Mich. Comp. Laws § 339.5933

Sec. 933. For purposes of this article, boiler operator and stationary engineer registrations are classified as follows: (a) Low pressure boiler operators, who operate low pressure boiler plants that have an aggregate of not more than 4,000 square feet of boiler heating surface. (b) High pressure boiler operators, who operate boiler plants that have an aggregate of not more than 4,000 square feet of boiler heating surface or not more than 10 steam engine-turbine horsepower. (c) Third-class stationary engineers, who operate boiler plants that have an aggregate of not more than 7,500 square feet of boiler heating surface or not more than 100 steam engine-turbine horsepower. (d) Second-class stationary engineers, who operate boiler plants that have an aggregate of not more than 20,000 square feet of boiler heating surface or not more than 200 steam engine-turbine horsepower. (e) First-class stationary engineers, who operate boiler plants that have an aggregate of 20,000 square feet or more of boiler heating surface or 200 steam engine-turbine horsepower or more.