What is the maximum allowable diameter for standpipe outlets?

The maximum allowable diameter for standpipe outlets being 2.5 inches aligns with the standards set for standpipe systems in buildings, particularly those outlined in the National Fire Protection Association (NFPA) guidelines. This size is designed to accommodate the needs of fire service personnel while ensuring compatibility with standard firefighting equipment. The 2.5-inch outlet is commonly used for handlines during fire combat, as it allows for a manageable and efficient flow of water while ensuring that hoses and nozzles fit appropriately.

Larger diameters, such as those listed in the other options, are not standard for standpipe outlets in most urban buildings. A 3-inch or larger outlet would be impractical for the intended firefighter use during emergencies, as it could complicate the deployment of hoses and the connection of firefighting apparatus. The standard practices prioritize a balance between water flow capability and functionality, making the 2.5 inches the most effective choice for maintaining an efficient and safe firefighting strategy.