N.Y. General City Law § 45

§ 45. Examinations; conducting business without certificate\nprohibited. A person desiring or intending to conduct the trade,\nbusiness or calling of a plumber or of plumbing in a city of this state\nas employing or master plumber, shall be required to submit to an\nexamination before such examining board of plumbers as to his experience\nand qualifications for such trade, business or calling, and it shall not\nbe lawful in any city of this state for a person to conduct such trade,\nbusiness or calling, unless he shall have first obtained a certificate\nof competency from such board of the city in which he conducts or\nproposes to conduct such business.\n