An open delta transformer is a three-phase transformer configuration that uses only two single-phase transformers instead of the usual three. By removing one transformer from a standard delta connection, the system can still deliver three-phase power, but at roughly 58% of the capacity a full three-transformer bank would provide. It’s a practical compromise between cost and capability, widely used by utilities serving areas where three-phase demand is relatively light.
How Open Delta Works
In a standard (closed) delta transformer bank, three single-phase transformers are wired in a triangle. Each transformer handles one leg of the triangle, and the three-phase output comes from the connection points between them. In an open delta configuration, one of those three transformers is simply removed, leaving two transformers connected end to end in a “V” shape rather than a closed triangle.
Despite the missing transformer, the two remaining units still produce three separate phase voltages. The physics of alternating current means the voltage across the open gap naturally appears as a valid third phase. So any three-phase motor or load connected to the output sees all three phases and runs normally, just with less total power available.
There’s an important quirk in the voltage, though. The line-to-line voltages across all three phases remain equal, but the line-to-neutral voltages do not. Two phases will have equal voltage to neutral, while the third (the “wild leg” or “high leg”) is 1.732 times larger. This uneven neutral voltage is something electricians have to account for when wiring panels, since plugging a 120V device into the high leg would expose it to roughly 208V instead.
The 58% Capacity Rule
You might expect that removing one of three transformers would drop the bank’s capacity to 67%. In reality, the drop is steeper. An open delta bank delivers only 57.7% of the capacity of a full three-transformer delta or wye system of the same size. That’s a 42% decrease in capacity even though the installed hardware only dropped by 33%.
The reason comes down to internal power factor. In a closed delta, each transformer operates at or near its full rated capacity. In an open delta, the two transformers don’t share the load symmetrically. One operates with a leading power factor and the other with a lagging power factor, each shifted 30 degrees from the load. Even when the connected load has a perfect (unity) power factor, each transformer’s internal power factor tops out at 86.6%. That means each transformer can only deliver 86.6% of its nameplate rating.
To put concrete numbers on it: two 300 kVA transformers in an open delta have a combined nameplate rating of 600 kVA, but the maximum load you can actually connect is about 520 kVA (600 × 0.866). This derating is a fixed characteristic of the configuration, not something that can be designed around.
Why Utilities Use Open Delta
Open delta configurations are common in rural and suburban areas where a location needs three-phase power but the demand doesn’t justify three full transformers. A small machine shop on a mostly residential street, for example, might need three-phase service for its equipment while the surrounding homes only need single-phase. The utility can install two transformers on the pole instead of three, saving on equipment, installation labor, and pole space.
The setup is also popular because it provides a built-in upgrade path. If three-phase demand in the area grows, the utility can simply add a third transformer of the same size to close the delta, boosting capacity by about 73% without replacing any existing equipment. This is far cheaper and easier than swapping out an entire transformer bank for a larger one.
Open delta is especially practical for providing both 240V three-phase and 120V single-phase service from the same bank. Residential loads pull 120V and 240V single-phase, while a nearby commercial customer can draw three-phase 240V for motors and compressors. One transformer bank handles both, which is why this configuration shows up so frequently on utility poles in mixed-use areas.
Drawbacks and Limitations
The biggest limitation is the reduced capacity. At 57.7% of a full bank’s output, open delta is only suitable for lighter three-phase loads. Heavy industrial operations that need significant three-phase power will almost always require a closed delta or wye configuration.
Voltage balance is another concern. Unbalanced single-phase loads connected to an open delta bank can cause voltage fluctuations across the phases. When one transformer ends up carrying significantly more load than the other, it heats unevenly, which shortens its lifespan and can degrade the voltage quality delivered to customers. Three-phase motors running on noticeably unbalanced voltage draw more current, run hotter, and wear out faster.
The wild leg issue mentioned earlier also limits flexibility. The high-leg phase carries a higher voltage to neutral, which means it cannot safely supply standard 120V circuits. Electricians must identify and mark this leg clearly in the panel, and it effectively becomes unavailable for single-phase loads. This reduces the usable single-phase capacity of the system.
Open Delta vs. Wye Configurations
The main alternative to a delta configuration (open or closed) is a wye connection, where three transformers are wired in a star pattern with a shared neutral point. Wye systems include a neutral wire, which makes them more versatile for distribution networks. They can supply both single-phase (line to neutral) and three-phase (line to line) loads without the wild-leg complication that delta systems introduce.
Wye configurations handle unbalanced loads more gracefully and work well with the mix of single-phase and three-phase customers found in most distribution networks. This is why wye connections have become the dominant choice for general power distribution. Delta connections, including open delta, tend to show up more in situations involving high starting torque requirements (like large motors), power transmission over shorter distances, or the cost-driven scenarios where two transformers need to do the work of three.
An open wye configuration also exists, using two transformers in a wye arrangement, but it’s less common than open delta. Open wye systems can introduce problematic voltage imbalances and don’t handle certain fault conditions as cleanly, so open delta remains the go-to choice when a two-transformer three-phase solution is needed.