Branch Length and the Other Sizing Methods. Episode seventy-nine of the Gas Service Animated series.

Same house, same appliances, same code — and one method says one-inch while another says three-quarter. Both legal. Sizing methods are lenses, not laws of physics: they all deliver the gas, they just spend the pressure budget differently. Today: branch length, hybrid pressure, two p s i systems, and the tubing that refuses to use these tables at all.

The branch length method is section 6.1.3, and it is two moves. Move one: size the longest run exactly as last episode — point of delivery to the most remote outlet, one row, section loads. Move two: for each remaining branch, re-enter the table at that branch's own length — point of delivery to the most remote outlet served through it. Short branch, shorter row, bigger capacities.

Watch it earn its keep. Add a tankless water heater pulling one hundred thirty c f h on a branch thirty feet from the meter. Under longest length, that branch reads the seventy-foot row: three-quarter carries one twenty-six — fails by four — so it becomes one-inch pipe. Under branch length, the branch reads its own thirty-foot row: three-quarter carries one ninety-nine. Passes with room to spare. Same tubing on the truck, one size smaller on the invoice — and the annex says it straight: this method could permit slightly smaller pipe in some segments.

Two honesty checks. The main run comes out identical under either method — the savings live only in the short branches. And every method answers to the same backstop, section 5.3.4: the pressure arriving at each appliance inlet must meet that appliance's minimum. If a lens ever disagrees with the pressure gauge, the gauge wins.

Method three: hybrid pressure, section 6.1.4 — the two p s i system's rulebook. The trunk runs at two p s i to line regulators near the loads; each regulator drops it to inches of water column for the last leg. So you size twice: the high-pressure trunk on the longest length to the most remote regulator, from the two p s i table — then each low-pressure leg on the length from its regulator to its most remote outlet, from the half-inch-drop table. Two pressures, two lookups, one system.

Why bother? Capacity. At one hundred feet, half-inch pipe on the half-a-column table carries fifty c f h. On the two p s i table — one p s i drop — the same half-inch carries four hundred sixty-two. Nine times the gas through the same steel. That is why long runs and big loads go two p s i. The checks: fifty-five point four inches of water column on the manometer upstream of the regulator, and section 5.4.4 capping in-building pressure at five p s i.

Corrugated stainless steel tubing never reads the schedule forty tables. Its tables are entered by E H D — equivalent hydraulic diameter, thirteen through sixty-two — from the listed manufacturer's sizing tables, and they already bake in four ninety-degree bends and two end fittings; extras add equivalent length. And the two p s i tubing table carries a warning in capital letters: it does not include the line regulator's pressure drop — where that drop exceeds three-quarters of a p s i, the table says do not use this table. Read the notes. The notes are load-bearing.

Adding an appliance to an existing system is a sizing method of its own. Section 5.1.2 makes checking capacity at the new total load a duty, before the connection — not after the callbacks. If the pipe comes up short, the code offers exactly three fixes: enlarge the system, run a separate line to the new appliance, or raise the operating pressure within the limits of what is installed. Pick one — hoping is not on the list.

The other methods in one breath. Branch length: longest run first, then every branch at its own distance — smaller pipe, legally. Hybrid pressure: two p s i trunk to the regulators, low-pressure legs after, two lookups. Tubing: the manufacturer's tables by E H D, warnings included. Existing systems: re-size before you connect. And under all of it, the same floor — minimum inlet pressure at every appliance. Next: the air the fire breathes.