The nation's leading retailers reported sales in July moderated from the exceptionally strong pace of June, but their cash register receipts continued to run about 10 percent ahead of year-ago levels.

The July sales gains were below the average 13 percent increase posted in June by the largest retailers because consumers rushed out earlier this summer to buy air conditioners and other seasonal merchandise to cope with the unusually hot weather, analysts said.

June sales were so strong, retail observers said, that even the final 10 percent cut in personal income tax rates, which took effect July 1, failed to stimulate consumer spending enough to outpace the June gains.

The sales gains in July were not as high as those in June but "June was an aberration," explained analyst Jeffrey Edelman of Dean Witter Reynolds Inc., with June's results higher than expected. Sales in July also exceeded expectations, he said, adding, "the current trend of sales will continue."

Even with the slowdown, July sales reports are still "quite favorable," agreed Jeffrey Feiner, retail analyst at Merrill Lynch Pierce Fenner & Smith.

But he warned that "some increases in interest rates added some cold water on this favorable economic backdrop.

"The July results reflect favorable trends in inflation, unemployment and income gains, while the recent rise in interest rates has placed some pressure on consumer attitudes," Feiner said. "We nonetheless believe the retailing environment will remain favorable throughout the remainder of 1983."

Sears, Roebuck & Co., the nation's largest retailer, said its July sales were up 9.8 percent from a year earlier to $1.65 billion. For the 26-week period ending July 30, Sears sales were up 7.5 percent.

Sears Chairman Edward R. Telling said all sections of the country posted sales increases, with the largest increase in the Midwest.

The steamy summer heated up sales of air conditioners and fans and gave Sears strong sales gains in major appliances for the 10th successive month, Telling said. Other strong sellers were in men's apparel, hardware, automotive items, recreation and home fashions.

The second-largest store chain, K mart, reported an 11.8 percent rise in July sales to $1.37 billion. Sales for the first half are running 11.1 percent ahead of last year.

But J. C. Penney Co., the third-largest retail group, reported a 0.7 percent decline in July sales to $675 million. At the half-way mark in the fiscal year, Penney's sales were up 2.8 percent. Penney's is dropping several departments from its stores and that strategic move had been expected to hurt sales.

Sales for Cincinnati-based Federated Department Stores, ranked fourth, jumped 14.8 percent in July to $535.6 million. For the first six months, sales were up 15.3 percent.

F. W. Woolworth, ranked fifth, had a 6.7 percent sales increase to $370.7 million. Dayton Hudson Corp., based in Minneapolis and the nation's sixth largest retailer, said sales surged 20.7 percent in July to $413.3 million. So far this year, sales are running 23.3 percent ahead of last year.