Pipe sizing by the longest length method. Episode five of the Gas Service Animated series.

Before a flame ever lights, the pipe decides whether every appliance can breathe. If a house line is too small, pressure sags along its length whenever everything fires at once. The appliance closest to the meter may run fine, while the furnace at the far end of the run starves at high fire. Pipe sizing exists so the farthest appliance still gets full pressure on the coldest day of the year.

First, every appliance load becomes a flow. Natural gas delivers roughly one thousand Btu for every cubic foot, so dividing the nameplate rating by one thousand gives cubic feet per hour. It is an approximation, but it is the one the sizing tables are built around. Our example house has a furnace at one hundred forty thousand, a water heater at forty thousand, a dryer at thirty five thousand, and a range at seventy thousand. That totals two hundred eighty five thousand Btu per hour — about two hundred eighty five cubic feet per hour.

Now map the system. From the meter it is twenty feet to tee A, and ten feet up from A to the furnace. Twenty more feet reach tee B, five feet drop to the water heater, fifteen feet reach tee C, five feet drop to the dryer, and ten feet finish at the range. The longest path runs from the meter through A, B, and C to the range: twenty plus twenty plus fifteen plus ten is sixty five feet. The tables have no sixty five foot row, so we round up to the seventy foot row — always up to the longer row, never down.

Here is the heart of the method: that one seventy foot row now sizes every section in the house, even the short ones. On the seventy foot row of the schedule forty table for natural gas, half inch pipe carries sixty cubic feet per hour. Three quarter inch carries one hundred twenty six. One inch carries two hundred thirty seven, and inch and a quarter carries four hundred eighty six. For each section we ask one question: what is the smallest size whose capacity meets or beats the load? The meter run carries two eighty five, so watch it scan the row until it finds inch and a quarter.

Work the sections, always on the seventy foot row. The meter run carries the whole house, two eighty five, so it takes inch and a quarter. The furnace branch carries one forty — three quarter inch only offers one twenty six on this row, so it fails, and the furnace branch takes one inch. Section A to B carries everything past the furnace, one forty five: also one inch. The water heater branch carries forty: half inch. B to C carries one oh five: three quarter. The dryer takes half inch, and the range takes three quarter. Seven sections, one row, one rule.

One preview before we move on. The code also allows the branch length method, which sizes each branch by its own distance back to the meter. The furnace branch sits only thirty feet out — twenty feet to tee A plus ten feet up. On the thirty foot row, three quarter inch pipe carries one hundred ninety nine cubic feet per hour, more than the one forty the furnace needs. So by that method the same branch legitimately drops to three quarter inch. Both answers are code compliant — what matters is that you pick one method, apply it consistently, and document which one you used.

Three traps catch people on exams and in the field. First, rounding down: a sixty five foot run is not a sixty foot job — the row must be as long as or longer than the run. Second, mixing table families: propane tables, different pressure drops, and tubing tables all give different numbers, so the whole job must come from the one table that matches the gas, the pressure, and the material. Third, sizing only for today — the code expects the system to account for appliances planned for the future.

On the job, the sizing sheet gets verified. Confirm every nameplate rating and the total connected load. Measure the actual longest run, with a reasonable allowance for fittings. Check that one method and one table were used throughout, and that the meter and regulator can deliver the total. And every final answer defers to the current code tables, the manufacturer, the authority having jurisdiction, and your company's procedures.

Total the load, find the longest run, round up, and let one row size every section. The printable study sheets in this series walk this exact example on paper, section by section. Next episode: leak investigation and the combustible gas indicator.