What dimension is used as a standard for nominal pipe size in a fuel oil system supply line?

In a fuel oil system supply line, the standard for nominal pipe size is based on the inside diameter. This is because the inside diameter directly affects the flow characteristics and capacity of the pipe when transporting fuel oil. The nominal pipe size designation refers to the size that conveys the internal capacity rather than the overall dimensions of the pipe.

By using the inside diameter as the standard, it provides a consistent method for determining the volume of fluid the pipe can handle, which is critical for ensuring efficiency and safety in fuel distribution systems. Other dimensions such as outside diameter, wall thickness, and length are important in contexts like pipe fittings and installation, but they do not define the nominal pipe size in terms of flow capacity.

This distinction is crucial for engineers and installers to appropriately select pipes that meet the specific hydraulic requirements of the system.