According to NFPA 13, standard-weight Schedule 40 steel pipe has sufficient wall thickness to be joined by threading without requiring a special listing for the fittings. The process of cutting threads removes material, and thinner-walled pipes, such as Schedule 5, 7, or 10, would be structurally compromised by standard threading methods. While thin-wall pipe can sometimes be joined with threads, it requires the use of fittings that are specifically listed for that particular pipe and application, which constitutes a "special listing" requirement. Schedule 40 pipe is the baseline standard for which conventional threaded fittings are designed and approved.

A. Schedule 5: This thin-wall pipe lacks the necessary wall thickness for standard threading and would require specially listed fittings for such a connection.

B. Schedule 7: Similar to Schedule 5, this is a thin-wall pipe that cannot be joined with standard threaded fittings without a specific listing.

C. Schedule 10: This pipe has a wall thickness less than Schedule 40 and is not permitted to be joined by threading unless using specially listed fittings.

1. NFPA 13, Standard for the Installation of Sprinkler Systems (2022 Edition).

Section 7.5.2.1: "Steel pipe with a wall thickness less than Schedule 30 (in sizes 8 in. and larger) or a wall thickness less than Schedule 40 (in sizes smaller than 8 in.) shall not be joined by threading." This establishes Schedule 40 as the minimum for standard threading in common pipe sizes.

Section 7.5.2.2: "Where the pipe is threaded and the fittings are listed for use with that specific pipe, the minimum pipe wall thickness shall be in accordance with the listing." This clause confirms that thinner pipes (like Sch. 10) require a special listing.

2. SFPE Handbook of Fire Protection Engineering (5th Edition).

Chapter 20, Section 20.4.1, "Pipe and Tube": This section discusses piping materials for automatic sprinkler systems, noting that "Schedule 40 is the most common pipe schedule used for threaded steel pipe systems." It further explains that lighter-weight pipes like Schedule 10 are typically joined by grooving or welding due to their thin walls. (Springer, 2016, pp. 20-12). DOI: https://doi.org/10.1007/978-1-4939-2562-420