You've asked a very important and nuanced question about a service change that is more complex than it initially appears. Here's a breakdown of the factors involved and considerations

The Core Issue: Can you convert a 240V Delta Corner-Grounded to 240/139V Wye with the Existing Transformers?

The simple answer is highly unlikely without replacing the transformers. While electrically you might be able to reconfigure the connections, there are several critical reasons why this is generally a bad idea and often outright impossible:1. Transformer Voltage Ratings and Winding Insulation



The Transformers are likely designed for 240V Delta

Corner-grounded delta transformers are designed to have 240V across each winding (phase-to-phase). A Wye system introduces the concept of phase-to-neutral voltage (139V in your case). Applying 240V across a winding intended for 139V will severely overstress the insulation and lead to rapid failure. The insulation between the windings and the core is designed for a specific voltage level.

Insulation to Ground

In a corner-grounded delta, one phase conductor is bonded to ground. The insulation on the transformer windings may be designed with the assumption that it's close to ground potential. In a Wye configuration, the phase conductors have a voltage to ground, requiring better insulation.2.

Transformer Impedance and kVA Rating



Designed for Delta Loads

Transformers are designed with specific impedance to limit fault current. Reconfiguring them may alter the impedance, potentially leading to issues with overcurrent protection and voltage regulation.

kVA Rating

While the kVA rating might seem the same, the way the kVA is delivered changes. In a delta system, kVA is phase-to-phase. In a wye system, it's phase-to-neutral.3.

Grounding Considerations



Corner-Grounded Delta

The corner-grounded delta system intentionally bonds one phase conductor to ground. This provides a ground fault return path.

Wye System

Wye systems typically have a neutral conductor bonded to ground at the transformer. You cannot simply convert a corner-grounded delta to a wye by just connecting one of the delta points to a neutral. This creates a direct short circuit.4.

Availability of a Neutral Point



Delta Transformers

Delta transformers do not inherently have a neutral point. A Wye system requires a neutral point. You cannot create a neutral point from a delta configuration without special (and costly) techniques like a zig-zag grounding transformer (which is specifically designed for that purpose, not for converting a service).

Why the Change to Wye Might be Desired



Availability of 120V/240V Loads

A 240/139V Wye allows for both 240V (phase-to-phase) and 139V (phase-to-neutral) loads. This is advantageous if you need to serve single-phase loads. (Although, 139V is an uncommon voltage, 120/208 is far more common)

More Stable Grounding

Wye systems, when properly grounded, often provide a more stable and predictable grounding environment than corner-grounded delta.

What Needs to Happen


New Transformers

The most likely and safest solution is to replace the three 240V delta transformers with three transformers designed for the specific 240/139V Wye system you need. This ensures the windings are properly rated and insulated for the voltage levels involved.2.

Neutral Conductor

Run a properly sized neutral conductor from the transformer secondary to the service entrance.3.

Proper Grounding

Establish a solid ground connection at the transformer and the service entrance, following all applicable codes.4.

Metering Changes

Yes, the metering will almost certainly need to change. Here's why:

CT Ratios

The current transformer (CT) ratios will likely need to be different to accurately measure the current in the Wye system. The expected current levels on the secondary side will change with the voltage.

Meter Type

The meter itself might need to be changed to accommodate the new voltage and system configuration. Delta and Wye meters are wired differently.

Consider smart meter implementation, if not already implementedImportant Considerations



Utility Approval

Crucially, you must coordinate any service change with your local utility (POCO). They will have specific requirements for transformer size, grounding, metering, and protection.

Load Calculations

Perform a thorough load calculation to determine the appropriate transformer size for the new system. Don't just assume the existing kVA is sufficient.

NEC Compliance

Ensure all work complies with the National Electrical Code (NEC) and any local codes.

In summary: Converting a 240V delta corner-grounded service to 240/139V Wye is not a simple re-wiring job. It typically requires replacing the transformers, establishing a proper neutral and ground, and updating the metering equipment. Always consult with your utility and a qualified electrical engineer or contractor before undertaking any service change.* Flag for review