A major credit agency downgraded more than $5 billion of Santee Cooper bonds Tuesday, meaning the state-owned utility will have to pay higher interest rates when it borrows money.

The move by Fitch Ratings lowered the grade assigned to the $5.02 billion in debt to AA- from AA.

The state-owned utility is the last of five that are adding nuclear-generating capacity to have its bonds downgraded, said Santee Cooper spokeswoman Laura Varn.

She said the change in outlook by Fitch's bond watchers "was not unexpected."

"They had signaled earlier they would look hard at utilities with new nuclear projects because they feel there is an inherent risk with new nuclear because it hasn't been done in 30 years," Varn said.

Santee Cooper is building two nuclear units at the V.C. Summer plant north of Columbia that it owns with South Carolina Electric and Gas. The reactors are set to come online in 2016 and 2019, at a cost of about $10 billion.

Santee Cooper owns 45 percent of the project and wants to cut that to 20 percent. The Moncks Corner-based utility is looking for one or more partners to pick up the rest of its stake.

While Fitch issued Santee Cooper a stable outlook, it downgraded the debt because it said the utility is likely to have excess capacity coupled with the possibility of higher rates, given its moderate customer growth. The ratings firm also cited a drop in sales to its biggest customer, Central Electric Power Cooperative.

Santee Cooper began evaluating its ownership in new power plants in 2010 after Central Electric said it would shift some of its power purchases to Duke Energy, beginning in 2013.

That decision -- combined with a recession-induced decline in demand and costs associated with more stringent environmental rules -- helped scuttle a coal-fired generator that Santee Cooper planned to build near Florence.

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So far, only Charlotte-based Duke Energy has issued a letter of intent to buy some the utility's stake in the new nuclear units. Letters of intent by Florida investors have been withdrawn, Varn said.

Fitch also rated as AA- nearly $236 million in debt Santee Cooper is issuing this month as part of a refinancing deal.

The bond-rating agency did not note the power-rate dispute between Santee Cooper and Alcoa, which consumes about 10 percent of the utility's power at its aluminum plant near Goose Creek.

Reach Warren L. Wise at 937-5524.