The minimum size for the horizontal member of a trapeze hanger is determined by the size of the pipe being supported and the span of the trapeze. According to NFPA 13, Standard for the Installation of Sprinkler Systems, specific sizing tables must be used. For a trapeze hanger constructed of Schedule 40 steel pipe with a span of 8 ft (2.4 m) that is intended to support a 4 in. (102 mm) sprinkler main, the standard requires the trapeze pipe to have a minimum nominal diameter of 2 in. (50 mm). This ensures the hanger can adequately support the load without excessive deflection or stress.

A. 1.5 in. (40 mm): Per NFPA 13 tables, a 1.5 in. Schedule 40 trapeze pipe with an 8 ft span can only support a maximum pipe size of 3 in., which is insufficient.

C. 2.5 in. (65 mm): While a 2.5 in. pipe would be strong enough, it exceeds the minimum size required by the standard, making it an incorrect choice for this question.

D. 3 in. (75 mm): A 3 in. pipe is significantly oversized for this application and is not the minimum required diameter according to the prescriptive tables in NFPA 13.

1. NFPA 13, Standard for the Installation of Sprinkler Systems (2022 Edition). Chapter 17, Hangers. Table 17.4.4.3.1, "Trapeze Hanger Sizing for Schedule 40 Steel Pipe as the Trapeze." The table shows that for a "Span of Trapeze" of 8 ft, a "Nominal Pipe Size of Trapeze Hanger" of 2 in. corresponds to a "Maximum Pipe Size Permitted" of 4 in.

2. NFPA 13, Standard for the Installation of Sprinkler Systems (2019 Edition). Chapter 17, Installation of Hangers. Table 17.4.4.3.1, "Trapeze Hanger Sizing for Schedule 40 Steel Pipe as the Trapeze." This table provides the identical requirement as the 2022 edition for this specific scenario.

3. NFPA 13, Automatic Sprinkler Systems Handbook (2019 Edition). Chapter 17, Section A.17.4.4.3.1. The handbook commentary explains that the values in Table 17.4.4.3.1 are based on engineering calculations to limit the deflection of the trapeze member to L/360, ensuring structural integrity. This confirms the basis for the required 2 in. pipe size.