That limit counts only personal investments and not agency contributions into the TSP accounts of employees under the Federal Employees Retirement System. They get an automatic employer contribution of 1 percent of salary, plus up to another 4 percent in matching contributions.
The TSP also mirrors general tax policy in allowing “catch-up contribution” investments by those who are age 50 or older in a year, if they reach the regular limit or are investing at a rate to hit it by the end of the year. That maximum will remain $6,000.
Regular investments continue year to year unless changed, so to take advantage of the higher limit you’ll have to make a new payroll withholding election in 2018. Catch-up contribution elections don’t continue; they must be made each year.