5 C.F.R. § 846.401 Refunds of excess contributions.
(a) An individual who elects FERS coverage is entitled to a refund of CSRS contributions made prior to the effective date of the election for service that is subject to FERS computation under § 846.304(c) (if not already refunded) which exceed the contributions required under FERS, as provided by this section.
(b) The refund is equal to—
(1) For service described in § 846.302(a) and performed on or after January 1, 1984, and before January 1, 1987, the amount by which the amount contributed exceeds 1.3 percent of basic pay;
(2) For service described in § 846.302(a) and performed on or after January 1, 1987, the amount by which the amount contributed exceeds the amount required under § 841.503 of this chapter; and
(3) For service described in § 846.302(b), the amount by which the amount contributed exceeds 1.3 percent of basic pay.
(c) A refund made under this section is payable with interest computed as prescribed under § 831.105 (d) and (e) of this chapter. Interest is payable regardless of the length of the period of service for which refund is being made or the total amount of service the employee has.
(d) A refund described in this section is payable upon the receipt of an application by OPM or its designee.