The Longest Length Method. Episode seventy-eight of the Gas Service Animated series.

Every pipe size on a gas drawing is a promise: with every appliance firing at once, each one still gets its gas at pressure. The code gives you a half inch of water column to spend across the whole system — and one method spends it so carefully it never needs an apology. This is the longest length method, step by step.

Where does sizing authority come from? Section 5.3.3 gives three legal routes: the chapter six tables or equations, a listed manufacturer's sizing tables, or engineering methods. Section 6.1 then names the ways to use those tables — and section 6.1.2 is the longest length method, the default the trade runs on. One worst-case length. One row of the table. The whole system.

Step one: the load inventory. Walk the house and write down every appliance's nameplate input. Furnace, one hundred thousand. Water heater, forty thousand. Range, sixty thousand. Two hundred thousand Btu per hour total — and per section 5.3.2, the sizing basis is the total connected load, everything at full fire at the same time. Each section of pipe carries the sum of everything downstream of it. The section at the meter carries the whole house.

Step two: convert to cubic feet per hour, because the tables speak in gas volume, not heat. Divide by the heating value. The customary default is one thousand Btu per cubic foot for natural gas — so two hundred thousand becomes two hundred c f h. That default is utility practice, not a law of nature: the real district value comes from the gas supplier.

Step three: the measurement that names the method. Tape the piping from the point of delivery — the meter outlet — along the pipe, to the most remote outlet. Here, seventy feet to the range. Enter the table at seventy — and if your length falls between rows, round up to the next longer row, never down. Annex B guidance says it plainly: that single row now sizes every section of the system.

Step four: read the row. At seventy feet, half-inch pipe carries sixty. Three-quarter, one twenty-six. One-inch, two thirty-seven. Now size each section for the load it feeds. The trunk carries two hundred — two thirty-seven passes — one-inch pipe. The furnace branch carries one hundred — three-quarter at one twenty-six. The water heater branch, forty — half-inch at sixty. The range branch, sixty — and half-inch reads exactly sixty. Legal, with zero margin. Remember that margin.

Step five: the fittings check, where zero margin dies. Annex B says segments with four or more fittings deserve an equivalent-length allowance from Table B.3.2 — elbows and tees are pipe you did not tape. Add a few feet and the design slides to the eighty-foot row, where half-inch carries fifty-six. Now the range's sixty fails, and the branch becomes three-quarter. Four elbows just bought a pipe size. Count them before the inspector does.

The field calculator runs steps two through five for you: enter the section load and the longest run, and it rounds the length up, picks the smallest passing schedule forty size, and shows you the next size down failing — the same table, 6.2.1(b), half a water column drop, sixty specific gravity. What it cannot do is walk the house: the inventory, the tape, the fittings count, and the section 5.3.4 backstop — every appliance still sees its minimum inlet pressure — stay yours.

The longest length method in one breath. Inventory every nameplate. Convert at the real heating value. Tape to the most remote outlet and round up. One row sizes everything — each section for its downstream load. Count the fittings, re-check the row. And verify the minimum inlet pressure at every appliance. Conservative by design — in the code's own words, maximum operating conditions as the norm. Next: the methods that trade that conservatism for copper and steel savings.