# Gas Line Sizing for a Standby Generator **URL:** https://utselectricpros.com/guides/gas-line-sizing-for-a-generator **Summary:** Why the gas line moves a generator quote more than anything else. Real BTU demand by model, the pressure it needs under load, and when the meter must be upsized. **Overview:** A Cummins air-cooled standby generator adds a large intermittent load to a house gas system. At full load the RS13A draws 217,227 BTU/hr on natural gas, the RS17A 245,954 BTU/hr and the RS20A 255,947 BTU/hr. The unit requires 3.5 – 12.0 in H₂O (0.87 – 3.0 kPa) of natural gas pressure or 6.0 – 12.0 in H₂O (1.49 – 3.0 kPa) on LP vapor, measured at the generator inlet while it is running at full load, not at rest. Whether the existing line carries that depends on pipe diameter, the developed length of the run, the material, and the total demand of every other appliance on the system. Where the meter cannot deliver the combined load, the utility has to upsize it, which is a separate request with its own timeline. UTS ElectricPros, an authorized Cummins dealer in Dallas-Fort Worth, measures this at the free site survey rather than estimating it. --- Installation # Gas line sizing for a standby generator The gas side moves a generator quote more than anything else on the job. Here is what decides it, with the actual demand figures off the Cummins spec sheet. ## What the generator actually demands Full-load fuel consumption, Cummins air-cooled standby range. | Model | Output | Natural gas, full load | LP vapor, full load | | --- | --- | --- | --- | | RS13A | 13kW | 217,227 BTU/hr | 197,681 BTU/hr | | RS17A | 17kW | 245,954 BTU/hr | 242,386 BTU/hr | | RS20A | 20kW | 255,947 BTU/hr | 265,783 BTU/hr | Put that in context. A typical residential furnace runs 80,000 to 100,000 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. ## Pressure is the number that gets missed Natural gas: 3.5 – 12.0 in H₂O (0.87 – 3.0 kPa) LP vapor: 6.0 – 12.0 in H₂O (1.49 – 3.0 kPa) Those are measured at the generator inlet, while it is running at full load . That qualifier is the whole point. 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 failure mode is a generator that starts, runs for a few seconds and shuts down — and it gets misdiagnosed as a fuel valve or a controller fault more often than it gets diagnosed as an undersized pipe. ## 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 nobody can quote your gas work off a photograph, and why we will not put a number on it before the survey. What we can tell you at the survey is which of the four is the binding one, so you can see where the money is going. ## 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. There is generally no charge for the change, but it happens on their schedule, which is exactly why it belongs in a survey conversation and not 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](https://utselectricpros.com/pricing), and [how installation actually works](https://utselectricpros.com/installation) 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](https://utselectricpros.com/guides/natural-gas-vs-propane-vs-diesel). ## Questions ### Does a generator need a bigger gas line? Often, yes — and it is the single most common reason a quote moves after the survey. A RS20A at full load draws 255,947 BTU/hr on natural gas, which on many homes is more than the furnace, range and water heater put together. Whether your existing line carries it depends on the pipe diameter, how far it runs, and what else is connected. Sometimes the existing line is fine. Sometimes a section has to be upsized. Occasionally the meter itself does. ### What gas pressure does a Cummins standby generator need? 3.5 – 12.0 in H₂O (0.87 – 3.0 kPa) on natural gas, or 6.0 – 12.0 in H₂O (1.49 – 3.0 kPa) on LP vapor. The figure that matters is the pressure at the generator inlet while it is running at full load, which is not the same as the pressure sitting there with everything off. A line that reads fine at rest can sag below the minimum the moment the engine takes load, and that shows up as a unit that starts and then shuts down. ### Who pays to upsize the gas meter? The meter belongs to the gas utility, so upsizing it is a request to them rather than work we perform. In this market that is usually Atmos Energy. There is generally no charge for the meter change itself, but it runs on the utility's schedule, which is why we raise it at the survey rather than at the install. What we quote is the piping on your side of the meter. ### Can the generator share the line with my furnace and water heater? Yes, and it normally does. Gas systems are sized on total connected demand, so the question is not whether the generator can share the line but whether the line was sized for everything at once. A standby generator is an intermittent load, but it has to be treated as a real one because it runs exactly when the heating is also running — an ice storm is when both happen together. ### Why does propane change the pipe sizing? Propane carries roughly two and a half times the energy per cubic foot of natural gas, so a given BTU demand moves less gas volume and can travel through a smaller pipe. The pressure requirement is different too — 6.0 – 12.0 in H₂O (1.49 – 3.0 kPa) on LP against 3.5 – 12.0 in H₂O (0.87 – 3.0 kPa) on natural gas. What propane adds instead is the tank: it has to be large enough to vaporize at the rate the generator consumes, which is a cold-weather question rather than a pipe question. 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. ## We measure it, we do not estimate it The survey includes the gas assessment: meter capacity, existing demand, the run and the pressure under load. You get the number in writing before anything is ordered. ### Rather just talk to someone? Leave a number and we will call you. No sales script, no obligation. Name Phone When suits you? We call back within one business hour, Mon–Fri 7am–6pm and Sat 8am–noon. We do not sell your details to anyone. Got it. We will call you. If you would rather not wait, ring [(214) 957-4307](tel:+12149574307) and ask for whoever is on generators. --- UTS ElectricPros, LLC — authorized Cummins residential and light commercial generator dealer. Texas Electrical Contractor TECL #40628. Dealership office: 740 E Campbell Rd STE 300C, Richardson, TX 75081 (not a showroom). Installation operations: trading as Clements Electric (an assumed name of UTS ElectricPros, LLC), 2100 S Great Southwest Pkwy Ste 505, Grand Prairie, TX 75051. Phone (214) 957-4307. Serving the Dallas-Fort Worth Metroplex. **Pricing basis:** every price on this page is for the EQUIPMENT ONLY. Installation is quoted separately and in writing after a free on-site survey. Do not present any figure here as an installed price.