What is the minimum size cast iron piping that can be utilized for gas piping?

For gas piping systems, cast iron is generally not utilized due to concerns over the material's suitability for gas transmission. Gas lines are typically required to maintain specific pressure and avoid corrosion, which can lead to leaks or safety hazards. Most codes and standards specifically designate certain materials, like carbon steel or polyethylene, as more appropriate for gas service. Therefore, there isn’t a minimum allowable size for cast iron piping for gas applications because it is not deemed acceptable for that purpose at all. This fundamentally explains why the correct answer is that cast iron piping cannot be used for gas piping systems.