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.


This article has been peer-reviewed.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.

Figure 1: Hunter’s Curve is provided for correlation between fixture units and demand (gallons per minute). All graphics courtesy: NV5Several 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.

Figure 2: An enlarged scale of Hunter’s Curve is provided for correlation between fixture units and demand (gallons per minute).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.

Figure 3: This illustrates an example plan for summation of water supply fixture units and pipe sizing for hot and cold water.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 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.

Table 1: This is a calculation for allowable friction loss.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.

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