It is not legal in the US per OSHA regulations for an employer to work an employee 8 hours without a scheduled break. In fact, the OSHA rules are to be posted in every break room.
Some employers try to avoid paying overtime by moving their employee's hours between workweeks or averaging it between two workweeks. For example, some employers will try to avoid paying overtime to an employee who works 50 hours by only having them work 30 hours the following week.
In South Carolina, there is no state-specific law that limits the number of hours an adult employee can work straight in a single day or week.
However, labeling an employee as part-time does not change the application of FLSA requirements. The IRS, meanwhile, considers a part-time employee to be someone who works less than 30 hours per week or less than 130 hours per month.
South Carolina is an at-will state, which means that employees may be terminated for any reason, a good reason, a bad reason, or no reason. The employee may also quit for similar reasons without providing notice to employer.
In South Carolina, work hours are typically governed by Title 41 of the state's Code of Law. Full-time employment is generally considered to be between 35 and 40 hours per week, although this isn't explicitly defined by law. This standard applies nevertheless to the majority of employees working in the state.