What is the longest length method for sizing gas pipe?

It sizes every section of the system from one length: the run from the meter to the farthest appliance. Phoenix's residential code, which follows the 2024 International Residential Code, puts it this way in Section G2413.4.1: "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."

Two terms in that rule matter:

• Point of delivery. For natural gas, the code defines it as "the outlet of the service meter assembly," so the measuring starts at the meter.
• Load of the section. Each piece of pipe is sized for the gas flowing through it, which is the total of every appliance downstream of that piece.

The load is figured at full blast. Section G2413.2 says "The total connected hourly load shall be used as the basis for pipe sizing, assuming that all appliances could be operating at full capacity simultaneously." The code does allow a lower figure where a diversity of load can be established. The goal behind all of it, from G2413.1, is to "supply gas to each appliance inlet at not less than the minimum supply pressure required by the appliance."

The International Code Council's CodeNotes on fuel gas pipe sizing lays the method out in four steps. In short form:

1. 1Find the longest run. Measure the pipe from the meter to the most remote outlet.
2. 2Find each appliance's demand. Take the input rating from the appliance label in Btu per hour and divide by the heating value of the gas to get cubic feet per hour (CFH). ICC uses about 1,000 Btu per cubic foot for natural gas and notes the gas supplier can give you the heating value.
3. 3Pick the right table. Each table covers one pipe material, gas, inlet pressure and allowed pressure drop.
4. 4Size each section. ICC's rule for the row is plain: "Select the row in the table that equals the longest pipe length determined in Step 1 (if the longest length is between table values, use the row with the next higher value). This row is used to size all gas piping in the system."

That last point is the heart of the method. Every section, even a short branch near the meter, is read off the same long-run row. ICC also warns that "If a designer fails to verify the sizing with the actual connected load values, the resulting system could be undersized."

An illustration, not a design. Take the code's table for Schedule 40 steel pipe with natural gas under 2 psi and a 0.5 inch water column pressure drop. Say the longest run is 55 feet. There's no 55-foot row, so the 60-foot row is used, where 1/2-inch pipe carries 65 CFH, 3/4-inch carries 137 CFH and 1-inch carries 257 CFH. A furnace needing 80 CFH and a water heater needing 40 CFH put 120 CFH on the pipe leaving the meter, so that section reads 3/4-inch. The furnace branch, at 80 CFH, is also 3/4-inch. The water heater branch, at 40 CFH, reads 1/2-inch. Your plumber's actual layout, pipe material and pressure will change the answer.

Equivalent length turns fittings into feet of pipe. Every elbow, tee and valve adds friction, and friction costs pressure the same way extra pipe does. The informative Appendix A published with the fuel gas code explains that the pressure loss across any piping component can be equated to the pressure drop through a length of pipe. So the measured run plus an allowance for fittings gives the length used to pick the table row.

It can change the answer more than you'd expect. Appendix A notes that "The equivalent length of a combination of only four elbows/tees can result in a jump to the next larger length row, resulting in a significant reduction in capacity." For steel pipe tables, the Seattle copy of Appendix A says an allowance "should be considered for any piping run with four or more fittings," and its Table A.2.2 lists equivalent lengths of pipe fittings and valves. Keep in mind the appendix labels itself: "This appendix is informative and is not part of the code." It's guidance, and your jurisdiction decides what it accepts.

Flexible tubing works a little differently. ICC's CodeNotes reproduces the note on the code's tables for corrugated stainless steel tubing (CSST): "Table includes losses for four 90-degree bends and two end fittings." Runs with more than that get extra length by the formula in that note, "L = 1.3n, where L is additional length (feet) of tubing and n is the number of additional fittings and/or bends." If you're choosing between the two materials, see CSST versus black iron gas pipe.

The longest length method is simple and on the safe side. It isn't the only option. Section G2413.3 lets gas piping be sized with the code's tables or equations, with the sizing tables in a listed piping system manufacturer's installation instructions, or with approved engineering methods.

• Branch length method. ICC calls it "a less conservative variation of the longest-length method." The main run is sized the same way, but each branch not already sized uses the length from the meter to the farthest outlet on that branch.
• Hybrid pressure. For systems with a higher-pressure section and a line regulator, the code sizes the high-pressure piping out to the most remote regulator, then sizes from each regulator to its outlets. Our page on 2 psi elevated pressure gas systems covers where that shows up.
• Engineered calculations. The TracPipe CSST design guide from OmegaFlex says "The longest run method was devised at a time when gas utilities could not always guarantee a constant pressure at every meter during times of high demands; it is a conservative approach," and adds that other engineered calculations are permitted by most codes.

Conservative has a cost, usually a size up in spots. It also leaves room when you add an appliance later.

Adding load is where undersized pipe shows up. A pool heater, a bigger tankless water heater or an outdoor kitchen can push a line that was fine for years past its table row. The City of Phoenix, in a gas clearance table written with Southwest Gas and the Arizona Corporation Commission, lists "Appliance upgrade with an increase in BTU consumption requiring piping capacity calculations" as work that needs a city permit, with inspection and clearance by the city. Other Valley cities run their own permit offices, so ask which one covers your address; our page on who can install a gas line in Arizona covers the licensing side.

Before new piping goes into service it gets pressure tested and inspected; see how a gas line pressure test works. The code section behind the sizing itself is summarized in our UPC 1215 gas pipe sizing page, and a real project example is running a gas line for a pool heater.

If you're planning new gas work, HQ Plumbing & Air handles gas line repair and installation. We're licensed and insured, we give free estimates, and you get upfront pricing before work starts. Smell gas right now? Southwest Gas gives a simple rule: "If you smell rotten eggs, leave the area, then call us for help."