Where water supply systems other than plastic are tested with air, the test pressure shall not be less than _____ psi.

When testing water supply systems that do not utilize plastic piping, using air as the test medium is a common practice for ensuring the integrity of the system. The requirement for a test pressure of no less than 50 psi is established to effectively determine if there are any leaks or weaknesses in the system. This pressure level is robust enough to challenge the reliability of the materials used in the installation, helping to identify any potential failures that could compromise the system's performance.

Testing at this higher pressure ensures that the components of the water supply system can withstand more than the typical operational pressures they would encounter under normal conditions. It provides a safety margin that helps confirm the integrity and reliability of the piping and fittings, which is critical for preventing future leaks and maintaining a safe and functional plumbing system.

The other options present lower pressure levels that would not adequately test the resilience and sturdiness of the materials used in the system. A pressure below 50 psi could result in a failure to detect leaks that might only be apparent under operational conditions, thus failing to meet the necessary standards for safety and performance in plumbing systems.