14.2 Pipe Sizing, Longest-Length Method & Pressure Drop

402.1 requires the piping system to supply the maximum demand and to deliver not less than the minimum inlet pressure each appliance needs. 402.2 sets the volumetric flow as the sum of the maximum inputs of the appliances served. Use the total connected hourly load and assume every appliance can run at full capacity at once. Diversity is allowed only where a diversity of load can be established. Adjust the volume for altitude above 2,000 feet. 401.4 is the existing-system twin: adding an appliance means you re-check the old pipe; if it is inadequate, enlarge it or run a separate pipe of adequate capacity. 401.3 says modifications still keep sizes in accordance with Chapter 4. 401.8 forbids a pipe that cannot supply the full number of outlets.

Convert Btu/h to the table’s units

IFGC Appendix A A102.1 (informative, but it is the conversion the tables assume): obtain cubic feet per hour by dividing total Btu/h input by the average Btu heating value per cubic foot from the serving gas supplier. The Appendix A sample calculations use 0.60 specific gravity and 1,000 Btu/ft³. On the exam, unless a stem gives a different heating value, convert natural gas as:

cfh = Btu/h ÷ 1,000

A 199,000 Btu/h tankless is 199 cfh. A 100,000 Btu/h furnace is 100 cfh. A 40,000 Btu/h water heater is 40 cfh.

Do not convert propane into cfh to use the propane tables. Appendix A A103.1 Step 2 is explicit: Tables 402.4(25) through 402.4(37) are selected with demand in thousands of Btu per hour. The same 199,000 Btu/h tankless is 199 on a propane table. Those tables already use specific gravity 1.50. Mixing a natural-gas cfh table with an LP nameplate, or dividing LP Btu/h by 1,000 and then reading a propane MBH column, is how candidates undersize a second-stage line.

402.3 allows three sizing paths: the Section 402.4 / 402.5 tables or equations; the sizing tables in a listed piping system’s manufacturer’s instructions (the CSST path); or engineering methods. 402.6 is the pressure-drop governor: from the point of delivery to every appliance inlet, remaining pressure shall be greater than or equal to the appliance’s minimum. Indoor systems are generally capped at 5 psig unless a 402.7 exception applies (welded or brazed joints, LC-4 fittings, flanged-and-welded joints, a ventilated chase, certain industrial/research/warehouse/boiler-room occupancies, temporary construction piping, agricultural use, or an LP-gas system over 20 psi that complies with NFPA 58).

Longest-length method — 402.4.1

402.4.1 is one sentence you should be able to write from memory:

The pipe size of each section of gas piping shall be determined using the longest length of piping from the point of delivery to the most remote outlet and the load of the section.

Measure meter (or other point of delivery) to the farthest appliance. That footage is the only length you take into the table. Every section — the trunk that carries everything and the 8-foot stub that carries one water heater — is read on that same row. You do not size the stub on 8 feet. Appendix A A103.1 adds the exam technique: if the measured length is not printed, use the next longer row; then, starting at the most remote outlet, find a capacity equal to or greater than that section’s load and read the size at the top of the column.

402.4.2 Branch Length Method is the allowed alternate. Size the longest run with the longest length, then size each remaining branch using the length from the point of delivery to the most remote outlet on that branch. 402.4.3 Hybrid Pressure sizes the high-pressure portion on the longest length from the point of delivery to the most remote line pressure regulator, then sizes each low-pressure zone on the length from that regulator to its most remote outlet.

Table 402.4(2) — the table to tab

Table 402.4(2): Schedule 40 metallic pipe, natural gas, inlet less than 2 psi, pressure drop 0.5 in. w.c., specific gravity 0.60, capacity in cfh. These are fetched 2024 values, not folklore:

Table 402.4(1) is the same pipe and gas at a 0.3 in. w.c. drop. Its 10-foot, 1/2-inch cell is 131 cfh — that 131 is real, but it is not the 0.5-inch-drop number. If the stem does not name the drop, the common low-pressure commercial table is 402.4(2).

Worked examples with those cells

Example 1 — longest length 60 feet, natural gas, Table 402.4(2). A mechanical room has a 100,000 Btu/h furnace (100 cfh), a 40,000 Btu/h water heater (40 cfh), and a 250,000 Btu/h rooftop (250 cfh). The most remote outlet is 60 feet from the meter. Stay on the 60-foot row: 1/2 = 65, 3/4 = 137, 1 = 257, 1-1/4 = 528.

• Water-heater branch at 40 cfh: 1/2 inch (65 ≥ 40).
• Furnace branch at 100 cfh: 3/4 inch (137 ≥ 100; 65 is short).
• Rooftop branch at 250 cfh: 1 inch (257 ≥ 250). That cell is tight; 249 cfh would still be 1 inch, 258 cfh would jump to 1-1/4.
• Trunk that serves all three (390 cfh): 1-1/4 inch (528 ≥ 390; 257 is short).

Example 2 — the 199k tankless field fail. A 199,000 Btu/h natural-gas tankless sits 80 feet from the meter on a lone 1/2-inch run. Demand is 199 cfh. Table 402.4(2) at 80 feet gives 1/2-inch only 56 cfh, 3/4-inch 117 cfh, 1-inch 220 cfh. The 1/2-inch pipe carries about 28 percent of the load. 1 inch is the first size that works (220 ≥ 199). A 3/4-inch “upgrade” still fails. This is the 401.4 addition problem dressed as a new house: the installer copied a 40-foot water-heater size onto an 80-foot tankless.

Example 3 — adding load to an existing 1-inch, 80-foot main. The same 80-foot 1-inch main already serves 100 cfh of other appliances (Table capacity 220 cfh). Adding the 199 cfh tankless makes 299 cfh. 220 < 299, so the existing 1-inch main fails 401.4. The 80-foot row needs 1-1/4 inch (452 ≥ 299).

Example 4 — undiluted propane, Table 402.4(28). That table is Schedule 40 metallic, 11.0 in. w.c. inlet, 0.5 in. w.c. drop, SG 1.50, intended for pipe between a single- or second-stage low-pressure regulator and the appliance, capacity in thousands of Btu per hour. At 80 feet: 1/2-inch = 94, 3/4-inch = 197, 1-inch = 372. The same 199,000 Btu/h tankless is 199 on this table. 197 < 199, so 3/4-inch propane at 80 feet fails by 2 MBH. The legal size is 1 inch (372). That one-cell miss is an exam item waiting to happen.

Example 5 — 10 psi first-stage propane, Table 402.4(25). Between the first-stage and second-stage regulators, 10 psi inlet, 1.0 psi drop, an 80-foot 1/2-inch steel line carries 1,080 thousand Btu/h. That is why two-stage LP systems can use small pipe outdoors and still feed a large second-stage regulator. Do not read Table 402.4(25) on the low-pressure house line.

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