When Kohann H. Whitney saw the operating costs for two Fairfax County libraries, she was astonished.

It just didn't make sense that annual budgets should be nearly identical for a 1,200-square foot library, Carter Glass, and another library more than 10 times its size, the 15,000-square-foot Hunters Woods regional facility, also in the Reston area.

Whitney, then a member of the county library board, asked staffers to check her suspicions. The result, officials said yesterday, is they now estimate Fairfax is owed $54,000 by the Chesapeake & Potomac Telephone Co. of Virginia for overcharges at Carter Glass since 1983.

Officials said the overcharge -- more than half Carter Glass's $87,000 annual operating budget -- is one of the largest ever that the county government has discovered.

"I'm delighted," said Whitney, who left the library board this month to join the School Board as the Centreville representative. "I hope we get interest."

C&P officials disputed the size of county's refund. "The net figure is $2,400," said Harry N. Doyle, the phone company's Fairfax County manager. He said the higher number may have been from an earlier estimate.

Fred K. Kramer, county director of general services, said $54,000 is only an estimate, but the overcharge is "a significant amount." County Supervisor Martha V. Pennino, in announcing the find to her fellow supervisors yesterday, estimated it at $60,000.

Officials said the overcharge apparently began when the Carter Glass library was linked to the library computer. Nobody noticed the error until debate began last winter over whether to close Carter Glass. Ultimately, the board decided to keep the library branch open.