5 CFR §1600.33

1. (a)An account balance can be combined with another once the TSP record keeper is informed (by the participant's employing agency) that the participant has separated from Government service.
2. (b)Tax-exempt contributions may not be transferred from a uniformed services TSP account to a civilian TSP account.
3. (c)A traditional balance and a Roth balance cannot be combined.
4. (d)Funds transferred to the gaining account will be allocated among the TSP core funds according to the investment election in effect for the account into which the funds are transferred.
5. (e)Funds transferred to the gaining account will be treated as employee contributions and otherwise invested as described at 5 CFR part 1600.
6. (f)A uniformed service member must obtain the consent of his or her spouse before combining a uniformed services TSP account balance with his or her civilian account, even if the civilian account is not subject to FERS spousal rights. A request for an exception to the spousal consent requirement will be evaluated under the rules explained in 5 CFR part 1650.
7. (g)A loan cannot be transferred between accounts. Before the accounts can be combined, any outstanding loans from the losing account must be closed as described in 5 CFR part 1655.