A natural gas standby generator needs a line sized for its full-load gas flow, ideally serving the generator alone. At the unit it needs a regulator sized for that flow, a flexible connector, a shutoff, a sediment trap and a test port. The meter must carry the generator's load too, so tell Southwest Gas early.
How big a gas line does a generator need?
Big enough to hold the pressure at full load. The generator's nameplate and data sheet list its maximum gas flow. Generac's gas design guide gives one example. Its SG500, set up for 7 to 11 inches of water column, "requires 6,000 CFH of gas at full load." At roughly 1,000 Btu per cubic foot, that's about 6 million Btu per hour.
The guide sets the performance targets the pipe has to meet:
- Stable pressure. The pressure at the generator's test port "should typically be less than 2" water column (w.c.) from no-load running to full-load running."
- Never below minimum. "The gas pressure must remain above the minimum specified for the generator set at all times." Falling short leads to "extended crank cycles, inability to carry full load, and unstable engine speed."
- Low pipe loss. It tells designers to size the pipe from the service regulator to the generator for less than 0.5 inch water column pressure drop, counting every elbow.
Section 402.2 of the 2024 International Fuel Gas Code, which Phoenix enforces, sizes gas pipe on the sum of the maximum input of everything it serves. Generac's sizing table shows the allowed pipe length shrinking fast as generators get bigger. It ends with a reminder: "Please verify adequate service and meter sizing."
### Should the generator have its own gas line?
Generac recommends it. Its guide says the generator "should have a dedicated fuel supply, which is not shared with any other appliances." It also says other facility loads "must be factored in" when sizing, and that "The emergency system must be before the facility shutoff," so closing the building's gas doesn't also shut down the emergency generator.
At least one Valley city puts a version of this in writing. Glendale has a technical guideline on gas piping to generators. It says that when a generator goes in, "there shall be a separate fuel gas pipe installed exclusively for the purpose of operating the generator." That line gets its own listed valve at the meter, "LOCKED OPEN" and labeled "GENERATOR NATURAL GAS (or LPG) SUPPLY." A second valve at the meter is labeled "BUILDING NATURAL GAS (or LPG) SUPPLY." Other cities may not require it, so check with your building department. Even where it's optional, a dedicated line makes the generator's supply easier to size and protect.
What goes at the generator end of the line?
A short list of parts, each with a job. Generac's typical layout shows a primary regulator, a fuel shutoff, a sediment trap, a fuel pressure test port and a flex hose to the base rail.
| Part | What it does | Rule behind it |
|---|---|---|
| Flexible fuel line | Isolates the pipe from engine vibration | Section 616.2: engine equipment "shall not be rigidly connected to the gas supply piping" |
| Primary regulator | Drops supply pressure to the generator's range | Generac: sized for about 1.5 times the generator's maximum flow |
| Shutoff valve | Isolates the generator for service | Generac: full-port ball valves the same size as the pipe |
| Sediment trap | Catches debris before the fuel inlet | Shown in Generac's typical layout |
| Test port | Lets a tech read pressure at full load | Generac: pressure must never drop below the nameplate minimum |
A few details from the Generac guide trip up installs often. The flexible line has to be "as straight as possible" and "is not intended to correct misaligned pipe sections or to serve as an elbow." The guide recommends about 10 feet of pipe between the regulator and the generator inlet, and straight pipe on both sides of the regulator, about 10 pipe diameters. And each generator in a multi-unit setup gets its own regulator; the guide says not to share one large regulator across several sets.
### Why does the regulator need so much extra capacity?
Because a generator's gas demand changes fast, and the regulator has to keep up without letting pressure sag. Generac says to pick a direct-acting regulator that delivers "approximately 1.5 times the maximum gas flow required by the generator." In its SG500 example, that means a regulator rated for about 9,000 cubic feet per hour. It also says to plan for seasonal swings in utility pressure, which can call for a larger regulator than the nominal numbers suggest. Section 410.1 of the fuel gas code requires a listed line pressure regulator wherever an appliance runs at lower pressure than the supply.
Does the gas meter need to change?
It may. The meter and service belong to Southwest Gas. Its tariff says "It is the customer's responsibility to notify the Utility of changes in their load requirements." A generator can be the biggest single gas load on the property, so give Southwest Gas its full-load flow early. Southwest Gas's commercial services page says operations running generation should contact its Key Account Management team.
For long runs, elevated pressure can help. Generac notes the supply side "may be raised as high as code allows and regulate it down to generator operating pressure at the generator," which can mean smaller pipe. Southwest Gas has to approve that pressure in writing first, and it grants it only for systems that would need 4 inch or larger pipe at standard pressure, or for equipment that needs more than standard pressure. Our page on 2 psi gas systems explains how it works, and our guide to a bigger Southwest Gas meter covers the utility side.
What does the install involve?
The gas piping is one piece of a bigger job:
- 1Load and pipe sizing from the generator's full-load flow, the run length and the other loads on the meter.
- 2A permit. New gas piping needs a permit, inspection and gas clearance in Phoenix. Our guide to who can install gas lines in Arizona covers the licensing side.
- 3Piping, regulator and connections set up the way the generator maker's instructions call for. Section 616.1 requires engine equipment to follow the manufacturer's instructions and NFPA 37, and stationary generator assemblies to meet UL 2200.
- 4A pressure test before the pipe is covered. Our guide to how a gas line pressure test works walks through it.
- 5A full-load check at the test port, so you know the pressure holds when it matters.
The electrical side (transfer switch, wiring, generator setup) belongs to the electrician and the generator dealer.
HQ Plumbing & Air sizes and installs commercial gas lines for standby generators across the Phoenix area, including dedicated supply lines, regulators, permits and pressure tests. We work alongside your electrician and generator dealer. You get a free estimate and the price up front.
A generator is only as reliable as the gas line feeding it at full load.