12.3 Part 3: Available Pressure, Elevation, Regulators, Device Losses, and the Length Column

Step 1: Start from the Right Pressure

• Minimum street pressure at the meter or other source (610.7 and 610.8). If the drawing gives a range, use the low end.
• Regulator installed? Use 80% of the reduced (set) pressure (608.2). A regulator set at 65 psi gives 0.80 × 65 = 52 psi to start from.
• Devices with pressure loss (610.2): if the drawing gives a loss, such as 10 psi through a backflow assembly or 5 psi through a softener, include it by subtracting it from the available pressure.

Step 2: Adjust for Elevation (1/2 psi per Foot)

Section 610.8(2): add or subtract 1/2 psi for each foot of elevation difference between the source and the highest water supply outlet. Subtract when the highest outlet is above the meter. Add when every outlet is below it, for example a building downhill from the meter.

Physics versus the table. Water actually exerts 0.433 psi per foot. The table method deliberately uses 0.5 psi per foot, which is more conservative. On Part 3, when you are choosing the Table 610.4 range, use 1/2 psi per foot.

Step 3: Pick the Pressure Range

Table 610.4 note 1 describes the ranges as "available static pressure after head loss":

Below 30 psi, Table 610.4 cannot be used. Size by Appendix A, or raise the pressure. Section 608.1 requires at least 15 psi residual at fixtures.

Step 4: Pick the Length Column

Columns: 40, 60, 80, 100, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1,000 feet. Choose the column equal to or longer than the developed length from the meter to the most remote outlet (610.8(4)). All segments of the system, the building supply and every branch, are sized in that same column (610.9: "the total developed length of the system").

Worked Examples

Length examples: 140 feet uses the 150 column, 95 feet uses 100, 100 feet exactly uses 100, and 205 feet uses 250.

Why the Range Choice Matters

With the same load and length, a lower pressure range allows fewer fixture units per pipe size. For the 150-foot column:

A 50-FU building at 150 feet needs a 1-1/2-inch supply in the 30–45 range (1 / 1-1/2 = 56), but only 1-1/4 inch in the 46–60 range (1 / 1-1/4 = 52).

Measuring Developed Length on the Drawing

Developed length follows the pipe, not the straight line. Suppose the drawing shows this path from the meter to the most remote outlet:

Every piece of pipe counts, including rises and drops. The 150-foot column applies. The elevation adjustment uses only the net height of the highest outlet above the meter, which in the example is 8 feet. It does not use the 10-foot rise to the ceiling, because that pipe comes back down before any outlet.

Two Elevation Factors in One Chapter

The UPC uses two different elevation factors, and the exam can test either:

• Table 610.4 method (610.8): add or subtract 1/2 psi per foot. This is more conservative than the physical value, and it keeps the arithmetic simple under exam conditions.
• Appendix A method (A 104.2): multiply the elevation difference by 0.43 to get the loss in static pressure. Appendix A then uses friction-loss charts and a chosen residual pressure at the highest fixture.

On Part 3 you are applying Table 610.4, so apply 610.8 exactly as written. If a Part 1 question asks for the static pressure lost to a given rise, it is asking for the physical value of about 0.433 psi per foot.

Checklist Before You Open Table 610.4

1. Minimum street pressure, or 80% of the regulator setting.
2. Minus device losses given on the drawing.
3. Plus or minus 1/2 psi per foot to the highest outlet.
4. Range: 30–45, 46–60, or over 60.
5. Length column at or above the meter-to-most-remote-outlet developed length.
6. Total fixture units from Section 12.2, with flushometers per Table 610.10.