Maryland Code, Business Regulation § 9A-103

(1) an individual owner of a single-family dwelling while that owner is practicing heating, ventilation, air-conditioning, or refrigeration services on or within a building or structure owned by the individual;

(2) an individual who is building a single-family dwelling in which that individual will reside while practicing heating, ventilation, air-conditioning, or refrigeration services on or within that dwelling;

(3) an employee of a public utility company regulated by the Public Service Commission, when engaged in:

(4) subject to § 9A-403(a) of this title, an employee of the United States government, the State government, a local government, or an independent agency while that employee is practicing heating, ventilation, air-conditioning, or refrigeration services on or within buildings or structures owned or solely occupied by the United States government, the State government, a local government, or an independent agency;

(5) an individual employed in the installation, maintenance, alteration, repair, or replacement of self-contained appliances requiring not more than 225 volts or 25 amperes of electrical current;

(6) an individual possessing a master electrician's license issued by the State or any county, when engaged in the installation, alteration, service, repair, or replacement of individually controlled electric resistance heat;

(7) an individual possessing a master plumber's license issued by the State, Baltimore County, or the Washington Suburban Sanitary Commission when providing heating, ventilation, air-conditioning, or refrigeration services on hydronic heating systems;

(8) an individual possessing a license to practice engineering when providing heating, ventilation, air-conditioning, or refrigeration services in connection with the practice of engineering;

(9) an individual who installs, alters, remodels, maintains, or repairs oil burners exclusively while employed by a supplier of home heating fuel; or

(10) an individual regularly employed by the owner of property, or the owner's agent, to engage in maintenance and repair work.