Which type of pipe must be used underground before it enters a structure?

In the context of the National Fuel Gas Code, the use of plastic pipe for underground installations is mandated due to its resistance to corrosion and ability to withstand various environmental conditions. Plastic pipes, such as those made from polyethylene, are specifically designed for gas distribution and are lightweight, flexible, and resistant to many chemicals and stresses that could be encountered underground.

These properties make plastic pipe an ideal choice for underground use, as they are less likely to degrade over time compared to metals like carbon steel or copper. Additionally, plastic pipes have lower friction loss and are easier to install, which contributes to their widespread acceptance and use in gas distribution systems. The National Fuel Gas Code outlines the appropriate materials for different applications to ensure safety, durability, and compliance with standards, further reinforcing the choice of plastic for underground installations.