Part II — Gas Piping

Learning Objectives

Upon completing this chapter, you will be able to:

4.Apply the Massachusetts Uniform State Plumbing Code (248 CMR 10.00) and NFPA 54/58 requirements to design, size, and inspect gas piping systems for residential and commercial occupancies.

5.Determine the correct pipe sizing methodology using the longest-run method and the branch-length method, including the application of tables for natural gas and LP-Gas.

6.Identify required pressure tests, purging procedures, and the master plumber’s responsibility for system integrity and documentation.

7.Recognize code-compliant materials, fittings, and installation methods, including CSST (corrugated stainless steel tubing) requirements.

8.Calculate gas demand for multiple appliances and apply diversity factors where permitted.

9.Distinguish between the roles of the master plumber, the gas fitter, and the utility company regarding service piping, meter sets, and pressure regulation.

1.1 Scope and Jurisdiction

Gas piping work in Massachusetts falls under the joint authority of the Board of State Examiners of Plumbers and Gas Fitters and the local inspector. The governing code is 248 CMR 10.00 (Uniform State Plumbing Code), which adopts by reference NFPA 54 (National Fuel Gas Code) and NFPA 58 (Liquefied Petroleum Gas Code). The master plumber is responsible for all gas piping from the point of delivery (meter outlet or LP-Gas container first-stage regulator) to the appliance connection.

Point of delivery is defined as the outlet of the meter for natural gas, or the outlet of the second-stage regulator for LP-Gas (or the first-stage regulator if only one stage is used). The master plumber does not install service lines upstream of the meter; that is the utility’s or licensed gas fitter’s scope. However, the master plumber must coordinate with the utility regarding pressure, meter location, and shutoff valve access.

Exam Trap: Questions often blur the line between service piping and building piping. Know that the code’s jurisdiction for the plumber begins at the meter outlet (natural gas) or the regulator outlet (LP-Gas). The meter itself is never installed by the plumber.

1.2 Materials and Approvals

Only materials listed for gas service and approved by the Board may be used. The 2023 code recognizes:

Black steel pipe (ASTM A53 or A106) — the default standard for rigid systems.

Galvanized steel pipe — permitted only above ground and only where not in contact with corrosive agents; prohibited for LP-Gas due to sulfur-based odorant degradation of the zinc coating.

Copper tubing — permitted for natural gas only, not for LP-Gas (due to the same sulfur issue). Must be Type K or L, with brazed or flared joints. Solder is prohibited.

CSST (Corrugated Stainless Steel Tubing) — must be installed per the manufacturer’s instructions and the specific listing. Requires bonding to the electrical grounding electrode system per NFPA 54 and the NEC. The bonding clamp must be attached to the CSST fitting or the rigid pipe connection, not to the tubing itself.

Polyethylene (PE) plastic pipe — permitted for underground exterior use only, never inside a building. Must be installed with a continuous tracer wire and proper burial depth (typically 12–18 inches, per manufacturer and code).

Field Point: The master plumber must verify that all materials bear the appropriate listing mark (e.g., ASTM, ANSI, or third-party certification). Unlisted fittings, especially imported malleable iron fittings without a listed rating, are a common inspection failure.

Exam Trap: A question may present a scenario with galvanized pipe on an LP-Gas system. The correct answer is rejection — galvanized is not permitted for LP-Gas service.

1.3 Pipe Sizing Fundamentals

Sizing is based on allowable pressure drop, not on pipe diameter alone. For natural gas systems with a standard 7-inch water column (WC) supply pressure, the code permits a maximum pressure drop of 0.5 inch WC from the meter to the appliance. For systems with higher supply pressure (2 psi or more), a line regulator is required at each appliance or at the point of entry, and the allowable drop is calculated differently.

The two accepted methods are:

30.Longest-Run Method (Standard): Determine the total gas demand (in cubic feet per hour, CFH) for the entire system. Identify the longest run from the meter to the farthest appliance. Use the appropriate sizing table (based on specific gravity 0.60 for natural gas) to select a pipe size that delivers the total demand over that longest length. All other branches are then sized individually based on their own demand and their own run length.

31.Branch-Length Method: More precise, used for large systems. Each section of pipe is sized based on the demand it serves and the length from the meter to the farthest appliance served by that section. This often results in smaller pipe sizes and is permitted by the code but requires careful tabulation.

Demand Calculation: Each appliance has a rated input in BTU/hour. Convert to CFH by dividing by the heating value of the gas (approximately 1,000 BTU/ft³ for natural gas; for LP-Gas, use 2,500 BTU/ft³ and size from the LP-Gas tables, which are based on a different specific gravity and pressure).

Example: A furnace rated at 120,000 BTU/hr requires 120 CFH of natural gas. If the longest run is 60 feet, and the total system demand is 250 CFH, the main line must be sized for 250 CFH over 60 feet.

Exam Trap: Do not size the main line for the sum of all appliances if a diversity factor applies. For dwelling units, the code allows a 75% demand factor for four or more appliances (excluding space heating). However, for commercial systems, diversity is rarely permitted unless specifically justified. The exam will test whether you apply the factor correctly.

1.4 Sizing Tables and Pressure Drops

The code provides sizing tables in NFPA 54 (Tables 6.2.1 through 6.2.6) and in 248 CMR 10.00. These tables are based on:

Specific gravity of the gas (0.60 for natural gas, 1.50 for LP-Gas vapor).

Pressure drop (0.5 inch WC for low-pressure systems; 1.0 inch WC or more for higher-pressure systems).

Pipe inside diameter (schedule 40 steel).

Key Table Columns: Each table lists pipe size (nominal diameter) and length of run (from 10 to 500 feet). The intersection gives the maximum CFH capacity.

Pressure Drop Rule of Thumb: For a low-pressure system (7-inch WC), the total drop from meter to appliance must not exceed 0.5 inch WC. This includes losses through fittings, valves, and the meter itself. In practice, the tables already account for standard fitting losses, so no additional calculation is needed for standard installations.

Higher-Pressure Systems (2 psi and above): These are common in large commercial buildings. The piping is sized for a 10% pressure drop (e.g., from 2 psi to 1.8 psi) and then a line regulator reduces pressure to 7-inch WC at each appliance or at a manifold. The master plumber must ensure that the regulator vent is piped to the outside and that the downstream piping is sized for the reduced pressure.

Exam Trap: The exam may provide a scenario where the supply pressure is 2 psi but the piping is sized using the 0.5-inch WC table. This is a critical error — always match the table to the actual supply pressure and allowable drop.

1.5 Installation Requirements

1.5.1 Piping Support and Protection

Hangers and Supports: Must be of a material that will not corrode the pipe. Steel pipe must be supported at intervals not exceeding every 10 feet for 1-inch and smaller, and every 12 feet for larger sizes. CSST must be supported per the manufacturer’s instructions, typically every 4 to 6 feet, and must not be kinked or bent tighter than the minimum bend radius.

Sleeves: Where piping passes through masonry or concrete, a protective sleeve is required. The annular space must be sealed to prevent moisture intrusion.

Corrosion Protection: Underground metallic piping must be protected with a listed coating or wrapping. Above-ground piping in contact with dissimilar metals must be separated by a dielectric fitting.

1.5.2 Prohibited Locations

Gas piping may not be installed:

In or through a duct (HVAC supply or return).

In a chimney or flue.

In an elevator shaft or hoistway.

In a solid wall or partition (unless the pipe is continuous and has no fittings).

In a location where it is subject to mechanical damage without protection.

1.5.3 Shutoff Valves

A gas shutoff valve is required at each appliance, located in the same room and within 6 feet of the appliance, upstream of the appliance connector. The valve must be accessible and of a listed type. A union must be provided downstream of the valve to allow appliance removal.

Field Point: The master plumber must ensure that the appliance connector (the flexible line from the valve to the appliance) does not exceed 3 feet in length for residential appliances, unless the manufacturer’s instructions permit a longer connector. Connectors must not pass through walls, floors, or ceilings.

1.5.4 CSST Bonding

CSST must be bonded to the electrical grounding electrode system with a minimum 6 AWG copper conductor. The bond must be attached to the CSST fitting or the rigid pipe connection at the point where the CSST terminates. This is a frequent source of inspection failures and a common exam question.

Exam Trap: A question may state that CSST is bonded at the meter. This is incorrect — the bond must be at the CSST system, not at the meter or the service entrance.

1.6 Appliance Connections and Venting

Each gas appliance must have:

A sediment trap (drip leg) installed upstream of the appliance control, unless the appliance is designed to prevent sediment entry.

A shutoff valve as described above.

A connector that is listed for the application and does not exceed the maximum length.

Venting: The master plumber is responsible for verifying that the appliance venting system is sized correctly. For natural draft appliances, the vent connector must rise at least ¼ inch per foot toward the chimney or vent. The total vent height and diameter must comply with the appliance manufacturer’s instructions and the code. Type B vent is required for most gas appliances; single-wall metal pipe may be used for connectors but must maintain clearances to combustibles (typically 6 inches).

Field Point: When installing a high-efficiency condensing appliance, the vent is typically PVC or CPVC and must be sloped back to the appliance to allow condensate drainage. The master plumber must ensure no trap in the vent line and that the termination is at least 3 feet from any building opening.

1.7 Pressure Testing and Purging

1.7.1 Test Requirements

All gas piping must be pressure-tested before it is concealed or covered. The test must be performed by the master plumber or under their direct supervision. The code requires:

Low-pressure test (7-inch WC or less): Test at 10 psi (or 1.5 times the maximum operating pressure, whichever is greater) for a minimum of 30 minutes. The test gauge must be a mercury manometer or a digital gauge with 0.1 psi resolution.

Higher-pressure test (above 7-inch WC): Test at 1.5 times the maximum operating pressure, but not less than 10 psi, for 30 minutes.

Important: The test must be conducted with air or inert gas, never with gas. The system must be isolated from the meter and appliance regulators during the test. All open ends must be capped.

Exam Trap: A question may state that the test is performed at the operating pressure. This is incorrect — the test pressure is always higher than the operating pressure.

1.7.2 Purging

After the test is passed and the system is connected to the gas supply, the piping must be purged of air before lighting appliances. The purge must be performed by discharging gas to the outdoors, not into the building. The master plumber must ensure that no ignition sources are present during the purge.

Field Point: For large commercial systems, purging may require a purge valve installed at the end of the line. The master plumber must calculate the volume of the piping to estimate purge time and must use a combustible gas indicator to verify that the purge is complete.

1.8 LP-Gas Systems (NFPA 58)

LP-Gas (propane) systems have additional requirements beyond natural gas:

Container placement: Containers must be located at least 10 feet from any building opening, ignition source, or property line (for containers up to 500 gallons). Larger containers require greater distances.

Regulation: A two-stage regulation system is standard. The first-stage regulator reduces container pressure (100–200 psi) to 10 psi; the second-stage regulator reduces to 11-inch WC. The vent of the second-stage regulator must point downward and be protected from water entry.

Piping: As noted, galvanized and copper are prohibited. Only black steel or listed LP-Gas-compatible materials may be used.

Testing: LP-Gas systems are tested at the same pressures as natural gas, but the test must be performed with an inert gas (nitrogen) to avoid introducing oxygen into the system, which can create a flammable mixture with residual propane.

Exam Trap: The exam may ask about the maximum pressure drop for LP-Gas systems. It is 1.0 inch WC for low-pressure systems (11-inch WC supply), not 0.5 inch. This is a common error.

1.9 Code Navigation

Concept	Primary Reference	Secondary Reference
Scope and definitions	248 CMR 10.00 (General)	NFPA 54, Chapter 1 & 3
Materials and approvals	248 CMR 10.03	NFPA 54, Chapter 4
Pipe sizing tables	NFPA 54, Tables 6.2.1–6.2.6	248 CMR 10.10 (Appendix)
Pressure drop limits	NFPA 54, Section 6.1	248 CMR 10.10
Installation (supports, valves)	NFPA 54, Chapter 4 & 5	248 CMR 10.10
CSST bonding	NFPA 54, Section 7.13	NEC 250.104 (referenced)
Appliance connections	NFPA 54, Chapter 5	248 CMR 10.10
Venting	NFPA 54, Chapter 13	Manufacturer’s instructions
Pressure testing	NFPA 54, Chapter 8	248 CMR 10.10
Purging	NFPA 54, Section 8.3	248 CMR 10.10
LP-Gas specific	NFPA 58, Chapters 5–7	248 CMR 10.10
Gas fitter licensing	MGL 142, Sections 3–5	248 CMR 3.00/4.00

Exam Strategy: For the open-book portion of the exam (if applicable), memorize the table numbers and chapter structure of NFPA 54. The exam will often reference a table by number, not by description. Knowing that Table 6.2.1 is for natural gas at 0.5-inch WC drop will save you time.

1.10 Practical Field Points for the Master Plumber

99.Documentation: Always provide the inspector with a written test report, including the date, test pressure, duration, and the name of the person who performed the test. This is a legal record.

100.Coordination with Utility: Before requesting the gas utility to turn on service, verify that all appliance valves are closed and that the system has been purged. The utility will not take responsibility for downstream piping.

101.Existing Systems: When extending an existing system, the master plumber must verify that the existing piping is adequately sized for the additional load. This requires a full demand calculation, not just a visual inspection.

102.Code Updates: The 2023 code edition includes updates to CSST bonding requirements and clarifies the use of flexible connectors. Stay current with Board bulletins and code interpretations.

1.11 Common Exam Traps Summary

Galvanized pipe on LP-Gas — always wrong.

Copper pipe on LP-Gas — always wrong.

Solder joints on copper gas piping — always wrong; must be brazed or flared.

Pressure test at operating pressure — wrong; test at 1.5× operating or 10 psi minimum.

0.5-inch WC drop for LP-Gas — wrong; use 1.0-inch WC.

CSST bonded at the meter — wrong; bond at the CSST system.

Sizing the main line for the sum of all appliances without diversity — wrong for residential with 4+ appliances.

Using a 0.5-inch WC table for a 2-psi system — wrong; use the higher-pressure table.

Appliance connector longer than 3 feet — wrong for residential unless manufacturer permits.

Gas piping in a return air duct — always prohibited.

1.12 Conclusion

Gas piping is a high-liability area of plumbing practice. The master plumber must not only know the code but also understand the engineering principles behind pipe sizing, pressure drops, and venting. On the exam, read each question carefully, identify the governing code section, and apply the specific requirement — not a general rule of thumb. The difference between a 0.5-inch and a 1.0-inch pressure drop, or between a 10-psi and a 1.5× test, is often the difference between a correct and an incorrect answer.

Final Review: Before the exam, re-familiarize yourself with NFPA 54 Table 6.2.1 (natural gas, 0.5-inch drop) and the corresponding LP-Gas table. Practice one sizing calculation daily. Know the CSST bonding requirements cold. And remember: the code is written for the protection of life and property — when in doubt, choose the safer, more restrictive option.

Part II — Gas Piping