Nov 26 (Reuters) - Constellation Energy CEG.N has called on the nation's largest grid operator to include in its tariff rules guidance for interconnected electricity suppliers to follow when supplying to data centers located next to power plants.

In a complaint filed against PJM Interconnection with the Federal Energy Regulatory Commission on Friday, Constellation alleged that some local utilities are using the lack of such rules to thwart competition from power generators by opposing the so-called co-located data centers.

The federal regulator earlier in November had rejected an agreement to increase the power capacity of a data center connected directly to a nuclear plant in Pennsylvania, citing risks to grid reliability and a rise in costs for consumers.

Constellation, which is the largest operator of U.S. nuclear power plants, had backed Talen Energy TLN.O in that regulatory battle.

Connecting large data centers directly with power plants has emerged as a major priority for Big Tech's plans to quickly access large amounts of power for its AI expansion instead of waiting for years to connect to the grid.

Constellation also said utility firm Exelon EXC.O, its former parent, refusing to finish interconnection work at the LaSalle nuclear plant to support co-location of a data center could add $15 million to $19 million in costs.

PJM, Constellation and Exelon did not immediately respond to requests for comment.

A time-consuming complaint would prove to be negative for the power companies, Jefferies analysts said in a note.

(Reporting by Sourasis Bose in Bengaluru; Editing by Sriraj Kalluvila)

((Sourasis.Bose@thomsonreuters.com;))