Reason for Credential: Except as provided under s. 145.06 (4) Stats., and pursuant to s. 145.06, Stats., no person may install plumbing unless the person holds a credential issued by the department as a licensed master plumber, licensed master plumber restricted, licensed journeyman plumber, licensed journeyman plumber-restricted, registered plumbing apprentice, registered plumbing learner-restricted, registered utility contractor or registered pipe layer.