Section 7.2.1/Gas Piping Sizing Methods
NFPA 54 Section 7.2 covers gas piping sizing using the longest length method or the branch length method, based on total BTU/h demand, pipe material, and inlet pressure.
Gas piping must be sized to deliver the required volume of gas to each appliance at the minimum supply pressure specified by the appliance manufacturer. NFPA 54 (National Fuel Gas Code) provides two sizing methods. The Longest Length Method (Section 7.2.2) sizes all pipe segments based on the longest run from the meter to the most remote appliance. This conservative approach oversizes some segments but simplifies the calculation. The Branch Length Method (Section 7.2.3) sizes each section of pipe based on its actual length and the demand it serves, resulting in smaller pipe sizes for shorter branches but requiring more detailed calculations. Both methods use the sizing tables in Appendix A, which provide pipe capacities in cubic feet per hour (CFH) or thousands of BTU/h based on pipe material (steel, copper, CSST), pipe size, length, and inlet pressure (typically 0.5 psi for low-pressure systems, 2 psi for medium-pressure). The total connected load is the sum of all appliance BTU/h ratings. A diversity factor may be applied where permitted by the authority having jurisdiction for buildings where not all appliances operate simultaneously.
Why this section exists
Undersized gas piping creates a pressure drop that starves downstream appliances of adequate gas flow, causing incomplete combustion, pilot outages, and appliance malfunction. Oversized piping wastes material and increases cost. The sizing tables are engineering calculations that account for gas density, friction losses, fitting equivalent lengths, and the minimum pressure required at each appliance. The two methods provide flexibility: the longest length method is faster for simple residential systems, while the branch length method is more economical for complex commercial systems with many branches of varying lengths. The sizing must account for the specific gas (natural gas at 1000 BTU/cf or propane at 2500 BTU/cf) because the tables are based on gas properties.
What plan reviewers look for
Plan reviewers check the gas piping plan for pipe sizes at each segment. They verify the riser diagram shows the total connected load (BTU/h) and the longest run or branch lengths. They check that the pipe sizes match the applicable sizing table for the pipe material, length, and demand. They verify the inlet pressure matches the utility supply. They check for shutoff valves at each appliance (within 6 feet, accessible). They verify the gas piping material is appropriate for the location (CSST requires bonding per the NEC, piping through concealed spaces must be protected). They check that the gas meter capacity is adequate for the total connected load. For commercial kitchen equipment, they verify the gas demand matches the equipment schedule.