The maximum allowed inside a building for threaded gas pipe is 5 lbs and 4”.
Section 5.5 of NFPA54 states “the maximum design operating pressure for piping systems located inside a building shall not exceed 5 psi….”
Table 18.104.22.168 gives the specifications for threading metallic pipe. For Iron pipe, the chart goes up to 4”. So the maximum allowable threaded pipe inside a building is 4” and 5 lbs.
You have the capabilities to going to higher pressures, but there are restrictions. Pipes would have to be welded or located in a ventilated shaft.