You've hit a common and important question in electrical engineering. The short answer is yes, you can feed a 480V Delta primary transformer from a 480Y/277V source, BUT with very careful considerations and understanding of the implications.

Here's a breakdown of the considerations

Why it seems counterintuitive:
Delta primaries need phase-to-phase voltage

A Delta-connected winding is designed to be fed with phase-to-phase voltage. In a 480V Delta system, the voltage between any two phases (A-B, B-C, C-A) is 480V.

Wye systems have phase-to-neutral and phase-to-phase

A 480Y/277V system provides both 480V phase-to-phase voltage and 277V phase-to-neutral voltage.

How it can work, and the pitfalls


Using Phase-to-Phase Voltages from the Wye Source

You connect the three phases (A, B, and C) from the 480Y/277V source to the three terminals (A, B, and C) of the 480V Delta primary. The neutral from the 480Y/277V service is not connected to the transformer primary. You are essentially ignoring the 'Y' characteristic of the source and only utilizing the 480V phase-to-phase component.2.

Consequences and Considerations



No Neutral Connection on the Delta Primary

This is the crucial point.

DO NOT CONNECT THE NEUTRAL from the 480Y/277V service to the Delta primary. Connecting the neutral would create a ground fault and severely damage the transformer and potentially cause a catastrophic failure. Voltage Imbalance Sensitivity

A Delta primary is more susceptible to voltage imbalances on the source side than a Wye primary. Even small voltage imbalances on the 480Y/277V source will translate to circulating currents within the Delta winding. Monitor the source voltage for imbalances and derate the transformer if needed.

Harmonic Current Circulation

Delta windings can trap triplen harmonics (3rd, 9th, 15th, etc.). If your loads on the 208Y/120V side are heavy in non-linear loads (computers, VFDs, etc.), those harmonic currents will circulate in the Delta primary winding, causing overheating and reduced transformer life. You may need a K-rated transformer or harmonic filters.

Overvoltage during Single-Phasing (loss of a phase)

A big risk. If one phase of the 480Y/277V source is lost (a blown fuse, a tripped breaker), the remaining two phases can backfeed the transformer, resulting in overvoltage on the 208Y/120V secondary side. This can destroy connected loads. You MUST have robust overvoltage protection on the 208Y/120V secondary side. Fuses or breakers alone are often not fast enough. Consider using surge protective devices (SPDs) that are specifically rated for overvoltage conditions, not just lightning strikes. Also, implement single-phase monitoring on the primary and secondary to quickly disconnect the transformer if single-phasing occurs.

Grounding

The secondary (208Y/120V) side must be properly grounded per NEC requirements. This is critical for safety and proper operation of overcurrent protection devices. Usually, the X0 (neutral) point of the 208Y/120V secondary is bonded to ground.

Transformer Sizing and Derating

Because of potential harmonic currents and voltage imbalances, it is wise to derate the transformer. Select a transformer with a higher kVA rating than your expected load. Consult the transformer manufacturer for their recommendations on derating for harmonic content and voltage imbalance. Oversizing helps dissipate heat and reduces the risk of premature failure.

Transformer Type

Consider using a transformer specifically designed for harmonic mitigation (K-rated) if you expect significant non-linear loads. These transformers have heavier conductors and core designs to handle harmonic currents.

Protection

Ensure adequate overcurrent protection on both the primary and secondary sides of the transformer. Use properly sized fuses or circuit breakers.

Transformer Testing

Upon installation of the transformer, conduct thorough testing, including voltage, current, and impedance measurements, to verify that the transformer is operating within specified parameters and that there are no hidden issues. Regularly schedule preventative maintenance to catch issues before they become major problems.

Why it might not be the best approach



Increased Risk

The potential for overvoltage during single-phasing and the sensitivity to voltage imbalances make this a riskier solution than using a Wye-Wye transformer.

Efficiency

Circulating currents in the Delta primary can reduce transformer efficiency and increase operating costs.

Complexity

The need for special overvoltage protection, harmonic mitigation, and careful monitoring adds complexity to the system design and maintenance.

Code Compliance

Ensure your design complies with all applicable codes and standards, including the NEC. A qualified electrical engineer should perform the design calculations and ensure that all protection devices are properly sized and coordinated.

POCO Restrictions

You mentioned dealing with the POCO. Be very careful to work with them. They may have specific requirements or prohibitions on this type of connection. Getting their approval in writing is essential.

Alternatives to Consider


Negotiate with the POCO

This is often the best first step. Can you work with them to get a 480V Delta service installed? Explain your needs and explore options. Sometimes, there are solutions they can provide that aren't immediately obvious.2.

Use Two Transformers (Zig-Zag or Scott-T)

More complex, but sometimes necessary if a standard Delta-Wye transformer is unsuitable. These involve specialized transformer connections that can handle phase conversion more effectively. This is unlikely the most economical option.3.

Autotransformer

This is usually only if the voltage change is slight. Autotransformers cannot provide isolation between primary and secondary, which can be a safety concern and violate code, depending on the application. You want to step down from 480V to 208V, so an autotransformer will not work.4.

Solid-State Transformer (SST)

Although expensive, SSTs can provide isolation and voltage conversion as well as improved power quality and control over power flow.

In summary

While technically you can use a 480V Delta-Wye transformer with a 480Y/277V source by only connecting the phase conductors and leaving the neutral disconnected, it introduces significant risks and requires careful consideration of potential problems such as voltage imbalances, harmonic currents, and overvoltage during single-phasing. It's generally not the best practice and should only be considered if other options are not feasible, and only with robust protection and monitoring.

Recommendation

Start by contacting the POCO again to explore all possible service options. If you must use a Delta-Wye transformer in this configuration, work with an experienced electrical engineer who understands the risks and can design a system with appropriate protection and monitoring.

Thoroughly document your design decisions and assumptions. This is critical for future troubleshooting and maintenance.

Disclaimer: This information is for educational purposes only and should not be considered a substitute for professional engineering advice. Always consult with a qualified electrical engineer for specific design and installation guidance. Improper installation can result in property damage, injury, or death. Flag for review