Which type of piping cannot be used for potable water lines?

Black steel piping is not suitable for potable water lines due to its susceptibility to corrosion and the potential for rusty particulates to contaminate the water supply. This type of piping is typically used for gas lines and not advisable for any application involving drinking water.

On the other hand, PVC, PEX, and stainless steel piping are specifically designed or approved for use in potable water systems. PVC is resistant to chemical leaching and is commonly used for various plumbing purposes. PEX, made from cross-linked polyethylene, is flexible and resistant to scale and chlorine, making it a popular choice for residential plumbing. Stainless steel offers excellent corrosion resistance, durability, and is often used in applications where water purity is essential.

Thus, these three types of piping are appropriate for potable water lines, while black steel piping is not, highlighting why it is the correct answer to this question.