A 2 psi system gets gas from Southwest Gas at about 2 pounds per square inch instead of the standard quarter pound, and regulators drop it back down near each appliance. Smaller pipe then carries the same gas. Southwest Gas approves it in writing only for systems that would need 4 inch or larger pipe, or equipment that needs it.
What pressure does Southwest Gas normally deliver?
Southwest Gas's Arizona tariff sets standard delivery pressure at "approximately 0.25 pounds per square inch gauge (7-inch water column) subject to variation under load conditions," measured at the meter outlet. Elevated pressure is an exception the utility grants. Under the same rule, when a customer wants more than standard pressure, Southwest Gas "may supply at its option such greater pressure" as long as it doesn't hurt service to other customers, and it "reserves the right to lower said delivery pressure or discontinue the delivery of gas at higher pressure at any time upon reasonable notice."
That last part matters for design. A building piped to depend on 2 psi is betting on the utility continuing to supply it.
Why does 2 psi allow smaller pipe?
Because the pipe is allowed to lose more pressure along the way and still deliver enough. The fuel gas code's sizing tables show the difference clearly. Compare the same Schedule 40 steel pipe over 100 feet:
| Pipe size | Standard pressure, Table 402.4(1) (0.3 in. w.c. drop) | 2 psi, Table 402.4(5) (1 psi drop) |
|---|---|---|
| 1/2 inch | 38 cubic feet per hour | 462 cubic feet per hour |
| 1 inch | 148 cubic feet per hour | 1,710 cubic feet per hour |
| 1-1/4 inch | 304 cubic feet per hour | 3,510 cubic feet per hour |
| 2 inch | 877 cubic feet per hour | 10,100 cubic feet per hour |
At 2 psi, a 1 inch line over 100 feet carries more gas than a 2 inch line at standard pressure. With natural gas at roughly 1,000 Btu per cubic foot, that's the difference between about 148,000 and 1.7 million Btu per hour through the same pipe. The table shows the physics. Whether you can use it depends on Southwest Gas's rules below, and they only allow it on big systems.
What equipment does a 2 psi system need?
Regulators, and the right kind. Section 410.1 of the 2024 International Fuel Gas Code, which Phoenix enforces, requires a line pressure regulator "where the appliance is designed to operate at a lower pressure than the supply pressure," listed to ANSI Z21.80/CSA 6.22, with access for service and protection from damage. The code calls a regulator that steps down from above 0.5 psi to 5 psi a medium-pressure (MP) regulator, and Section 410.2 adds requirements for it:
- It must hold a reduced outlet pressure when nothing is flowing, called lock-up.
- Its capacity has to cover the appliances it serves.
- Indoors, it's vented outdoors or fitted with a vent-limiting device (Section 410.3).
- A capped tee goes upstream, for a gauge and as a sediment trap, and another at least 10 pipe diameters downstream.
Maxitrol's line regulator guidebook describes how its 2 psi regulators work: certified for "2 psi inlet pressure, with an outlet pressure range of 7 to 11 inches w.c.," located "upstream of appliances already fitted with a regulator." It also notes that appliance regulators "should no longer be used as line regulators." Its vent limiters are made for indoor use and "eliminate the need to run vent piping to an outside area."
Above 2 psi, more protection kicks in. Section 416.1 requires overpressure protection devices when the supplier delivers more than 2 psi to appliances rated at 14 inches of water column or less. Maxitrol notes the regulator standard itself requires one at supplies above 2 psi, to hold the downstream side to 2 psi if the line regulator fails.
### Where does the pressure drop down?
Before the gas reaches anything built for low pressure. This trips up restaurant layouts most often. Commercial gas hoses for equipment on casters are limited to low pressure. Dormont's instructions restrict its movable-equipment connectors to piping "not in excess of 1/2 psi." So in a 2 psi kitchen, the line regulator sits upstream of the cooking manifold, and every hose downstream sees normal appliance pressure. Our page on gas hoses for equipment on casters covers those connectors.
There's also a ceiling inside buildings. Section 402.7 caps indoor piping at 5 psig unless joints are welded or brazed, the piping is in a ventilated chase, or the space is something like a boiler room or industrial area. Two psi sits well under that.
How do you get 2 psi from Southwest Gas?
You ask early and get it in writing. Southwest Gas's Central Arizona commercial package says the request has to come in when the project starts, and it "will require written approval by Southwest Gas' Engineering Department prior to installation of your houseline. Failure to receive written approval will eliminate any meter set." It also says any downstream regulation is not the utility's responsibility, so the regulators are part of your plumbing job.
Timing and paperwork to plan for:
- Lead time. The same package says elevated pressure adds six weeks, for a minimum of 12 weeks, to underground work.
- City plans. Phoenix's gas piping plan review checklist says that for natural gas delivered at 2 psig or more, you "obtain written confirmation letter from Southwest Gas," and place it in the drawing package. The same checklist wants the supply pressure and the pressure after each regulator shown on the drawings.
- Billing. The tariff says meter volumes at higher pressure are corrected to that pressure for billing, so the meter reading is adjusted rather than taken at face value.
When will Southwest Gas approve 2 psi?
Only for one of two reasons. Southwest Gas's Request for Elevated Gas Delivery Pressure form, in its Central Arizona commercial package, makes the applicant check one:
- To shrink very large pipe. The request must be "Required to reduce interior pipe size smaller than 4"." The form adds: "if interior pipe size is smaller than 4" at 7" w.c. (1/4 PSI), pressure will not be granted." So 2 psi is for systems that would need 4 inch or larger pipe at standard pressure, such as very large kitchens, big multi-tenant buildings, or heavy loads like large boilers or standby generators on long runs.
- For equipment that needs it. The other reason is equipment "which requires pressure greater than 7" w.c. (1/4 psi)," with a manufacturer spec sheet for approval.
For the pipe-size reason, Southwest Gas wants a drawing of the interior gas piping "approved by an authorizing municipality or stamped by a P.E. (Professional Engineer)." For the equipment reason, it wants the manufacturer spec sheet. The form also says its review "is NOT a certification of adequacy of the design," and the applicant agrees not to install the house line before written approval.
What that means in practice:
- A small shop or a typical restaurant whose piping works out under 4 inch at standard pressure won't get 2 psi to save on pipe size.
- A large building that would need 4 inch or bigger pipe can ask, with a stamped or city-approved schematic.
- An existing building follows the same test. Converting also takes new regulators and a permit.
A licensed plumber or engineer sizes the system at standard pressure first. That size decides whether a request is even possible. Our guide to what's involved in a restaurant gas line install shows where this decision fits in the project.
HQ Plumbing & Air designs and installs commercial gas piping at standard and elevated pressure across the Phoenix area, including regulators, permits and pressure tests. You get a free estimate and the price up front.
Higher pressure in the main, normal pressure at the burner. That's the whole idea.