[TheRealThing]

Wrong. At least when I ran it in MI, no plumbing license required.
You can argue this until you turn blue, and you'll still be wrong. Do you think I didn't take the class and learn the rules of the time?

Here it is in blue and it was in effect in 2006. All I'm saying here is that my statements are all backed up by the various agreements and codes I have quoted from. According to the Michigan code you may well have run med gas piping in Michigan but, if you did it was under the supervision of a licensed master plumber who was responsible to make sure your work met code requirements again, as was according to their code specifics of the day.

MEDICAL GAS PIPING INSTALLATIONS
Issue
What individuals have the responsibility for obtaining permits, providing supervision, making installations, conducting inspections, and conducting verifications for medical gas piping systems in accordance with State of Michigan requirements? This Technical Bulletin will identify the various documents addressing these matters and provide a brief summary of who performs the various functions related to medical gas piping installations.

Statute
The State Plumbing Act, 2002 PA 733 (Act 733) has several applicable references in the statue, which address medical gas systems.
Section 9(b) states:
"Plumbing" means the practice, materials, and fixtures, in or adjacent to a
building, structure, or premises, used in the installation, maintenance, extension, or alteration of all piping, fixtures, plumbing appliances, plumbing appurtenances, as defined by the code, in connection with the sanitary drainage or storm drainage facilities, plumbing venting systems, medical gas systems, backflow preventers, and public or private water supply systems. (Emphasis added by author)

Section 9 identifies and includes medical gas systems within the scope of plumbing addressed by Act 733.

Section 15 states:
(1) A person shall not engage in or work at the business of a plumbing
contractor, master plumber, journey plumber, or apprentice plumber unless
licensed or registered by the department. Except as provided in subsections
(2), (3), (4), and (5), plumbing shall be performed by a licensed master or
journey plumber. A licensed master plumber shall be in charge and
responsible for proper installation and conformance with the code.
Plumbing shall not be performed unless the plumbing contractor who is
responsible has secured a permit from the state or a governmental
subdivision authorized to issue permits.
(2) A license is not required for the following work:
(a) Minor repair work.
(b) The installation of a building sewer or water service pipe provided that
a permit is secured from the responsible enforcing agency and
inspections are performed. The installations shall comply with the
applicable code.
3
© The installation of domestic water treatment and filtering equipment
that requires modification to an existing cold water distribution supply
and associated waste piping in buildings if a permit is secured,
required inspections performed, and the installation complies with the
applicable code. If the enforcing agency determines a violation exists,
it shall be corrected by the responsible installer.
(3) A homeowner may install his or her own plumbing, building sewer, or
private sewer in his or her single-family dwelling if a permit is secured.
(4) The installation of medical gas piping providing the installation shall be
performed under the supervision of a licensed plumbing contractor.
(5) This act does not prevent a person from performing any activities within the scope of licensure or registration under any other licensure or registration
act or applicable codes for that licensed or registered professional adopted
pursuant to law. (Emphasis added by author)

Section 15(1) the second sentence clarifies that a license is not required for items addressed in subsections (2), (3), (4), and (5). Subsection(4) includes medical gas systems and clarifies that they “shall be performed under the supervision of a licensed plumbing contractor.” Thus, the installer, while not being required to be licensed, shall be employed by the licensed plumbing contractor.

Section 15(1) further requires the licensed plumbing contractor to obtain a permit for plumbing, which was defined earlier to include medical gas systems.
Finally, Act 733 establishes a licensed master plumber shall be in charge and responsible for proper installation and conformance with the code. The requirement of having a master plumber for supervision assigned to the contractor is implemented and stated in Sections 7(d), 9©, 15(1), 21(4), and 41(2).

Penalties for non compliance of Act 733 are identified in Sections 47 and 49.
The Stille-DeRossett-Hale Single State Construction Code Act, 1972 PA 230 (Act 230) establishes the authority of the director, commission, and boards to develop the appropriate international codes through the rules process to adequately protect the health, safety, and welfare of Michigan citizens. It also establishes code enforcement authority for those jurisdictions seeking the authority.
The Michigan Building Officials and Inspectors Registration Act, 1986 PA 54 (Act 54) establishes and regulates the registration of building officials and various inspectors. Rules which regulated conduct and minimum training requirements were also developed.