Code of Federal Regulations Title 5. Administrative Personnel § 5.551.204 Nonexemption of certain employees

(1) Employees in equipment operating and protective occupations, and most clerical occupations;

(2) Employees performing technician work in positions properly classified below GS–9 (or the equivalent level in other white-collar pay systems) and many, but not all, of those positions properly classified at GS–9 or above (or the equivalent level in other white-collar pay systems);  and

(3) Employees at any grade, or equivalent level, in occupations requiring highly specialized, technical skills and knowledge that can be acquired only through prolonged job training and experience, such as in the Air Traffic Control series, or in the Aircraft Operations series unless such employees are performing predominantly administrative functions rather than the technical work of the occupation.