5 C.F.R. § 848.201 – Eligibility.

(a) A retirement-eligible employee, as defined in paragraphs (b) and (c), may elect to enter phased retirement status if the employee has been employed on a full-time basis for not less than the 3-year period ending on the effective date of phased retirement status under § 848.203.
(b) Except as provided in paragraph (c) of this section, a retirement-eligible employee means an employee who, if separated from the service, would meet the requirements for retirement under subsection (a) or (b) of 5 U.S.C. 8412.
(c) A retirement-eligible employee does not include—
1. (1) A member of the Capitol Police or Supreme Court Police, or an employee occupying a law enforcement officer, firefighter, nuclear materials courier, air traffic controller, or customs and border protection officer position, except a customs and border protection officer who is exempt from mandatory separation and retirement under 5 U.S.C. 8325 pursuant to section 535(e)(2)(A) of Division E of the Consolidated Appropriations Act, 2008, Public Law 110-161;
2. (2) An individual eligible to retire under 5 U.S.C. 8412(d) or (e): or
3. (3) An employee covered by a special work schedule authority that does not allow for a regularly recurring part-time schedule, such as a firefighter covered by 5 U.S.C. 5545b or a nurse covered by 38 U.S.C. 7456 or 7456A.