The thrift savings plan that lets U.S. workers invest and shelter 15 percent of pay from taxes is apparently the most generous plan of its kind in the nation. The question is: Can it stay that way?

One of the key features of the plan is the high percentage of pay that workers can contribute, plus the high matching contribution they get from Uncle Sam.

Amounts that workers may now contribute (10 percent for those covered by the new Federal Employees Retirement System and 5 percent for those under the old retirement system) could be reduced next year unless Congress exempts the federal investment plan from rules that apply to similar private sector plans.

Those so-called antidiscrimination rules limit the percentage of salary that higher-paid ($50,000 a year and up) workers can contribute to no more than 2 percent above the average amount contributed by lower-income workers eligible to participate in the plan.

Congress is considering such an exemption for the federal thrift savings plan.

Although modeled after 401(k) plans in the private sector, the federal plan, which started in April, is already bigger than most other plans with nearly half a billion dollars in assets and growing by $5 million a day.

That it is one of the best was confirmed by a recent study by Hewitt Associates of plans offered by 371 of the biggest corporations. Hewitt specializes in tracking benefit programs. Its study showed:Eighty-four percent of the corporations made a contribution to employe thrift accounts, while 16 percent did not.

Most of the private plans matched 50 cents for every $1 contributed by workers.

Typically, private plans limit employe investments to 6 percent of salary, with 3 percent matched by the company. In government, the contribution rate is 10 percent for fully covered employes with 5 percent matched by the government.

None of the firms in the Hewitt study offered employes an automatic match. In the government plan, all workers covered by the FERS pension plan (currently about 600,000 people) automatically get 1 percent of salary contributed by the government to their accounts, regardless of whether they contribute.

The move to exempt the federal plan from rules covering similar private sector plans could be politically unpopular. But there are good reasons to do it. Among them:

The plan was designed in part to lure federal workers into joining the new pension plan, which, because of its lower benefit levels, will cost taxpayers less. The investment program is designed to let federal workers finance a bigger share of their own retirement income.

Thrift plans available to state and local government workers are subject to less stringent rules linking the investment of high-income employes to participation by lower-paid workers.

The antidiscrimination test was designed to ensure that investment plans are not designed as tax breaks for upper-income workers. There is little danger of that in government because only about 5 percent of the federal work force makes $50,000 or more a year.

Tomorrow on WNTR radio (1050 AM) Federal Times reporter Susan Kellam will talk with two experts about the current and future status of the federal thrift savings plan. Her guests will be Tom Trabucco of the Thrift Investment Board and Raymond Cardinal of the Energy Department, which has the highest sign-up rate of any major federal agency.