I need further clarification. You say you are returning to federal service, are you retired and returning as a reemployed annuitant or just being rehired? If you are being rehired, it depends on the type of position you are offered whether you must pay into FERS or not. If it is a temporary or seasonal position, no FERS is permitted, otherwise most other hiring authorities require you to pay into FERS. It is not a voluntary election. If you are a reemployed annuitant, FERS contributions are mandatory even if the position is temporary. Regarding the contribution rate you will be required pay into FERS depends on how much previous service you have, if any. If you are a new hire or rehired and have less than 5 years of potentially creditable FERS service, then the contribution rate would be 4.1% for regular employees. There are three exclusions from FERS-FRAE coverage; 1) If on December 31, 2013, you were covered under FERS or FERS-RAE; 2) On December 31, 2013, you were performing civilian service which is creditable or potentially creditable service under FERS or FERS-RAE; or 3) On December 31, 2013, you were not covered under FERS or FERS-RAE and were not performing civilian service which is creditable or potentially creditable service under FERS or FERS-RAE, but as of December 31, 2013, had performed at least five years of civilian service creditable or poten¬tially creditable under FERS-RAE, including service creditable for CSRS and/or CSRS Offset.