Medium-pressure piping will only need to be welded for piping 4" sizing and larger:
403.1.1 Pipe Size and Pressure Limitations
Gas distribution piping operating at a pressure of over 1/2 psig (3.5 kPa gauge) to 5 psig (34.5 kPa gauge) and size 4 inches (102 mm) or larger shall be welded.
Exception: Manufactured and listed gas trains provided with the appliance may be threaded.
Medium-pressure is permitted inside the building for commercial and industrial occupancies. The only issue I see is that we are less than 4000 CFH in the distribution:
402.6 Gas Distribution Pressures
No gas distribution piping containing gas at a pressure in excess of 1/2 psig (3.5 kPa gauge) shall be run within a building.
Exceptions:
Pressure not exceeding 5 psig (34.5 kPa gauge) is permitted for: commercial and industrial occupancies where fuel requirements for appliances exceed 4,000 cubic feet per hour (113.2 m3/h) and such large volume use is supplied through separate gas distribution piping.
The piping would be subject to the testing you recommended previously:
406.4 Test Pressure Measurement Distribution pressures over 1/2 psig (3.5 kPa gauge) through 5 psig (34.5 kPa gauge). The completed piping is to be tested at 50 psig (340 kPa gauge) for a minimum of 30 minutes.
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