Okay, let's break down this corner-grounded delta situation and how to safely and correctly integrate your transformer for the house. This is a critical situation, and miswiring can lead to equipment damage or safety hazards.

Understanding Corner-Grounded Delta Systems In a corner-grounded delta system, one of the phase conductors (in your case, Phase B) is intentionally connected to ground at the service entrance. This differs from a solidly grounded wye system, where the neutral point is grounded. The voltage between the grounded phase (Phase B) and ground is zero. The voltage between the ungrounded phases (Phases A and C) and ground is equal to the phase-to-phase voltage (480V in your case). Overcurrent protection devices (fuses or breakers) are installed in all three phase conductors. Corner-grounded delta systems are less common than other grounding methods, but they are still used in some industrial applications, often due to legacy installations.

Addressing Your Questions1. Corner-Grounded Delta and Delta-Wye Transformer Compatibility Yes, there are issues that need careful consideration. You cannot simply connect a standard delta-wye transformer to a corner-grounded system without modifying it. Standard delta-wye transformers are not designed to have one of the delta phases directly connected to ground. If you ground the H1 terminal as you would for a standard, ungrounded delta, it will short out Phase B. The entire system is predicated on a phase already being the ground reference. Options



Isolation Transformer

The safest and best approach is to use an isolation transformer (480V Delta to 480V Delta) on the supply side. This would allow you to create an ungrounded delta system, then use your 480V Delta to 208Y/120V transformer as a separately derived system. This isolates the corner grounded system from the new system.

Consult Transformer Manufacturer

The only other option is to explicitly consult the transformer manufacturer. You might be able to get a transformer designed specifically for corner-grounded delta input. However, this is less common and likely more expensive than the isolation transformer approach. They may offer a modified winding configuration, or special instructions, but you need to get it in writing from the manufacturer. Do

NOT proceed without that.2. Transformer Connections (Assuming you use the Isolation Transformer approach) H1 Grounding

With the isolation transformer approach: Yes, on the delta-wye transformer (the second one, the 480-208/120V), you'd ground H1 as normal, making it a separately derived system.

Phase B Connection

With the isolation transformer approach: The isolation transformer will provide a standard 480V Delta service for the primary side of your Delta-Wye transformer.

Manufacturer Instructions

Always follow the transformer manufacturer's instructions and nameplate diagram.3.

Grounding Electrode Conductor (GEC) Separately Derived System

With the Isolation Transformer setup, the secondary (208Y/120V) side of your delta-wye transformer is a separately derived system per the NEC.

Dedicated GEC

You absolutely must have a dedicated grounding electrode conductor (GEC) for the secondary of the Delta-Wye transformer, connected to a suitable grounding electrode (ground rod, building steel, etc.). See NEC Article 250 for details.

Bonding

You must also properly bond the transformer enclosure, the neutral point of the wye-connected secondary, and the grounding electrode(s) together with appropriate bonding jumpers.

Reason

The GEC provides a low-impedance path for fault current to return to the source (the transformer secondary), allowing overcurrent protection devices to operate and clear the fault. This is crucial for safety.

Key Considerations and Warnings Safety First

Working with 480V is dangerous. Hire a qualified electrician experienced with corner-grounded delta systems.

NEC Compliance

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

Coordination

Coordinate with the local utility company. They may have specific requirements for connecting to their system.*

Documentation

Keep thorough documentation of all work, including transformer nameplate information, wiring diagrams, and grounding electrode information.

In summary, the isolation transformer approach is the safest and most generally recommended way to proceed. Trying to adapt a standard delta-wye transformer to a corner-grounded delta system without explicit manufacturer approval is extremely risky. Flag for review