When is it permissible to use a single sanitary T fitting in a nominally horizontal soil-or-waste pipe?

Using a single sanitary T fitting in a nominally horizontal soil or waste pipe is permissible primarily to connect a vent pipe. This configuration allows for proper air venting, which is essential in ensuring that wastewater flows smoothly through the pipes without creating a vacuum that could impede drainage or cause siphoning of traps.

In plumbing systems, vent pipes are crucial as they allow for equalization of air pressure within the drainage system, facilitating the proper flow of water and waste. When a sanitary T fitting is installed to connect a vent pipe, it provides a direct pathway for air to enter the drainage system above the water level, helping to maintain the system's efficiency.

Other options may relate to various configurations in plumbing systems, but they do not conform to the standards outlined in the National Plumbing Code for the use of a sanitary T in horizontal applications. For instance, using a single sanitary T fitting to serve as a trap or connect multiple outlets is not appropriate, as these applications would require different fittings or configurations to ensure effective wastewater management and avoid complications in drainage or venting. Similarly, for waste line transitions, other specialized fittings are preferred to accommodate the change in direction or diameter safely.