In which situation is a threaded connection permitted between black iron pipe and plastic pipe?

The situation where a threaded connection is permitted between black iron pipe and plastic pipe is when the connection is made outside and underground. This is primarily because certain types of plastic piping, such as PVC or CPVC, can be used in underground applications where they may be exposed to various environmental elements, as well as to certain chemicals and moisture that may be present.

Using threaded connections in these scenarios must adhere to local plumbing codes, which often specify that black iron pipe is compatible with plastic piping when it comes to underground and outdoor applications. This ensures that the integrity of the system is maintained and that there is proper sealing and support to prevent leaks or damage due to soil movement or exposure to elements.

Threaded connections are not typically allowed in other scenarios such as inside buildings, in commercial settings, or for temporary connections because of potentially different stress factors, potential exposure to heat or chemicals, and the need for more secure and reliable connections in plumbing systems. Each of these other situations may require alternative joining methods that are more appropriate and safe for the specific conditions of use.