Must we base benefits on the number of hours an employee works?

Q: “We are under 50 employees, so we aren't mandated to provide healthcare to 30-hour employees. However, we have been providing health insurance to our 30-hour employees (they all happen to be a manager status). Can we base benefits on employee class irrespective of hours, or if we give any 30-hour employee benefits would we need to do it for all of them? The question relates to our paid interns that we move to a 30-hour status.” – Greg, Florida