Any master plumber, gas fitter, journeyman plumber or journeyman gas fitter regularly employed for public work or by any institution, industrial establishment or public utility shall be exempt from this article, but he shall not carry on any outside business of plumbing or gas fitting unless he complies with this article. The plumbing or gas fitting business so carried on shall be in strict accordance with the plumbing and gas fitting regulations. (Mont. Co. Code 1965, § 95-31.)