If you receive an annuity from a job where you didn’t pay Social Security taxes, such as from the Civil Service Retirement System, and you also have enough Social Security credits to be eligible for Social Security retirement or disability benefits, a modified formula may be used to figure your benefit amount. This reduces the Social Security benefit, but not your CSRS annuity.
The reduction applies if you have fewer than 30 years of Social Security-covered earnings deemed substantial, $22,050 in 2015, a figure that rises slightly most years. The reduction is imposed on a phased-in basis, with the maximum being a reduction of about $400 a month.
There are some exceptions. See Chapter 3, Section 9 of the Federal Employees Almanac.