9.3 Approved Gas Piping Materials, CSST & Bonding

9.3 Approved Gas Piping Materials, CSST & Bonding

Quick Answer: A gas piping system containing any segment of non-arc-resistant (yellow) CSST must be bonded to the electrical service grounding electrode system with a jumper not smaller than 6 AWG copper and not longer than 75 feet, clamped to a rigid pipe, pipe fitting or CSST fitting — never to the corrugated tubing itself (IFGC 2018 Sections 310.2 through 310.2.5).

Chapter 4 of the IFGC 2018 answers two questions on almost every materials item: is this material approved at all, and is it approved in this location. Master the second half — aluminum indoors but never outdoors, plastic outdoors but never indoors, copper only in low-sulfur gas.

Approved Metallic Pipe (IFGC 403.4)

• Cast-iron pipe is prohibited outright (403.4.1). No exceptions, no locations.
• Steel, stainless steel and wrought-iron pipe must be not lighter than Schedule 10, meet ASME B36.10M dimensional standards, and comply with ASTM A53/A53M, ASTM A106 or ASTM A312 (403.4.2). Black iron in Schedule 40 is the workhorse. Note the joint consequence in 403.10.1: Schedule 40 and heavier pipe may be threaded, flanged, brazed, welded or press-connected, but pipe lighter than Schedule 40 may only be press-connected, flanged, brazed or welded — you may not cut threads into it.
• Copper and copper alloy pipe shall not be used if the gas contains more than an average of 0.3 grains of hydrogen sulfide per 100 standard cubic feet (scf) of gas, equal to 0.7 mg per 100 liters (403.4.3). Hydrogen sulfide (H₂S) attacks copper and forms flakes of copper sulfide that plug orifices and jam gas valves. Threaded copper, copper-alloy and aluminum-alloy pipe may never be used with gases corrosive to those materials.
• Aluminum-alloy pipe must comply with ASTM B241 (alloy 5456 is prohibited), must be marked at each end of each length, must be coated where it contacts masonry, plaster or insulation, and may not be used in exterior locations or underground (403.4.4).

Approved Metallic Tubing (IFGC 403.5)

Tubing may never be used with gases corrosive to the tubing material. Steel tubing follows ASTM A254 and stainless tubing ASTM A268 or A269. Copper tubing must be Type K or Type L of ASTM B88 or ASTM B280 and carries the same 0.3-grains-per-100-scf hydrogen sulfide restriction (403.5.3). Aluminum-alloy tubing (403.5.4) is barred from exterior and underground locations just like the pipe. Corrugated stainless steel tubing (CSST) must be listed in accordance with ANSI LC 1/CSA 6.26 (403.5.5).

Plastic Pipe: Outdoors, Underground, Nowhere Else

Polyethylene (PE) pipe, tubing and fittings must conform to ASTM D2513 and be marked "Gas" and "ASTM D2513"; polyamide must conform to ASTM F2945 (403.6). PVC and CPVC are flatly prohibited for supplying fuel gas. The only PVC the code recognizes is ANSI/UL 651 pipe carrying a regulator vent to a remote termination, and even that shall not be installed indoors (403.6.3).

Section 404.17.1 controls installation: plastic pipe shall be installed outdoors underground only, shall not be used within or under any building or slab, and shall not operate above 100 psig for natural gas or 30 psig for LP-gas. Three exceptions matter:

1. It may terminate above ground outside a building in a premanufactured anodeless riser or service-head-adapter riser — a factory-made casing that carries the PE up out of the trench with no sacrificial anode required.
2. It may terminate with a wall head adapter inside a building where the PE is inserted into an approved indoor piping material.
3. It may run under an outdoor patio, walkway or driveway slab at the burial depth of 404.12.

Underground PE also needs a tracer: 404.17.3 requires a yellow insulated copper tracer wire not smaller than 18 AWG, insulated for direct burial, run adjacent to the nonmetallic pipe, with access provided or the wire terminating above ground at each end. Minimum burial is 12 inches (404.12), reduced to 8 inches for individual lines to outdoor lights, grills and similar appliances where approved and not subject to damage. Metallic-to-plastic connections made outdoors and underground require transition fittings conforming to ASTM D2513 Category I or ASTM F1973 (404.17.2). And 404.11.1 is a favorite one-liner: galvanizing is not adequate corrosion protection for underground gas piping.

Approved Materials and Where They May Be Used

CSST: Listing, Sizing and the Bonding Rule

Corrugated stainless steel tubing (CSST) is thin-wall, flexible, jacketed tubing pulled in long continuous runs from a manifold to each appliance, eliminating most concealed joints. Section 404.2 governs: CSST shall be installed in accordance with the terms of its approval, the conditions of its listing, the manufacturer's instructions and the code.

That is why you never size CSST from the Schedule 40 steel tables. Section 402.3 permits sizing from the tables published in a listed piping system's manufacturer's installation instructions, and CSST tables are indexed by EHD (equivalent hydraulic diameter) flow designation rather than nominal pipe size. Because of the corrugations, a CSST run carries substantially less gas than steel pipe of similar nominal size over the same length.

Bonding exists because lightning striking on or near a building energizes every metallic system inside it — water piping, structural steel, raceways, coax. If the gas piping sits at a different potential, the difference can be resolved by an arc, and yellow CSST has a wall on the order of a hundredth of an inch that a single arc can perforate, releasing gas exactly where an ignition source already exists.

IFGC 2018 Section 310, Line by Line

• 310.1 — Pipe and tubing other than CSST. Each above-ground portion likely to become energized must be electrically continuous and bonded to an effective ground-fault current path. Steel pipe counts as bonded where it connects to an appliance served by a circuit containing an equipment grounding conductor.
• 310.2 — CSST. Applies to CSST not listed with an arc-resistant jacket to ANSI LC 1/CSA 6.26. Such systems, and any system containing one or more segments of that CSST, must be bonded to the electrical service grounding electrode system or, where provided, the lightning protection grounding electrode system.
• 310.2.1 — Point of connection. The jumper connects to a metallic pipe, pipe fitting or CSST fitting — rigid pipe, a malleable-iron fitting, a prefabricated manifold or a brass CSST fitting, downstream of the point of delivery. The corrugated tubing is never the attachment point, and the clamp must land on clean, bare, unpainted metal.
• 310.2.2 — Size and material of jumper. Not smaller than 6 AWG copper wire or equivalent (aluminum permitted at equivalent size), solid or stranded, indoors or outdoors.
• 310.2.3 — Bonding jumper length. Not more than 75 feet between the gas piping and the grounding electrode system. Any additional electrode installed to meet that limit must itself be bonded to the service grounding electrode system.
• 310.2.4 and 310.2.5 — Connections and devices. Connections comply with NFPA 70 (National Electrical Code); the clamp must be listed to UL 467 and listed for its location, including weather exposure.
• 310.3 — Arc-resistant CSST. Black-jacketed CSST listed as arc-resistant to ANSI LC 1/CSA 6.26 and marked "AR" needs no extra jumper. Its jacket absorbs or conducts the arc energy, so it is treated like steel pipe and is considered bonded through an appliance equipment grounding conductor.

Only one bonding connection is required no matter how large or branched the system, because the piping is electrically continuous. Two retrofit rules follow: adding any length of yellow CSST to an existing steel or copper system triggers bonding retroactively, and mixing yellow CSST into an arc-resistant system pulls the whole system back under 310.2. A grounding electrode established just for the gas piping is not permitted.

Joints, Threads and Thread Compound

• Threads must be taper pipe threads complying with ASME B1.20.1 (403.9). A running thread — a nipple threaded straight along its whole length, often paired with a locknut and gasket — cannot develop the wedging seal a tapered NPT thread depends on, so it is not an acceptable fuel gas joint. Close nipples and stacked (nested) bushings fail for the same reason: no reliable seal, no mechanical strength.
• Damaged threads are not repairable (403.9.1). Stripped, chipped or corroded threads mean the pipe is scrapped, and if a weld seam opens during cutting or threading, that length is scrapped too. Section 403.7 says the same about pipe generally: defects shall not be repaired, only replaced.
• Thread engagement follows Table 403.9.2: field-threaded ½-inch and ¾-inch pipe gets about ¾ inch of thread and 10 threads cut; 1¼-, 1½- and 2-inch about 1 inch and 11 threads; 2½- and 3-inch 1½ inches and 12 threads; 4-inch 1⅝ inches and 13 threads.
• Thread joint compound (pipe dope) must be resistant to the action of liquefied petroleum gas and to any other chemical constituent of the gas conducted (403.9.3). Ordinary water-service dope softens in propane service; use a gas-rated compound or gas-rated PTFE tape.
• Copper tubing joints must use approved gas tubing fittings, be brazed with filler melting above 1,000°F, or use press-connect fittings listed to ANSI LC-4/CSA 6.32, and brazing alloys may not exceed 0.05 percent phosphorus (403.10.2). Soft solder is never acceptable on fuel gas.
• Flared joints are permitted only in systems of nonferrous pipe and tubing, and only where experience or testing shows the joint suits the conditions and the design prevents separation (403.10.4).
• Metallic fittings (403.10.5): threaded fittings larger than 4 inches are not permitted; fittings on steel, stainless or wrought iron must be steel, stainless, copper alloy, malleable iron or cast iron; fittings on copper must be copper or copper alloy; and zinc-aluminum alloy fittings may not be used in systems containing flammable gas-air mixtures.

Marking and Identification

Every length of pipe or tubing and every fitting must bear the manufacturer's identification (401.9), with narrow exceptions for steel pipe pieces 2 feet or less cut from longer lengths, steel fittings 2 inches and smaller, and identification supplied on packaging or in approved documentation. For piping other than steel, exposed gas piping must carry a yellow label marked "Gas" in black letters at intervals not exceeding 5 feet (401.5) — not required on pipe in the same room as the appliance it serves.

Exam Trap: The most-missed CSST item puts the bonding clamp on the corrugated tubing, or sizes the jumper at 8 AWG because that is a familiar equipment grounding conductor size for an appliance circuit. Both are wrong: the clamp lands on rigid pipe, a pipe fitting or a CSST fitting, and the jumper must be 6 AWG copper or larger. A second trap restates the copper limit as "0.3 grains per standard cubic foot." The code says per 100 standard cubic feet — a hundredfold difference that flips a prohibition into a permission.