delta-Delta vs. Delta-Wye

The most frequent mistake junior engineers and DIYers make is assuming all 240V three-phase systems are wired the same. They confuse Delta-Delta with Delta-Wye (Dyn11), which is the standard for 90% of commercial buildings.

In a Delta-Wye setup, the secondary windings are tied together at a central star (Wye) point, which is bonded to ground to create a neutral. This gives you dual voltages: 208V line-to-line for HVAC, and 120V line-to-neutral for standard outlets. A pure delta to delta connected transformer cannot do this. It only provides 240V line-to-line.

The High-Leg Exception: Sometimes, a 240V delta secondary will have one of its three windings center-tapped to ground to provide 120V for small control circuits. This creates a "High-Leg" (or Stinger Leg) delta. While L1 and L3 will read 120V to ground, the center-tapped L2 (the high leg) will read 208V to ground. Per NEC 110.15, this high leg must be identified with orange insulation or orange tagging. Connecting a standard 120V appliance to the high leg will instantly destroy the appliance and create a fire hazard.

Decision Tree: Should You Specify Delta-Delta?

Use this framework to determine if a delta to delta connected transformer is the correct specification for your project. Do not default to Delta-Wye out of habit.

Project Condition	If Yes...	If No...
Do you need a neutral for 120V/277V single-phase lighting or receptacles?	Stop. Specify a Delta-Wye transformer.	Proceed to next question.
Is the load purely 3-phase (motors, VFDs, heaters, industrial rectifiers)?	Proceed to next question.	Specify Delta-Wye or single-phase.
Is the environment prone to high 3rd-harmonic distortion (e.g., large VFD clusters, arc furnaces)?	Specify Delta-Delta. The primary delta will trap harmonics.	Proceed to next question.
Do you require open-delta redundancy for critical uptime?	Specify Delta-Delta.	Either topology works; choose based on cost.

Frequently Asked Questions