5 CFR 1620.21: Contributions.

(a) An individual covered under this subpart can make contributions to the TSP from basic pay in the amount described at 5 CFR 1600.22(a)(1). Unless stated otherwise in this subpart, he or she is covered by the same rules that apply to a CSRS participant in the TSP.

(b) The following amounts are not basic pay and no TSP contributions can be made from them:

(1) An annuity or salary received by a justice or judge of the United States (as defined in 28 U.S.C. 451) who is retired under 28 U.S.C. 371(a) or (b), or 372(a);

(2) Amounts received by a bankruptcy judge or a United States magistrate judge under a judges' annuity described at 28 U.S.C. 377;

(3) An annuity or salary received by a judge of the United States Court of Federal Claims under 28 U.S.C. 178; and

(4) Retired pay received by a judge of the United States Court of Veterans Appeals under 38 U.S.C. 7296.