The Difference Between Schedule 40 and Class 200 PVC
Growing up, I knew there was a difference at the local home improvement store between schedule 40 PVC and schedule 80 PVC when I held the pipe in my hands. Other than one being heavier and gray, I never really understood the difference and when schedule 40 or 80 should be used. No big deal, I knew the white pipe was what I wanted for everything I was going to do anyways, at least in that point in my life.
But to confuse the issue even further, I discovered other pipe that had a class rating. There weren’t just two types and they were all white! It wasn’t until later I learned the difference and hopefully I can shed some light on the confusing topic with you now.
A pipe schedule (abbreviated as SCH) is just a unit of measure of pipe wall thickness. It is a historic rating system known as IPS (Iron Pipe Sizes) that carried on from the metallic pipe ratings to PVC. Being that PVC is inherently weaker than steel, the overall thickness of the pipe is less obvious of its working strength compared to the alternative PVC rating, the PIP (Plastic Irrigation Pipe) class system.
While the IPS schedule rating system is widely used in construction and the most common applications of PVC pipe, the class rating system is the primary method for irrigation and agriculture systems.
The system of IPS ratings, there are 3 categories of pipe strength. Standard (STD), Extra Strong (XS) and Double Extra Strong (XXS). For PVC pipe, STD is schedule 40, XS is considered schedule 80 and XXS is a bit more undefined but generally considered schedule 120. Anything above schedule 80, however, is considered Double Extra Strong.
It’s important to note with the IPS system, that pipe strength in a particular schedule decreases as the pipe size increases. For example, 2” PVC in schedule 40 has far greater burst strength than 12” PVC also in schedule 40.
Frankly, I wish all PVC pipe ratings transitioned to the class system. Let me tell you why:
Class 125 pipe has a maximum sustained pressure rating of 125 psi. Likewise, class 200 PVC pipe has a maximum sustained pressure rating of 200 psi. The key word here is sustained. All classes for PVC pipe have a factor of safety for allowable static pressure. Typically, though not always, that ratio is 2:1. That means class 125 pipe can theoretically contain a surge up to 200 – 250 psi, though it will fail if that pressure is maintained for any length of time.
Factor of safety is a design mechanism to account for manufacturing faults, installation faults, material faults, etc. While it could be possible to use class 125 pipe where class 200 pipe is required, there are no guarantees that a 200 psi surge will even be contained in the best of circumstances. Always use the pipe rating recommended for your pressure application.
Pretty easy, right? If I am designing a water catchment system to be used to irrigate my lawn, I could use class 125 PVC for a gravity fed line because I know it will have little to no pressure, assuming the elevation change of the pipe is minimal. But if I know the required application pressure will be 180 psi, then you may go with class 200 pipe. What is so fantastic about PIP class ratings is the class applies to any pipe size.
The problem with the IPS schedule system for rating pipes, is that the rating does not immediately correlate to a working pressure. Or rather, it’s not plainly obvious. IPS pipe schedule system is set by the wall thickness of a pipe. Clearly, a pipe with thicker walls are stronger than those with thin walls, but how much so? What pipe schedule do I need to use for a design calling for 225 psi of pressure? You have to consult tables for that, which is why it is more difficult to use than the PIP class system.
One major advantage to using the IPS system, and why it is still so popularly used in construction and industrial applications is how common the pipe is. Schedule 40 pipe is used in the vast majority of all residential plumbing systems. It is stocked in every hardware store in America. It is easy to find, cheap to buy and has any fitting you could need.
For the same reason America still uses the imperial system for units of measure instead of the metric system, the IPS system is still in place because that is what is common and used everywhere else, despite the advantages of the metric system. And we will continue to do so as long as everyone else does it.
Ok – so now we have established there are two different rating systems for pipe and when they should be used. Let’s quickly discuss how the pressure ratings affect the pipe dimensions.
In the U.S., nominal pipe sizes are in inches. These can range from fractions of an inch to over 36”. These sizes correspond to the outside diameter of the pipe, though it is not a true dimension. Up until about 14” pipe, the actual pipe size is larger than the nominal size.
The outside diameter (OD) on PVC pipe is consistent between the two ratings (PIP and IPS) so the only way to get differential strength for different pipe sizes is to adjust the pipe walls. If your outside diameter is constant, then the inside diameters are the variable between different pressure strengths. The trade off for the added pressure strength is a decrease in flow rate, since the cross-sectional area of the pipe is smaller. This cross-sectional area is determined by the inside diameter, called the nominal bore.
Consistent outside diameters can make it tempting to mix and match different ratings on pipe, but it’s important to avoid doing so where possible. There are several situations with irrigation systems where you may use a variety of pipe ratings but the transition takes place at valves where there will already be a considerable amount of pipe friction.
The contractor or DIYer prefers have a consistent outside diameter for ease of construction but as far as fluids are concerned, they flow much better with consistent inside diameters, keeping pipe friction to a minimum.
So now that you know the difference between the IPS and PIP rating systems, the big question is when to use which type of pipe. Unfortunately, due to local ordinances and building codes, there is no one answer for everything. But as a simple rule, you should use schedule 40 PVC pipe for residential projects, class 200 (or higher if needed) and whatever you want for non-pressure and non-structural projects.
The cheaper options are going to be your class 63 and class 125 but these pipes are very flimsy and can break with minimal force. While their pressure capacity is sufficient for most applications, they can easily break with any exterior forces or if buried, the slightest knick of a shovel. They are so cheap because of the thin walls and less amount of material used.
So hopefully you now know that you can’t compare schedules and classes without knowing the pipe size. The class rating has an obvious pressure rating, but a schedule pressure rating depends on the pipe size. Whatever you use, make sure you make clean, straight cuts and secure joints. No pipe can hold pressure with loose or leaky joints. For more information, check out the proper way to make joints.