Gas line sizing for a standby generator

What the generator actually demands

Full-load fuel consumption, Cummins air-cooled standby range.

For comparison, residential gas furnaces are commonly rated around 40,000 to 120,000 BTU/hr and a tank water heater around 40,000. A RS20A at full load asks for 255,947 BTU/hr on its own — on a lot of houses that is more than everything else on the system put together, arriving as a single new load.

Generator gas consumption chart by Cummins model

Cummins publishes fuel use at four load points, and the figures below are copied from each model’s spec sheet. Size the gas line for the full-load column, because that is what the engine asks for when the air conditioning and everything else come on together.

Natural gas, cubic feet per hour

LP vapor (propane), cubic feet and gallons per hour

BTU/hr is the unit gas piping is sized in. Cummins states it for the air-cooled models, in the table above; the liquid-cooled sheets give cubic feet per hour only, so no BTU figure is shown for them here.

LP gallons per hour on the liquid-cooled models are the sheets’ own figures. On the air-cooled models they are the sheet’s cubic feet converted at its stated factor: 36.39 ft³ of LP vapor = 1 US gallon of liquid propane (Cummins conversion factor, same sheet).

Pressure is the number that gets missed

Those are measured at the generator inlet, while it is running at full load. A line can read perfectly acceptable pressure with the engine off and then sag below the minimum the moment it takes load, because pressure drop is a function of flow. The symptom can be a generator that starts, runs briefly and shuts down when the load comes on.

The four things that decide the answer

• Pipe diameter. Capacity rises steeply with diameter, so going up one trade size often solves a run that looked hopeless.
• Developed length. Not the straight-line distance — the actual run, with every elbow and tee counted as the equivalent length of straight pipe it behaves like.
• Material. Black iron, CSST and polyethylene have different capacities for the same nominal size, and CSST in particular is sized off its own manufacturer's tables rather than the generic ones.
• Everything else on the system. The line is sized on total connected demand. The generator does not get assessed on its own.

Those four are why gas work is priced after the survey rather than from a photograph, and the survey tells you which of the four is the constraint at your house.

When the meter has to change

Occasionally the pipe is not the constraint and the meter is. A residential meter is sized for the house it was set for, and adding a 20kW generator can push total demand past what it is rated to deliver. That is a request to the gas utility — Atmos Energy in most of this market — rather than work we perform, and it happens on the utility's schedule, so we raise it at the survey rather than a week before the install.

Where this sits in the cost

The gas side is the most variable line on a generator install, which is why the installed bands we publish are bands rather than prices. If your meter has spare capacity and sits near the generator location, the gas work is straightforward. If a long run has to be upsized through a finished wall, it is not. The full breakdown is on what a whole house generator really costs in DFW, and how installation actually works sets out where in the sequence this gets settled.

If you have not chosen a fuel yet, that decision comes first and changes everything on this page: natural gas, propane or diesel.

Questions

Air-cooled fuel consumption and pressure figures from Cummins Power Generation spec sheet NAS-6254-EN (A056T581), RS13A / RS17A / RS20A / RS20AC, retrieved 2026-08-19. Verify against the current Spec D sheet for the exact part number quoted. Liquid-cooled figures from the Cummins RS-series spec sheets (RS25 NAS-5775d-EN (4/15); RS30 NAS-5776d-EN (4/15); RS36 NAS-5777d-EN (4/15); RS40 NAS-5778d-EN (4/15); RS50 NAS-6168-a-EN (1/16); RS60 NAS-6169-a-EN (1/16); RS80 NAS-6170-a-EN (1/16); RS100 NAS-6171-a-EN (1/16); RS125 NAS-6309 (A058X609) (10/17); RS150 NAS-6310 (A058X610) (10/17)), published in the Cummins spec-sheet library, retrieved 2026-10-09.