Section 6.1.1/Gas Piping Sizing
NFPA 54 Chapter 6 covers gas piping sizing methods using the longest length method to determine minimum pipe diameters.
Gas piping must be sized to deliver the required gas volume to each appliance at the minimum supply pressure. The most common sizing method is the longest length method: determine the total connected BTU/h load, measure the longest pipe run from the meter to the most remote appliance, and use the sizing tables to find the minimum pipe diameter for each segment. The tables account for the pipe material, inlet pressure, and allowable pressure drop (typically 0.5 inches water column for low-pressure residential systems). Each branch must be sized for the load it serves using the longest run from that branch to the meter.
Why this section exists
An undersized gas pipe cannot deliver adequate gas volume to the appliance at the required pressure. The appliance receives less fuel than needed, causing incomplete combustion, reduced efficiency, nuisance shutdowns, and potentially dangerous operating conditions including carbon monoxide production. The sizing tables are derived from fluid mechanics equations for gas flow through pipes, simplified into lookup tables that account for the friction loss based on pipe length, diameter, and flow rate.
What plan reviewers look for
Plan reviewers check the gas piping plan for pipe sizes at every segment. They verify the sizing against the applicable table for the pipe material, pressure, and longest run length. They check that the total connected load matches the appliance schedule. They verify that each branch segment is sized for its downstream load using the total longest length from the meter.