Under federal law, there is no limit on the overtime hours an employer can force, as long as the employer obeys pay laws and employee safety is not at risk. However, many states enforce regulations that limit the hours an employer can require mandatory overtime for certain occupations.
Union contracts or other types of employment contracts can also put limits on the number of hours an employer can require. Breaching these contracts is means for a civil suit against the employer. If an employee is not working under such a contract or is not working in a state with a limit in place, he cannot refuse mandatory overtime without discipline, or possibly termination. Employers typically win mandatory overtime cases unless they have violated a contract or agreement, failed to compensate employees for their overtime work, or mandated work that posed safety or health risks.