Maryland Code, Business Occupations and Professions § 12-602

(a) Except as otherwise provided in this title, a master plumber, holder of a limited master plumber license, or other person who engages in the business of providing plumbing services may not employ an individual to provide or assist in providing plumbing services unless the individual:

(b) A person may not employ an individual to provide or assist in providing plumbing services under a public work contract subject to Title 17, Subtitle 2 of the State Finance and Procurement Article unless the individual is licensed by the Board, the Baltimore County Plumbing Board, or the Washington Suburban Sanitary Commission.

(c) A person may not classify an employee under a public work contract subject to Title 17, Subtitle 2 of the State Finance and Procurement Article who is licensed under this title at a specific work classification that is higher than the employee's license type.