Utility transformers are generally regulated in the US using the National Electrical Safety Code. Local code officials have no authority over electrical utility systems and would not be involved in specifying the protection of transformers with deluge sprinklers (or any other form of active fire protection).
Now if a utility wants to install a deluge system, and the jurisdiction has adopted a model fire code like the International Fire Code, then the utility is required to submit the shop drawings and hydraulic calculations to the fire code official for review and approval. Installation of a fire protection system, even if it is not required, necessitates a construction permit in IFC Section 105.8.
From an insurance co. point of view, yes they are required depending on the size of the unit, back up unit availability, etc. Sometimes we will also ask for a 4 hour fire wall separating the transformers from each other AND the deluge protection. Most times we will also ask for air release for the deluge valve via 1/2'pipe with fusible sprinklers to increase reliability. Bottom line ask who the carrier is and see what they want.
A deluge system is not usually required for a system servicing a small manufacturing facility, but is typically required for a power plant (see NFPA-850)
NFPA-850 covers many other requirements for power plant fire protection.
NFPA 850 (Thermal power plants), NFPA 851(hydroelectric power plants) contains the requirement, IEEE 979 for substations, but it is more a company/local/insurers issue, NFPA 15 has specific design requirements for deluge systems for transformers.
