Okay, let's break down these two questions regarding NEC and supply-side PV interconnections.

Question 1: 250.25 and Transformer Characteristics for Supply-Side Interconnections Your Interpretation of 250.25

You're correct in pointing out that 250.25 doesn't explicitly demand a perfect match of all phase and neutral characteristics to the existing service. It requires compliance with 250.24(A) through (D), which provides flexibility. However, the real-world application of this can be tricky and depends heavily on local utility requirements and accepted industry practice.

Your Example: 2000A 208/120V Service to 480/277V PV with 480/277V-208V Transformer

The scenario you've described is possible under the NEC, but highly unlikely to be acceptable by the utility. Here's why:

NEC Allows It (Potentially)

Technically, as you stated, a step-down transformer is allowed. The 208V winding can be wired in Delta as long as the system grounding and bonding methods are installed correctly.

Utility Concerns

Utilities are very protective of their grid. They are likely to reject this design due to many reasons. Voltage flicker, voltage imbalance on the grid, and the fact that a 3Ph 3W system cannot supply any single phase loads are all concerns. It is highly likely that the utility company would require a transformer that is 208/120V wye, and the inverters would have to match that output.

Key Considerations for Acceptability



Utility Interconnection Agreement

This is the MOST IMPORTANT FACTOR. You are at the utility's mercy. They have the ultimate say on what is allowable. Their engineering review will dictate what they'll allow. This will often override the NEC, especially regarding protective relaying, grounding configurations, and harmonic distortion limits.

Neutral Availability

The existing service is 208/120V, so it has a neutral. If the transformer's utility (208V) side is Delta-connected, the utility would need to evaluate how that affects system grounding and potential overvoltages, etc.

Harmonics

480V inverters, especially older ones, can inject significant harmonics into the system. A Delta-Wye transformer can help mitigate some of those harmonics, but the utility will still need to analyze them.

Fault Current Contribution

The utility will be extremely concerned about the fault current contribution of the PV system, especially on the 208V side. They will want to ensure their protective devices can still clear faults adequately.

Voltage Rise

Excessive backfeed from the PV can cause voltage rise on the utility's distribution system. They will want to see a thorough voltage study.

Best Practice

As you mentioned,

default to utility new service guidelines for supply-side interconnections. This typically avoids surprises later and ensures a smoother approval process.

In Summary

While the NEC might allow this from a strictly code-compliance standpoint, practically, it's highly probable the utility will require a 208/120V transformer configuration to properly align with their existing service characteristics. Check with them FIRST.

Question 2: 250.30(C) and Grounding Electrode for Separately Derived System (Transformer) Your Scenario

Transformer outside the building, Point of Interconnection (POI) inside the building, PV on the roof. You're asking if the existing building GES, tied to the service disconnect EGC, satisfies 250.30(C)'s grounding electrode requirement.

The Answer is NO, it does not satisfy the requirement. 250.30(C) specifically says "if the source of the separately derived system is located outside the building or structure supplied, a grounding electrode connection shall be made at the source location to one or more grounding electrodes in compliance with 250.
50." The grounding electrode system (GES) at the service equipment in the building and the grounding electrode connection at the transformer would need to be bonded together. Why This Matters

250.30(C) is designed to reduce voltage potential between different grounding systems. Because the transformer is outside, it's likely to have a different ground potential than the existing building. This can create circulating currents in the grounding conductors and potential safety hazards during a fault.

Your Assumption About Grounding Pad Connection

The fact that the grounding pads on the primary and secondary sides of the transformer are electrically connected inside the transformer is relevant for equipment grounding, but it doesn't replace the need for the grounding electrode system at the transformer's location if outside the building. That internal connection ensures that the transformer enclosure is properly bonded, but it doesn't address the potential ground potential difference issue.

How to Comply


Install a Grounding Electrode System at the Transformer Location

Drive ground rods, connect to a metallic water pipe (if available and permitted), etc., meeting the requirements of 250.50. This establishes a local ground at the transformer.
2.

Connect the New GES to the Existing GES

Run a grounding electrode conductor (GEC) from the new GES (at the transformer) back to the service equipment (where the building's main GES is located). The size of this GEC is determined by Table 250.66 based on the size of the largest ungrounded conductor of the PV system.
3.

The EGC Alone is Not Enough

The EGC run from the transformer back to the service equipment's grounding bus is there for fault clearing, not for establishing a robust, independent grounding electrode system at the transformer location.

In Summary

You must install a grounding electrode system at the transformer location (if the transformer is outside the building) per 250.30(C) and connect that new grounding electrode system back to the building's main grounding electrode system with a GEC. The EGC alone is not sufficient.

Important Considerations for Both Questions



AHJ and Utility Override

You are absolutely right. Always prioritize the Authority Having Jurisdiction (AHJ) and the utility's rules and requirements. What the NEC allows, they may not.

Coordination is Key

Talk to the utility and the AHJ early in the design process. Get their feedback on your plans before you commit to a specific configuration. This can save you time and money in the long run.*

Documentation: Thoroughly document your design, including grounding and bonding details, and provide it to the utility and AHJ for review.I hope this comprehensive explanation clarifies your questions. Please let me know if you have any further questions. Flag for review