Which type of pipe is explicitly prohibited for gas piping according to NFPA standards?

The correct answer pertains to the fact that cast-iron pipe is explicitly prohibited for gas piping due to safety concerns and the characteristics of the material. While cast iron is known for its durability and strength in various applications, it is not suitable for gas systems.

The primary reasons include the potential for cracking and brittleness, especially under stress or impact, which could lead to dangerous leaks. Gas piping systems require materials that can withstand pressure and environmental conditions without significant risk of failure.

In contrast, other materials like plastic, steel, and wrought-iron pipe may be permissible under certain conditions and applications when properly installed and used according to relevant codes and standards. Plastic pipes, such as those made from polyvinyl chloride (PVC) or polyethylene (PE), can be used in specific gas applications where conditions are right. Steel pipe is commonly used for gas applications due to its strength and resilience, and wrought-iron pipe can also be suitable, provided it meets specific industry standards.