Idaho Code § 54-2610

It shall be unlawful for any person or firm, copartnership, association or corporation, to engage in the business, trade, practice or work of plumbing in this state after the adoption of this chapter, unless such person, or responsible person representing such firm, copartnership, association or corporation, has successfully passed an examination as provided herein and has issued to him a state certificate of competency, which shall not be transferable, and said certificates of competency shall not be required for sewer contractors, sewage disposal contractors, or any excavating or utility contractors, or for their employees, as set forth and defined in section 54-2602(1)(f), Idaho Code.