An open delta transformer is a three-phase power configuration using only two single-phase transformers to deliver three-phase voltage, typically serving as an emergency backup or a cost-saving measure for light loads. When a facility needs three-phase power but the total load is small, or when one transformer in a standard three-transformer bank fails, dropping to an open delta (also known as a V-V connection) keeps the motors turning and the panels energized. However, because you are missing the third leg of the physical delta, the electrical vectors shift, fundamentally changing how the remaining transformers share the load and limiting your total system capacity.
The Math Behind the 57.7% Capacity Rule
The most critical concept to grasp about an open delta transformer bank is that two transformers do not provide 66.7% (two-thirds) of the capacity of a three-transformer bank. They provide exactly 57.7%. Think of it like a three-cylinder engine forced to run on two cylinders: you lose more than just one-third of your power because the remaining cylinders have to work out of their optimal firing phase to keep the crankshaft turning smoothly.
In a standard closed delta bank, the total capacity is the simple arithmetic sum of the three transformers. In an open delta, the two transformers must supply the vector sum of the three-phase load. Because the voltage and current vectors are 120 degrees apart, the math relies on the square root of 3 (1.732).
Worked Numeric Example: Sizing a 75 kVA Bank
Suppose you are installing two identical 75 kVA single-phase transformers (such as standard Howard Industries or ABB pole-top units) in an open delta configuration to feed a small 480V irrigation pump.
- Closed Delta Capacity (if you had 3 units): 75 kVA + 75 kVA + 75 kVA = 225 kVA
- Open Delta Capacity (with 2 units): 75 kVA × 1.732 = 129.9 kVA
Notice that 129.9 kVA is exactly 57.7% of the 225 kVA closed-delta capacity. If your load draws 140 kVA, your two 75 kVA transformers will overheat and fail, even though 140 kVA is less than their arithmetic sum (150 kVA).
The Hidden Power Factor Penalty
Here is the detail that burns out undersized open delta banks: the two transformers do not share the load equally unless the load power factor is exactly zero (purely inductive or capacitive). At a standard unity power factor (1.0), one transformer operates at a power factor of 0.866, and the other also operates at 0.866. This means the transformers are internally stressed by reactive power even if the load itself is purely resistive. You must size your wire and overcurrent protection based on this internal phase shift, a requirement detailed in All About Circuits' three-phase transformer theory guide.
What Changes in Your Installation (and Common Confusions)
Switching from a closed delta to an open delta changes three physical realities on the jobsite:
- Cost and Weight: You eliminate one-third of the iron, copper, and oil. A 300 kVA closed delta padmount might cost $18,000 and weigh 4,500 lbs, while an open delta bank using two 150 kVA units drops the equipment cost to roughly $11,000 and cuts the weight significantly, allowing for simpler concrete pad pours.
- Voltage Unbalance: Open delta banks are notoriously sensitive to single-phase line-to-neutral loads. If you tap a lighting circuit off one of the transformers, the voltage on the other two phases will sag or swell, potentially tripping VFDs (Variable Frequency Drives) on the unbalance fault.
- Protection Sizing: Under NEC Article 450, primary and secondary fusing must be calculated based on the individual transformer nameplate, but the feeder conductors must be sized for the 57.7% bank capacity limit.
What People Commonly Confuse It With
The most frequent error made by junior engineers and apprentices is the 66% Myth. Because you are using 2 out of 3 transformers, human intuition assumes 66% capacity. As proven above, the vector math limits it to 57.7%.
Secondly, open delta is often confused with a Broken Delta. An open delta (V-V) is a power delivery configuration used to supply three-phase loads. A broken delta is a specific grounding and protection topology used in generator windings and relay protection schemes to detect ground faults; it does not supply power to loads. Finally, it is sometimes confused with a Scott-T connection, which uses two transformers to convert three-phase power to two-phase power (a legacy setup for old furnaces), whereas open delta delivers standard three-phase power.
Where You Meet This in Practice
You will rarely see an open delta transformer bank specified for a new, large-scale commercial build. Instead, you will encounter it in three specific scenarios:
- Rural Utility Distribution: A utility needs to power a single 20 HP three-phase grain auger at the end of a long dirt road. Running a full three-transformer bank is overkill. They will hang two 25 kVA single-phase cans on the pole in an open delta, saving thousands in equipment and structural pole reinforcement.
- Emergency 'Limp-Home' Mode: A manufacturing plant relies on a 500 kVA closed delta padmount. A lightning strike blows the primary winding on the 'B' phase transformer. Instead of shutting down the plant for three weeks while waiting for a replacement, the utility disconnects the blown can, cross-straps the secondary, and runs the plant on the remaining two transformers at 57.7% capacity until the replacement arrives.
- Temporary Construction Power: Builders setting up temporary service for tower cranes and temporary lighting often use open delta configurations to save on rental costs for temporary padmounts, provided the crane motor starting currents don't exceed the reduced kVA limit.
Decision Tree: Open Delta vs. Closed Delta vs. Wye
Choosing the right three-phase transformer topology dictates your installation cost, fault tolerance, and load flexibility. Use this decision matrix to spec your next project.
| Criteria | Open Delta (V-V) | Closed Delta (Δ-Δ) | Wye-Wye (Y-Y) / Delta-Wye (Δ-Y) |
|---|---|---|---|
| Transformer Count | 2 Single-Phase | 3 Single-Phase (or 1 3-phase) | 3 Single-Phase (or 1 3-phase) |
| Capacity vs Nameplate | 57.7% of sum | 100% of sum | 100% of sum |
| Single-Phase Lighting Loads | Poor (causes severe unbalance) | Good (if center-tapped lighting tap used) | Excellent (dedicated neutral) |
| Fault Tolerance | Zero (lose one, lose all) | High (can limp home as open delta) | Zero (lose one, lose all) |
| Relative Cost | Lowest | High | Medium to High |
The Concrete Pick
If you are designing a permanent installation: Default to a Delta-Wye (Δ-Y) configuration (like the Eaton V75D150T 75 kVA padmount) to provide a stable neutral for mixed 3-phase and 1-phase loads.
If you are designing a temporary site or rural pump station under 100 kVA: Spec an Open Delta using two identical single-phase units (e.g., two ABB 50 kVA pole-top transformers).
Never spec an open delta for permanent commercial buildings with mixed lighting and HVAC loads; the voltage unbalance will destroy sensitive electronics and trip modern AFCI/GFCI breakers.
FAQ: Sizing, Protection, and Upgrades
Can I fuse the primary of an open delta bank at the full 3-phase load current?
No. Under NEC Article 450, transformer overcurrent protection is based on the continuous current rating of the individual transformer windings, not the theoretical system load. Because of the 30-degree phase shift internal to the open delta, the primary current on each transformer will be higher than a simple $I = P/V$ calculation suggests. Always size primary fuses at 125% to 250% of the individual transformer's primary full-load amp rating, consulting the manufacturer's fuse coordination tables.
What happens if I add a third transformer later?
If you wire the third identical transformer into the bank, you transition from an open delta to a closed delta. Your capacity instantly jumps from 57.7% to 100% of the arithmetic sum (e.g., from 86.6 kVA to 150 kVA in a three-50kVA bank). This is a common utility upgrade path: they install an open delta to serve a new facility's initial low load, then drop a third can on the pole three years later when the facility expands, avoiding the need to replace the entire padmount or pole structure.
Do I need a grounding bank for an open delta?
An ungrounded delta system can develop severe transient overvoltages during an arcing ground fault. If your open delta bank is ungrounded, it is highly recommended to install a zig-zag grounding transformer or use corner-grounded delta practices (per NEC 250.36) to stabilize the phase-to-ground voltages and provide a reliable path for ground-fault current to trip your breakers.
