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One of the most confusing aspects of working with plastic piping, whether for pressurized water supply or non-pressure applications like drain pipe plumbing, is understanding how the nominal pipe size relates to the actual dimensions. The quick answer is that for standard connections, the pipe size you buy is designed to fit snugly inside a fitting of the same size. For instance, a 1-inch pipe fits into a 1-inch fitting. However, the precise physical dimensions that dictate this fit are based on an industry standard called Nominal Pipe Size (NPS), which is crucial for selecting the correct PVC plumbing fittings.
The Nominal Pipe Size (NPS) System
The naming convention for PVC is based on the Nominal Pipe Size (NPS) standard, which refers to the pipe’s approximate inside diameter (ID). This means that a 2-inch pipe has an ID that is roughly two inches. Crucially, as the pipe’s schedule (wall thickness) increases—moving from Schedule 40 to Schedule 80—the Outer Diameter (OD) remains constant, but the Inner Diameter shrinks. Regardless of the schedule, all PVC plumbing fittings are manufactured to accept the constant Outer Diameter of the pipe.
The Critical Role of Outer Diameter (OD)
All solvent-weld PVC plumbing fittings are sized based on the pipe’s Outer Diameter (OD), which is the most critical measurement for a strong, reliable connection. For any given size, the OD is standardized across all schedules so that a Schedule 40 pipe and a Schedule 80 pipe of the same nominal size (e.g., 1.5 inches) can use the exact same fitting. The fitting provides a socket that chemically welds to this standardized OD, creating the pressure-tight seal that is essential for effective PVC plumbing fittings.
Understanding Insertion and Fitting Types
Standard PVC plumbing fittings—such as couplers, elbows, and tees—are typically designed as female sockets. This means the pipe slides into the fitting. The interior of the fitting is precisely molded to receive the exterior of the pipe. If you encounter a situation where you need one pipe to slide inside another (e.g., for certain repair couplings or adapting different materials), you must use a specialized component like a reducing bushing or specific repair fitting designed for inner dimension insertion, rather than relying on standard solvent-weld PVC plumbing fittings.
Measuring for Accurate Connection
To verify that you have the correct size pipe for your PVC plumbing fittings, the most reliable method is to measure the OD with calipers, although this is usually unnecessary if the size is stamped on the pipe. Always ensure that the pipe is square-cut and deburred before dry fitting. The pipe should insert into the fitting socket about one-third of the way before a noticeable resistance begins. This interference fit guarantees that when primer and cement are applied, the components are tightly pressed together, ensuring a complete chemical fusion across the entire joint.
Conclusion
The sizing system for PVC is based on the nominal inner diameter, but the secure connection relies on the standardized Outer Diameter of the pipe fitting snugly into the socket of the PVC plumbing fittings. Understanding this distinction is key to a leak-free installation. Delivering essential building materials across the North American market is the core focus of LESSO America. Their reliance on cutting-edge injection mold technology and a robust, coast-to-coast distribution network ensures superior quality in every product, including PVC plumbing fittings.
