9.4 Gas Pipe Sizing Tables: Longest-Length & Branch-Length Methods

9.4 Gas Pipe Sizing Tables: Longest-Length & Branch-Length Methods

Quick Answer: Under International Fuel Gas Code (IFGC) 2018 Section 402.4.1, the longest length method sizes every section of a gas piping system from a single table row - the developed length from the point of delivery to the most remote outlet, rounded up to the next tabulated length - and each section is then sized for its own load by picking the first pipe size whose tabulated capacity equals or exceeds that load.

Gas Appliances and Piping is 40 of the 100 scored questions on the Maryland Journey Plumber/Gas Fitter examination, and pipe sizing is the most arithmetic-heavy skill inside it. The exam is open book, so you are not memorizing capacities - you are being tested on whether you can find the right table and walk the right row under time pressure.

Step 1 - Convert input ratings to the units the table uses

Every appliance carries a nameplate input rating in British thermal units per hour (Btu/h). Divide that by the heating value of the gas to get a volumetric demand:

Demand (cubic feet per hour, cfh) = Input (Btu/h) / Heating value (Btu/ft3)

Natural gas is taken as 1,000 Btu/ft3, so 1 cfh = 1,000 Btu/h. Undiluted propane is about 2,500 Btu/ft3 (2,516 Btu/ft3 at 60 degrees F).

Where the appliances are not yet selected, IFGC Table 402.2 gives approximate inputs for typical appliances: 100,000 Btu/h for a single-family furnace or hydronic boiler, 50,000 Btu/h for a 50-gallon storage water heater, 65,000 Btu/h for a domestic range, 35,000 Btu/h for a clothes dryer, 80,000 Btu/h for a gas log, 2,500 Btu/h for a gas light. A nameplate always overrides the table, and the design must be re-verified once actual equipment is known.

Step 2 - Size for the total connected load, not a diversified load

IFGC 402.2 is blunt: the total connected hourly load is the basis for pipe sizing, assuming all appliances could operate at full capacity simultaneously. Diversity is the narrow exception - diversified loads may be used only where a diversity of load can be established. A gas system is therefore sized very differently from an electrical service, where demand factors are routine. Section 402.2 also requires the flow rate to be adjusted for altitude above 2,000 feet, which in Maryland reaches only parts of Garrett County.

The meter and the utility service regulator are supplied and sized by the serving gas utility; your job is to hand the utility a correct total connected load in cfh. IFGC 409.2 requires a shutoff valve on the supply side of every meter.

Step 3 - Identify the correct table before you read a single number

IFGC 402.3 allows three sizing routes: the code's tables and equations (402.4 and 402.5), the sizing tables in a listed piping system manufacturer's instructions, or other approved engineering methods. Tables 402.4(1) through 402.4(37) are keyed by four header lines, and misreading those headers is the most common sizing error on the exam:

1. Gas - natural, or undiluted propane
2. Inlet pressure - "less than 2 psi," 2.0 psi, 5.0 psi, or 11.0 in. w.c.
3. Pressure drop - 0.3 in. w.c., 0.5 in. w.c., 1.0 psi, 3.0 psi, or 3.5 psi
4. Specific gravity - 0.60 for natural gas, 1.50 for undiluted propane

Burn this in: Tables 402.4(1) through 402.4(24) are read in cubic feet per hour; Tables 402.4(25) through 402.4(37) - the undiluted propane tables - are read in thousands of Btu per hour (IFGC Appendix A, Section A.3.1). Appendix A Table A.2.4 supplies specific-gravity multipliers when the delivered gas is not 0.60: a 0.50 gravity gas multiplies capacities by 1.10, a 1.50 gravity gas by 0.63.

The workhorse table for Maryland residential natural gas is Table 402.4(2) - Schedule 40 metallic pipe, natural gas, inlet less than 2 psi, 0.5 in. w.c. drop, 0.60 specific gravity, capacity in cfh:

Step 4 - Measure the length and account for fittings

Measure the developed pipe length, then add an equivalent length allowance for fittings. IFGC Appendix A directs that an allowance from Table A.2.2 (Equivalent Lengths of Pipe Fittings and Valves) be considered for any run containing four or more fittings, and warns that only four elbows or tees can push a run into the next longer table row with a significant loss of capacity. ICC guidance offers a widely used shortcut: multiply the actual pipe length by 1.50 as an all-inclusive fitting allowance.

CSST is handled differently. Table 402.4(15) already includes losses for four 90-degree bends and two end fittings; extra bends or fittings add L = 1.3n feet of equivalent tubing, where n is the number of additional fittings or bends.

Two rounding rules govern every lookup, and they run in the same direction:

• Length rounds UP to the next tabulated row. An 88-foot developed length uses the 90-foot row.
• Capacity rounds UP to the next larger figure in that row. Never select a pipe whose tabulated capacity is below the section load.

Step 5 - The three length methods (IFGC 402.4.1, 402.4.2, 402.4.3)

• Longest length method (402.4.1) - the size of each section is determined using the longest length of piping from the point of delivery to the most remote outlet, together with the load of that section. One length, one row, whole system. Most conservative, and the default assumption on exam questions.
• Branch length method (402.4.2) - sections on the longest run are still sized from the longest-run row, but each remaining branch is sized from the length measured from the point of delivery to the most remote outlet in that branch. Less conservative, which is why Appendix A stresses fitting allowances here.
• Hybrid pressure method (402.4.3) - for 2 psi systems. Higher-pressure piping is sized from the longest length to the most remote line pressure regulator; piping downstream of that regulator is sized from the regulator to the most remote outlet it serves.

Worked example - longest length method

A Maryland single-family home is served with natural gas at 0.5 psi or less through Schedule 40 steel pipe. Connected appliances: a 100,000 Btu/h furnace, a 40,000 Btu/h water heater, a 65,000 Btu/h range, and a 35,000 Btu/h dryer.

Longest run = A + C + E + G = 20 + 18 + 14 + 12 = 64 ft. Apply the all-inclusive fitting allowance: 64 x 1.50 = 96 ft. Round up to the next tabulated length: the 100-foot row of Table 402.4(2) sizes the entire system.

The 100-foot row reads 50 / 104 / 195 / 400 / 600 / 1,160. Walk it once per section:

Section A fails at 1 inch because 195 cfh is less than the 240 cfh it must carry, so the next column - 1-1/4 inch at 400 cfh - is the answer. Section G passes at 3/4 inch by only 4 cfh, which is exactly why the fitting allowance matters.

The same job by the branch length method

Sections A, C, E and G lie on the longest run and do not change. Branch B, the range, is only 20 + 12 = 32 ft from the meter; 32 x 1.50 = 48 ft rounds up to the 50-foot row, where 1/2-inch pipe carries 72 cfh. Because 72 exceeds the 65 cfh range load, that branch drops from 3/4 inch to 1/2 inch. Branch D (48 x 1.50 = 72 ft, 80-foot row, 56 cfh) and branch F (60 x 1.50 = 90 ft, 90-foot row, 52 cfh) both stay at 1/2 inch.

Why 2 psi systems permit smaller pipe

A 2 psi system carries gas from the meter at 2 psi to a line pressure regulator near the appliances, which drops it to 7 to 11 in. w.c. Because that higher-pressure zone tolerates a 1.0 psi drop instead of 0.5 in. w.c. - and 1.0 psi equals 27.7 in. w.c., roughly 55 times the allowable loss - the same pipe carries dramatically more gas.

The 240 cfh house above needs 1-1/4-inch pipe leaving the meter on a low-pressure system, but only 1/2-inch pipe from the meter to the line regulator on a 2 psi system. Appendix A attaches three limits to the hybrid method: the maximum drop in the 2 psi section is 1 psi, the drop across the line regulator must not exceed 3/4 psi, and the calculated load may be increased up to 50 percent for future appliances. IFGC 402.7 caps piping inside buildings at 5 psig unless one of seven listed conditions - welded or brazed joints, a ventilated chase, certain occupancies - is met.

Propane sizing: same method, different units

Move that 240,000 Btu/h house to propane and the method is unchanged, but the table is not. Table 402.4(28) - Schedule 40 metallic pipe, undiluted propane, 11.0 in. w.c. inlet, 0.5 in. w.c. drop, 1.50 specific gravity, intended for sizing between the single- or second-stage regulator and the appliance - is stated in thousands of Btu per hour. Its 100-foot row reads 84 / 175 / 330 for 1/2, 3/4 and 1 inch (94 / 197 / 372 is the 80-foot row - a classic off-by-one-row slip). Entering with 240 yields 1 inch: a full size smaller than the 1-1/4 inch the identical load required on natural gas, because propane's 2,500 Btu/ft3 heating value more than offsets its higher specific gravity.

Exam Trap: Candidates convert the propane load to cfh (240,000 / 2,500 = 96) and then read 96 against Table 402.4(28). The 100-foot row shows 175 at 3/4 inch, so the answer comes back 3/4 inch - one full size undersized - because that table is already expressed in thousands of Btu per hour and needed the number 240, not 96. The mirror-image trap lives in the pressure-drop header: at the 100-foot row a 1-inch line carries 195 cfh in Table 402.4(2) at a 0.5 in. w.c. drop but only 148 cfh in Table 402.4(1) at a 0.3 in. w.c. drop, so a 180 cfh load sizes as 1 inch on one table and 1-1/4 inch on the other. Read all four header lines before you read the grid.