Good morning! This is a common and crucial point to clarify in electrical design. Let's break it down.

Your understanding is largely correct. Since your utility meter and demarcation point are upstream of the 34.5kV fused switchgear, and this switchgear feeds only the 3000kVA transformer, then the 34.5kV fused switchgear acts as your service disconnecting means.

Here's the reasoning and code context (primarily based on the NEC)



Service Equipment Definition

NEC Article 100 defines "Service Equipment" as "The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to a building or other structure or an otherwise designated area or space. The service equipment is intended to constitute the main control and cutoff means of the supply."

Service Conductors

NEC Article 230 covers service conductors. These are the conductors from the utility's distribution system to the service equipment. In your case, those conductors are the ones running to your 34.5kV switchgear.

Overcurrent Protection

NEC 230.90 and similar sections require overcurrent protection for service conductors. In your situation, the fuses in the 34.5kV switchgear provide that protection.

Service Disconnecting Means

NEC 230.70 and similar sections require a service disconnecting means. This is a way to shut off all power to the building or structure. Again, your 34.5kV fused switchgear effectively provides this function, as it isolates the entire transformer and downstream equipment.

Why you likely DON'T need a service-rated ATS

Since the transformer is fed from the 34.5kV switchgear (your service equipment), the 480V side of the transformer is considered a separately derived system (NEC Article 250.30). Because it's a separately derived system, the 480V switchboard isn't directly connected to service conductors. Therefore, the ASCO 7000 ATS on the 480V side does not need to be service-rated, provided it meets all other applicable codes and standards (e.g., listing, interrupting rating, etc.).

Important Considerations and Best Practices



Review Local Codes and AHJ (Authority Having Jurisdiction)

Always confirm with your local inspector or AHJ. They have the final say and may have specific interpretations or requirements. They might prefer a service rated ATS even if it's not technically required by the NEC.

Transformer Overcurrent Protection

Ensure the 3000kVA transformer has proper overcurrent protection on both the primary (34.5kV) and secondary (480V) sides, according to NEC 450 and any applicable local codes.

ATS Interrupting Rating

Verify that the interrupting rating of the ASCO 7000 ATS is adequate for the available fault current at its location on the 480V system. Perform a fault current study to determine the available fault current.

Grounding

Properly ground the separately derived system (transformer secondary) per NEC 250.30.

Labeling

Clearly label the 34.5kV switchgear as the "Service Disconnecting Means" and the 480V switchboard as the "Separately Derived System Distribution Panel."*

Coordination Study

Performing a coordination study is HIGHLY recommended to ensure proper operation of all overcurrent protective devices, from the 34.5kV fuses down to the branch circuits in the 480V distribution system.

In summary

Based on the information you've provided, a non-service rated ATS is likely acceptable in your application, but always verify with your local AHJ and ensure you meet all other applicable code requirements, especially regarding overcurrent protection, interrupting rating, and grounding.

Disclaimer: I am an AI Chatbot. This information is for general guidance only and does not constitute professional engineering advice. Consult with a qualified electrical engineer for specific design and code compliance. Flag for review