Instructions for Completing Water Calculation Worksheet

Instructions for completing Water Calculation Worksheet, SBD 6479 (R11/11)

1. Demand of building in water supply fixture units (WSFU). Add up WSFU's (SPS Tables 382.40 - 1 & 2).      1.a. Demand of building in WSFU converted to Gallons Per Minute. Convert WSFU's to GPM (SPS Table 382.40 - 3). 

2. Determine difference in elevation from main or external pressure tank to building control valve. Ask purveyor depth of main in street, or ask pump installer depth of pipe at connection to external  pressure tank.

3. Size of meter (if applicable). Ask purveyor for meter size for GPM demand.

4. Developed length in feet from main or external pressure tank to building control valve. Measure actual  distance.

5. Determine low pressure at main in street, or at external pressure tank. Ask purveyor for the low residual pressure of water at address, or ask pump installer low pressure setting on switch. 

6. Low pressure at main in street, or external pressure tank (as determined at # 5 above).

7. Determine pressure loss due to friction in the water service. Refer to SPS 382 Appendix Graphs A382.40 (7) - 2 thru 11. 

8. Determine the pressure loss or gain due to the difference in elevation between the main or external pressure tank and the building control valve. Measure difference in height (ft.) from the main or external pressure tank to the building control valve. Multiply height (ft.) by .434. 

9. Available pressure after the building control valve (enter in line "B"). 

1. = Pressure available for uniform loss. This number is only an indicator for using the pipe sizing SPS Tables 382.40-4 thru 11.

2. Available pressure after the building control valve (from line "9"). 

3. Determine pressure loss of water meter, SPS 382 Appendix Graph A382.40 (7)-1 or provide   manufacturer's loss curve.

4. Pressure at controlling fixture. This is the most demanding pressure required for a fixture to  properly operate. Compare; 1. Required fixture pressure, 2. Elevation of fixture, 3.  Developed length to fixture.

5. Determine difference in elevation between the building control valve and the controlling  fixture. Measure difference in height (ft.) from the building control valve to the   controlling fixture. Multiply height (ft.) by .434.

6. Pressure loss due to water treatment devices (water softeners, filters, etc.), and backflow  preventers which serve the controlling fixture. Add up the WSFU's downstream of the  water treatment device and convert to GPM using SPS Table 382.40-3, or, Table 382.40-3e when serving an individual dwelling. Refer to manufacturer's graph to convert gpm to  pressure loss through the WTD, and or a backflow preventer.

7. Pressure loss through tankless water heaters, combination boiler / hot water heaters, heat  exchangers which serve the controlling fixture. Add up WSFU's downstream of the  heating appliance and convert to GPM using SPS Table 382.40-3. Refer to manufacturer's pressure loss graph to determine loss at the required GPM.

8. Developed length from building control valve to controlling fixture in feet X 1.5. This is the  measured length (ft) of pipe between the building control valve and the controlling  fixture. Multiply the length (ft) by 1.5.

Table 82.40-1

Water Supply Fixture Units for Non-Public Use Fixtures

Table 82.40-3e

Conversion of Water Supply Fixture Units to Gallons Per Minute for
Water Treatment Devices(a) Serving an Individual Dwelling (b)

1.  Treatment devices providing treatment for compliance with Table 82.70-1 shall use Table 82.40-2 for conversion

2. Table shall not be used for converting hose bibb, high flow fixtures, or hydrant WSFU

Table 82.40-2

Water Supply Fixture Units for Public Use Fixtures

1.  For fixtures not listed, factors may be assumed by comparing the fixture to a listed fixture which uses water in similar quantities and at similar rates. 

2. Load factors in gallons per minute, gpm, based on manufacturer’s requirements

Table 82.40-3

Conversion of Water Supply Fixture Units To Gallons Per Minute

Note: Values not specified in the table may be calculated by interpolation