5 C.F.R. § 1600.22

A participant's employee contributions are subject to the following limitations:

(a) The maximum employee contribution will be limited only by the provisions of the Internal Revenue Code.

(b) A participant may make traditional contributions and Roth contributions during the same year, but the combined total amount of the participant's tax-deferred employee contributions and Roth contributions cannot exceed the applicable Internal Revenue Code elective deferral limit for the year.

(c) A participant who has both a civilian and a uniformed services account can make employee contributions to both accounts, but the combined total amount of the participant's tax-deferred employee contributions and Roth contributions made to both accounts cannot exceed the Internal Revenue Code elective deferral limit for the year.