Some of that 4.8 percent federal pay raise due next month will be eaten up by health insurance premium jumps next year of 10 percent or more, and by benefit cutbacks that will force many of the area's 360,000 feds to pay more out of pocket for hospitalization, mental health care, abortions and dental services.

There is also the very real possibility that U.S. workers hit with higher premiums and reduced benefits may be locked into their present health insurance plan if the government delays or cancels an open season period scheduled for November when workers could normally shop around for new and better insurance deals for 1982.

Office of Personnel Management chief Donald J. Devine yesterday notified key members of Congress that the Federal Employees Health Benefits (FEHB) program must take drastic action to avoid even bigger premium jumps -- of 30 percent or more -- in plans that cover most of the 9.2 million workers, dependents and retirees enrolled in the nation's biggest employer-employe health program.

Devine said that costs of operating the program, for which the government pays 60 percent of premium costs and employes 40 percent, were seriously underestimated during the Carter years. The result, he said, is that premiums next year would have to go up an average of 30 percent to maintain the current level of benefits. To avoid this, Devine said OPM will force insurance plans to cut back benefits and hold 1982 premium increases in the neighborhood of 10 percent.

As reported here Sept. 4, OPM experts estimated costs of the FEHB next year could exceed income by $500 million and perhaps drive some of the 130 insurance carriers out of the program.

In a special report to Congress, federal unions and insurance carriers, Devine said OPM is considering some drastic actions. Some have been decided on, others are being studied and will be announced shortly. They are:

Cuts or elimination of benefits for some major items to keep 1982 premium increases to around 10 percent.

A decision soon whether to delay or cancel open season this year. (Changes in benefits and health plan premiums will be announced shortly and employes are supposed to have an open season from Nov. 10 to Dec. 5 to make changes in coverage).

Reduction or perhaps outright elimination of benefits for some dental, mental and abortion services and cuts in other areas not specified. Officials said the extent of the cuts will be announced after discussions with health insurance carriers.

OPM has given health insurance carriers five days to bring plans they will offer for 1982 into conformity with OPM's benefit reduction guidelines. Those who don't meet the deadline, or whose health insurance packages do not meet with OPM approval, will be dropped from the 1982 program.

The average Washington white-collar federal worker gets $25,644 a year and pays anywhere from $600 to $800 per year for high-option family insurance coverage depending on which plan he is enrolled in. The 4.8 percent raise will be worth just over $1,000 before taxes for the average worker.

Although OPM feels it can prevent a projected 30 percent premium jump, employes can expect to pay substantially higher premiums in 1982 over '81. And, because some benefits will be reduced and deductibles raised, many people will end up paying a lot more for medical services next year, plus higher premiums.

Devine, a University of Maryland professor named to the personnel agency this year, said OPM in the past had underestimated health care costs and the number of retirees (who often have higher individual medical bills than younger workers) who would be covered by the FEHB.

He said OPM will investigate and improve its actuarial and accounting methods. But none of that will comfort feds who have seen pay raises cut back, thousands of jobs eliminated (with more cuts to come), frequency of retiree raises reduced and automatic longevity pay adjustments eliminated for 125,000 supervisors and managers.