Eight steps to determine plumbing system requirements

In nonresidential buildings, engineers should pay close attention to local codes, the Uniform Plumbing Code (UPC), and International Plumbing Code (IPC) when sizing water supply piping systems.

By Jeremiah Johnson, CPD, Revolution Engineering and Kris Kalkowski, PE, NV5 October 18, 2017


Learning Objectives

  • Explain water supply and distribution system sizing methods used in plumbing codes.
  • Provide basic calculations and examples for engineers to use when sizing water supply systems for various types of commercial buildings.

Several codes and standards are used when sizing water supply piping systems for commercial buildings. Various local authorities have adopted codes and standards that dictate methods of sizing systems. Currently, two of the major codes used in many jurisdictions within the United States are the 2015 editions of the Uniform Plumbing Code (UPC) and the International Plumbing Code (IPC). As there are multiple sizing methods and various system conditions, this is not intended to be an exact guideline for sizing all water piping distribution systems. There are multiple published standards for plumbing systems and water distribution systems that explain conditions and problems that arise when sizing various water piping systems.

The first step in determining plumbing system requirements and pipe sizing is to understand the building occupancy and plumbing fixture requirements. Plumbing fixture quantities are determined by the project architect based on code requirements as well as project-specific requirements that may exceed code. Building-occupancy types and associated plumbing fixture quantity requirements are dictated in the UPC, IPC, and the International Building Code (IBC). Each of these codes have slight differences in regards to plumbing fixture quantities based on occupancy types and the quantity of people that will be occupying the space. Once the quantity of required plumbing fixtures is determined, the architect will be able to design the various restrooms and associated plumbing fixtures for the building. Restroom groups may not be the only fixtures/appliances in the building that will require water supply. Food service areas, equipment make-up water, washing systems, and other appliances may also require water supply. Determining the required flow for all water supply fixtures will be required in order to properly size the water supply piping.

Starting with the basics in water pipe sizing, the basic flow equation is Q = VA, (Q = flow, V = velocity, and A = Area). This equation can be used to determine the required pipe size based on flow rate and velocity limitations. The UPC and the IPC dictate velocity limitations in water supply systems, and the values in the codes range from 4 to 5 ft/second for domestic hot water and a maximum of 8 ft/second for domestic cold water. The values in the UPC are better defined for the specific application. It is important to note that other factors may contribute to velocity limitations, such as acoustical requirements for sound-sensitive areas and corrosion and erosion in piping due to water quality.

The UPC and IPC provide similar methods in sizing water distribution systems. The sizing methods noted below conform with the UPC and IPC and note the key differences.

As previously mentioned, the first step in sizing any water distribution system is to work with the project architect to understand the building-use type, occupancy type, and quantity of people that will be occupying the building. Once the building program is developed and the architect has provided the required quantity of plumbing fixtures and appliances, the next step is to develop the diagrammatic piping layout in the building to serve each fixture/appliance as required. With the piping layout complete, pipe sizing can then be determined using the appropriate plumbing code section.

The 2015 edition of the UPC provides multiple sizing methods. Method one is outlined in Chapter 6, Section 610.0, and uses Appendix A. This method is used in this article for medium to large commercial-type projects. It is worth noting that Chapter 6 also provides sizing methods for flushometer valve piping systems; however, this typically applies to smaller projects.

The 2015 edition of the IPC provides sizing criteria in line with the UPC Appendix A. This information can be found in Chapter 6, Section 604, and using Appendix E. The UPC and the IPC’s sizing methods can be broken down into eight steps:

Step 1: Available water pressure

The first step in sizing water supply pipes is to determine the available pressure, static, and residual, if available. In many cases, this can be determined by calling the local water authority and requesting the domestic-water service pressure at either the required area or cross streets for the project site. Based on the available pressure at the city’s connection location, hydraulic calculations can then be completed to determine the available pressure at the building. Plumbing engineers typically deal with the plumbing systems within the building up to a point of 5 ft past the exterior wall. Therefore, it is good practice to discuss available pressures with the civil engineer, who may be completing a hydraulics analysis of the water piping from the city’s connection point to the building. The civil engineer may be able to provide the available high (static) pressure and expected low (residual/dynamic) pressure at the building, which already would have taken into account any site piping losses, meters, and backflow-prevention devices being provided. The anticipated high and low pressures are important to understand so the plumbing systems are operating properly. High system pressures can damage piping, equipment, and fixtures or, more importantly, exceed the maximum allowable pressure (80 psi) dictated by the plumbing codes. Low system pressures can affect fixture performance or system flow during peak periods. If this information is not available from the civil engineer, then the plumbing engineer can check with local utility authorities for site pressure information and then complete a hydraulics calculation to estimate the pressure losses through the site piping, including meter and backflow-prevention losses as required. For the purpose of this article, it is assumed that the civil engineer will provide the high and low water pressures at the building by completing their own hydraulic calculations for site piping and components.

Step 2: Determine the pressure requirements

The second step is to determine the pressure required for the building and all plumbing fixtures. As previously stated, plumbing codes dictate a maximum pressure of 80 psi to any plumbing fixture. Minimum pressures depend on the fixture or service type. For example, flush valve water closets can require as low as 25 psi for proper operation, as opposed to flush tank water closets, which can operate at much lower pressures. Mechanical make-up water systems may require 30 to 40 psi for proper make-up. For plumbing fixture requirements, it is recommended to review the manufacturer requirements for minimum operating pressures. If no specific pressure is required, a general guideline is to select 30 psi as a minimum pressure to each fixture. For the purpose of this article and the sample calculations, the assumption is that the required pressure is to be between 30 and 80 psi. Flush valve water closets and shower valves are the most stringent fixtures that require a minimum of 30 psi.

Step 3: Water supply demand

Next, the required water supply demand needs to be calculated for the entire building. The 2015 UPC, Table 610.3, and the 2015 IPC, Table E103.3(2), provide water supply fixture unit values for various types of plumbing fixtures. To determine the total demand, first tabulate and summate all of the water supply fixture units for all fixtures within the building. Water supply fixture-unit values can be converted into a flow rate using Hunter’s Curve, which takes into consideration the plumbing fixture flow, duration of operation, and the probability of simultaneous operation of all fixtures. The curve was developed by Roy B. Hunter in 1940 for the U.S. Department of Commerce and has been used in water supply pipe sizing ever since. This is a big topic of discussion in the plumbing community, as Hunter’s Curve is very conservative and tends to oversize water supply piping systems—especially taking into consideration how plumbing fixtures have evolved over the years and low-flow fixtures are commonly used in many buildings. Hunter’s Curve is provided in Figures 1 and 2 for reference.

An example for using Hunter’s Curve is as follows:

  1. The project architect determined the required quantity of plumbing fixtures: (20) gravity tank water closets, (30) lavatories, and (4) mop sinks.
  2. Total fixture units for these fixtures from UPC Table 610.3 for a public occupancy equals 92; however, using IPC Table E103.3(2) for a public occupancy results in 172.
  3. Using Hunter’s Curve, 92 fixture units with a flush tank system is equal to approximately 41 gal/minute per the UPC. Using Table E103.3 from the IPC, the building would require 58 gal/minute. The IPC’s Table E103.3(3) converts water supply fixture-unit values to flow rates. This table is like Hunter’s Curve as described above.

As shown in the example above, fixture-unit values differ between the UPC and IPC. It is imperative to confirm the correct code that will be used based on locally adopted codes to properly size piping systems to conform with the local code.

Step 4: Pressure losses through building supply systems

The fourth step is determining pressure losses through the interior-building supply systems. As mentioned above, it is assumed that the civil engineer is providing the high and low water pressures at the building connection. Additional losses through the interior-building supply system will include piping friction losses, elevation losses, equipment losses, and other miscellaneous components with pressure losses.

Piping friction losses can be calculated by knowing the piping material, pipe size, and flow rate. The Darcy-Weisbach equation provides a method for calculating friction loss in a pipe. This formula was used to derive the charts in the UPC’s Appendix A and the IPC’s Appendix E, which provides friction loss in head (psi) per 100-ft pipe length. The UPC and IPC include charts for copper tubing smooth pipe (type M, L, and K), fairly smooth pipe, fairly rough pipe, and rough pipe. These charts can be used to determine the velocity in feet per second and the friction loss per 100 ft of pipe length. These charts will be used in the next step to determine pipe sizing based on flow and allowable friction loss.

Elevation losses (or gains) occur when there is a physical change in elevation in the piping system. Each foot of vertical rise is equivalent to 0.434-psi pressure drop or vice versa (each foot of vertical drop in elevation is equivalent to 0.434-psi pressure gain). For example, if the incoming water supply pipe is at an elevation of -4 ft below the finished floor and a plumbing fixture is being served at Level 2 with an elevation of 16 ft above the finished floor of Level 1, then this will be equal to 20 ft x 0.434 = 8.68-psi pressure drop. Therefore, if you have an entering pressure of 60 psi, this would result in 51.32 psi at the Level 2 fixture (assuming static flow with no friction losses or other losses in the system).

Equipment losses are determined based on the type of equipment and associated pressure drops per the manufacturer. Common equipment that may have pressure drops include water-softening equipment, water-filtration devices, instantaneous water heaters, etc. It is common for water-softening system equipment to have a pressure drop between 15 and 25 psi for continuous to peak flow. Pressure drops through all equipment need to be coordinated with the manufacturer based on the required flow rates.

Miscellaneous component losses include various appliances, fixtures, equipment, etc. Common items include backflow-prevention devices, water meters, point-of-use water filters, etc. Backflow-prevention devices and water meters can result in a large pressure drop that needs to be accounted for in the overall building pressure-loss calculations. These pressure drops are typically indicated in the manufacturer’s literature based on the required flow rate.

Step 5: Longest developed pipe length

This step will determine the longest developed pipe length to the furthest hydraulically remote fixture/appliance. It is important to note that the furthest fixture from the main-entry water service may not be the furthest hydraulically remote fixture. For example, a fixture on Level 2, which is closer to the main-entry water service, may still be more hydraulically remote than a fixture further away on Level 1. Consider a water closet on Level 2 that is approximately 100 ft away from the main-entry water service versus a water closet that is 200 ft away from the main-entry water service on Level 1. The water closet on Level 1 will have a longer pipe length; however, the water closet on Level 2 will have a higher pressure drop to reach this fixture due to the elevation losses. Another example would be a flush valve water closet versus a flush tank water closet. Again, the flush valve water closet will require a higher pressure to operate than a flush tank water closet, therefore it may be the furthest hydraulically remote fixture. This is worth considering when evaluating longest developed lengths and required water pressures at various fixtures.

The longest developed length is calculated by determining the overall piping distance from the main-entry water service to the furthest hydraulically remote fixture. For example, the furthest hydraulically remote fixture is a water closet on Level 2, which is approximately 500 ft away from the main-entry water service.

In addition to the overall piping distance to the furthest hydraulically remote fixture, pipe fittings need to be considered to determine the overall developed longest run. Depending on the piping materials and required fittings, additional losses will occur at each fitting. Manufacturers typically provide information for pressure losses in fittings and valves, which is represented in the equivalent length of pipe. For example, a 1-in. copper pipe with a standard 90-degree elbow will add approximately 2.5 ft of equivalent pipe length. Therefore, calculations can be completed based on the piping layout design and anticipated fittings, types of fittings, and pipe sizes to determine the additional losses through fittings and valves.

Calculating friction losses through every fitting and valve can be time-consuming, especially when the pipe sizing needs to be determined to review the losses through each fitting and valve. Not to mention that the final installation of piping from the plumbing contractors may differ from the diagrammatic design drawings, which will change the friction losses through the piping system. A good rule to follow is to use between 15% and 50% of the overall piping distance. For example, a standard commercial project with minimal changes in direction may only require an additional 15% added to the overall piping distance to determine the overall developed longest run. However, a project with a significant quantity of fittings and changes in direction may require upwards of 50% added to the overall piping distance. This total will provide the overall developed longest run.

Step 6: Allowable friction loss

The next step involves using the information from previous steps to determine the allowable friction loss in the piping system. The allowable friction loss will be used with the charts from Appendix A of the UPC and Appendix E of the IPC to determine pipe sizes and flow-rate requirements.

Reference the previous example of a commercial building with (20) gravity tank water closets, (30) lavatories, and (4) mop sinks. Assuming this is a 2-story commercial building with fixtures on both levels, the first step is to determine the available pressure. For this example, the civil engineer has provided the plumbing engineer with a 3-in. domestic-water service at an assumed low pressure of 60 psi (dynamic) and a high of 70 psi (static). The high static pressure is within acceptable pressure limitations per the UPC and IPC (does not exceed 80 psi). As there is no drop-in elevation (no basement or lower level within the building), there will not be an increase in pressure due to elevation drop. This example will focus on the low pressure of 60 psi and will use this value to determine the allowable friction loss. A meter loss of 10 psi is assumed.

Next will be to determine the required pressure from Step 2. Although all of the water closets are gravity tank type, it is good practice to maintain a minimum of 30 psi at the most remote fixture. For this example, 30 psi will be used for the minimum pressure required.

Now using Step 4, friction losses can be determined for the building supply system. At this time, piping friction losses will not be calculated as this step is to determine the allowable friction loss over the entire system. Elevation losses as described in Step 4 will be used for this example, with a vertical rise of 20 ft which is equal to an 8.68-psi pressure drop.

Step 5 will then be used to determine the overall developed longest run. For this example, 500 ft will be used as the overall piping distance from the main-entry water service to the furthest hydraulically remote fixture on Level 2. This building has various changes in direction but an overall majority of straight pipe runs. Therefore, the plumbing engineer will agree to add 25% to the overall piping distance to account for losses in fittings and valves. Therefore, the overall developed longest run is equal to 625 ft.

Finally, calculating the allowable friction loss per 100 ft of pipe run is completed by multiplying the pressure available after all losses and required pressure at furthest hydraulically remote fixture by 100 ft, then dividing the total by the longest developed run; see the calculation in Table 1.

For this example, the allowable friction loss per 100 ft is equal to 1.8112 psi. This value will be used in Step 7 to develop the pipe sizing chart per the tables from the UPC and IPC.

Step 7: Pipe size and flow requirements

This step uses Charts A 4.1, A 4.1(1), A 4.1(2), and A 4.1(3) from Appendix A of the UPC and Figures E103.3(2), E103.3(3), E103.3(4), E103.3(5), E103.3(6), and E103.3(7) of the IPC. These charts provide the correlation between pipe size, velocity, flow, and friction loss per 100 ft for various pipe materials (copper tubing, fairly smooth pipe, fairly rough pipe, and rough pipe). Depending on the piping material to be used for the project, the correct chart can be used. For domestic-water piping, fairly rough piping is used frequently. This chart will be used to further develop the pipe-sizing requirements based on the example used in Step 6.

Using the allowable friction loss per 100 ft, which from the example above is 1.8112 psi/100 ft, the chart can be used to find associated flow-rate limitations and velocities for various pipe sizes. Keep in mind that the velocity limitation for cold water is 8 ft/second and for hot water is 5 ft/second.

These values can then be converted into a table for reference related to pipe size, flow, and velocity using Hunter’s Curve to convert flow to water supply fixture units, as shown in Table 2.

Step 8: Pipe sizing

The final step is finalizing the pipe sizing on the associated plans. This includes summing up the water supply fixture units for all fixtures and totalizing the water supply fixture units through the entire piping system. The example markup plan in Figure 3 shows summation of the water supply fixture units using the UPC through the hot- and cold-water piping systems back to the main water supply. The same method would apply for the IPC, with changes to water supply fixture-unit values as required. For fixtures with cold-water supply only (i.e., urinals, water closets, hose bibs, etc.), this will be equal to the total water supply fixture-unit value from Table 610.3 of the UPC. For fixtures with hot and cold supplies, the notes at the bottom of UPC, Table 610.3, permits using 75% of the total supply fixture units to calculate flow. The IPC provides fixture-unit values for both cold- and hot-water-serving fixtures.

Once the water supply fixture units are totaled through the entire piping system, then the table developed in Step 7 can be used to provide the proper pipe size for each pipe segment.

The 2015 editions of the UPC and the IPC provide similar water-pipe-sizing methods for large commercial buildings, although there are clear differences between methods used in each code. Numerous published standards are also available for support in sizing of plumbing systems and conditions/problems that can arise. Using the correct code adopted by the local jurisdiction is required to complete proper sizing of the domestic-water systems. Understanding the basics behind pipe-sizing principals is vital in understanding how to use the proper code and properly design the water distribution systems for commercial buildings.