53 Pa. Stat. § 15321
On and after the first day of January, one thousand nine hundred and thirty-six, it shall be unlawful for any person or persons to carry on the business or work at the trade of plumbing, defined as drainage, water piping, or the construction, alteration, and repairing any drainage, cesspools, trap, waste vent, or water piping, and fixtures attached thereto, directly or indirectly, connected to the city service water supply, or any connection or connections in relation thereto, except gas or electric water heaters in cities of the first class, having a system of sewerage and water supply, of this Commonwealth, until a master plumber's license and certificate of registration to engage in or work at said business shall have been granted said persons by the Department of Health of such cities: Provided, That nothing in this act shall be construed to prevent the employment of apprentices, except as hereinafter provided, journeymen and licensed plumbers under the direction of duly certified and licensed master plumbers, or under the direction of registered journeymen plumbers: And provided further, That master plumbers, duly certified and licensed and having a bona fide place of business in said cities of the first class at the time of the passage of this act, and registered journeymen plumbers, shall not be required to undergo an examination, and they shall be entitled to apply for registration annually thereafter; Provided, That the term plumbing shall not be construed to include the laying of sewers and water pipes (including the laterals) by said cities, either directly or by its duly authorized contractors or agents, or constructed under the supervision of said cities, or under the supervision of a registered professional engineer in those cases where a complete set of plans of said sewers to be constructed, and water pipes to be laid, shall have been filed with the appropriate office of said cities.
53 P.S. § 15321