Tapping a building transformer's secondary conductors or bus bar to add a BESS. The system is 12kV and is a campus with distribution transformers throughout the facility. A BESS is being installed on the secondary of one of the building's 1MVA transformers. The "tap" would originate in the buildings main switchboard which is fed from the 1MVA transformer via an underground duct. The design was to install lugs on the line side of the main switchboard and run a tap to nearby disconnecting means.
Since the service connection is in a completely different location, the transformer is just a regular transformer, not a service transformer. From the transformer to the building, the transformer must conform to outside taps of unlimited length, the source originates outdoors, and terminates as soon as it enters the building; please note, 240.21(C)(4), applies to this connection. This might not be a tap if the secondary of the transformer has internal fusing.
If this is a tap, then the connection for the battery system can’t be made at the location specified unless an additional OCPD is added at the transformer, or directly outside of the building, 240.21.