Natural Gas and Fuel Line Pipe Marking: ASME A13.1 Requirements
Natural gas and fuel lines are some of the most hazardous pipe systems in any building. A misidentified valve, an unlabeled branch, or a wrong connection can lead to gas leaks, explosions, or fire.
Correct pipe marking is one of the simplest things you can do to reduce that risk.
This guide covers the correct ASME A13.1 colors, label text, and placement rules for all types of combustible fuel piping — natural gas, propane, fuel oil, and diesel.
The ASME A13.1 Color for Combustible Fuel Lines
Under ASME A13.1, pipes that carry flammable or combustible gases and liquids use a specific color to signal their hazard level.
The correct color for natural gas and fuel line pipe markers is:
Brown background — White text
This applies to:
- Natural gas supply lines
- Propane supply lines
- Liquefied petroleum gas (LPG) lines
- Fuel oil supply and return lines
- Diesel fuel lines
- Heating oil supply lines
- Generator fuel lines
Brown/white tells anyone who reads the label that the pipe carries something combustible. Combined with clear label text, this prevents dangerous confusion between gas lines and water, air, or other non-hazardous systems.
What About High-Pressure or High-Hazard Gas Lines?
ASME A13.1 has a separate category for extra-hazardous materials — pipes carrying toxic, flammable, or otherwise dangerous gases that pose an immediate threat at any leak.
Orange background — Black text is used for pipes carrying highly toxic, highly corrosive, or highly flammable materials where even a small leak is immediately dangerous.
In most commercial and light industrial settings, natural gas and fuel oil stay in the brown/white category. In industrial environments — such as petrochemical plants, refineries, or facilities handling hydrogen or high-pressure flammable gases — check with your AHJ (authority having jurisdiction) about whether the orange/black designation applies.
When in doubt, use the more cautious option and consult the project engineer.
Standard Label Text for Gas and Fuel Lines
Clear label text is essential on gas and fuel lines. The color tells you the hazard category. The text tells you exactly what's in the pipe.
| Pipe | Recommended Label Text |
|---|---|
| Natural gas supply (low pressure) | NATURAL GAS or NAT GAS |
| Natural gas supply (high pressure) | HIGH PRESSURE NATURAL GAS or HP NAT GAS |
| Propane supply | PROPANE or LP GAS |
| LPG supply | LIQUEFIED PETROLEUM GAS or LPG |
| Fuel oil supply | FUEL OIL SUPPLY or #2 FUEL OIL |
| Fuel oil return | FUEL OIL RETURN |
| Diesel supply | DIESEL FUEL |
| Generator fuel | GENERATOR FUEL SUPPLY |
| Heating oil | HEATING OIL SUPPLY |
For facilities with multiple gas pressures — which is common in industrial settings — always include the pressure level in the label text. "HIGH PRESSURE NATURAL GAS" and "LOW PRESSURE NATURAL GAS" communicate very different risks to a technician working in the mechanical room.
If the facility stores multiple fuel types (natural gas for boilers, diesel for generators, for example), the distinct label text on each system prevents cross-connection errors.
Flow Direction Arrows on Gas and Fuel Lines
ASME A13.1 requires flow direction arrows on all pipe systems — and gas lines are no exception.
On a gas supply system, the arrow shows which direction gas flows from the meter or regulator toward the appliances and equipment. On a fuel oil system, the supply arrow and return arrow confirm the direction of flow in each pipe.
Flow direction is especially important on gas systems because:
- A technician adding a branch or tapping the line needs to know whether they're on the supply side of a regulator or the outlet side.
- A service tech working on a boiler or furnace needs to know which valve isolates their equipment from the gas main.
- Emergency responders shutting off gas in a fire or leak situation need to quickly identify which direction leads to the shutoff.
Use integrated arrow labels that are part of the pipe marker, or add standalone brown/white arrow labels at each label location.
IFGC and NFPA 54 — Other Codes That Apply
Gas piping isn't only governed by ASME A13.1. Depending on the jurisdiction and facility type, other codes also apply:
IFGC (International Fuel Gas Code) — The IFGC covers installation of gas piping systems and requires that gas piping be identified in buildings where piping could be confused with other systems. Most states and municipalities have adopted the IFGC or a similar code.
NFPA 54 (National Fuel Gas Code) — Similar requirements for gas piping identification, adopted in many jurisdictions as an alternative to the IFGC.
NFPA 58 (LP-Gas Code) — Covers liquefied petroleum gas (propane) systems, including identification requirements for LPG piping.
NFPA 30 (Flammable and Combustible Liquids Code) — Applies to fuel oil, diesel, and other liquid fuel piping, and includes identification requirements.
In practice, meeting ASME A13.1 requirements with proper brown/white labels and accurate text satisfies the identification requirements of all of these codes in most applications.
Pipe Marker Sizing for Gas and Fuel Lines
Gas supply lines vary widely in size — from small ½" branch lines feeding individual appliances to large 4" or 6" gas mains feeding industrial equipment.
Use the ASME A13.1 size chart:
| Pipe OD | Label Size |
|---|---|
| ¾" to 2" | 8" × 1⅛" |
| 2½" to 7⅞" | 14" × 2¼" |
| 8" to 10" | 24" × 4" |
| Over 10" | 32" × 4" |
For most commercial natural gas branch lines (¾" to 1½"), the 8" × 1⅛" label is the right choice. Gas mains serving larger buildings or industrial facilities (2" to 4") typically use the 14" × 2¼" label.
Fuel oil piping is usually smaller — most return and supply lines in commercial settings are ¾" to 1½" — so the 8" label is most common.
Where to Place Gas and Fuel Line Markers
All ASME A13.1 placement rules apply to gas and fuel lines, with some additional priorities specific to fuel systems:
At the gas meter or regulator — The first label goes right at the meter or pressure regulator. Label both the inlet and outlet sides. If the regulator steps down from high pressure to low pressure, label both sides with the appropriate pressure designation.
At every shutoff valve — Every manual shutoff valve needs a label on both sides. This includes the main gas shutoff, appliance shutoffs, and zone isolation valves. In an emergency, the person shutting off gas needs to find the right valve immediately.
At every branch — When the gas main branches to serve different appliances or equipment, label each branch at the takeoff. A branch going to the boiler and a branch going to the water heater both need separate labels.
At every penetration — Both sides of every wall, floor, and ceiling penetration. Gas lines entering a mechanical room from the exterior or running between floors need labels on both sides of each penetration.
At equipment connections — Label at the connection to every gas-fired appliance: boilers, furnaces, water heaters, rooftop units, kitchen equipment, generators, and any other gas-fired equipment.
Every 25–50 feet on straight runs — In long corridors, pipe chases, or mechanical rooms, repeat labels at intervals. For gas lines, many AHJs and facility managers prefer 25-foot spacing rather than 50 feet.
Special Considerations for Fuel Oil Systems
Fuel oil systems have a few additional marking situations to think about:
Supply and return lines run together. Fuel oil systems have both a supply line (fuel going to the burner) and a return line (excess fuel returning to the tank). Both run side by side and both need labels — supply and return arrows pointing in opposite directions.
Day tank systems. Some large buildings have a main fuel storage tank and one or more day tanks (smaller tanks near the equipment). Label the fill line, the supply line, and any overflow or vent lines separately.
Vent and overflow lines. Fuel oil storage tanks have vent pipes and overflow pipes. These carry fuel vapor and overflow fuel — and both need to be clearly labeled. Don't leave vent pipes unlabeled just because they normally don't carry liquid.
Tank level gauge connections. In some installations, gauge lines connect to the tank. Label these as fuel oil system components even if they're small.
The Most Common Gas Line Labeling Mistakes
No labels after rough-in. Gas lines are often labeled during rough-in, then covered in insulation or wall finishes. The label needs to be on the final exposed surface — not hidden under wrap.
"GAS" only — no pressure info. In a building with both high-pressure gas mains and low-pressure branch lines, "GAS" alone isn't enough. Label the pressure.
Missing shutoff valve labels. The most important label on a gas system is the one on the shutoff valve. Don't skip it.
Skipping fuel oil return lines. The supply gets labeled. The return gets forgotten. Both need markers.
Wrong color. Green/white on a gas line is a serious error — it implies the pipe is a safe fluid. Brown/white is required for combustible fuels. Check every label.
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Frequently Asked Questions
What color are natural gas pipe markers? Natural gas pipe markers are brown background with white text under ASME A13.1. Brown/white is used for all combustible fuel piping including natural gas, propane, LPG, fuel oil, and diesel.
What is the correct label for a natural gas line? The most common labels are "NATURAL GAS" or "NAT GAS." For systems with multiple pressures, use "HIGH PRESSURE NATURAL GAS" or "LOW PRESSURE NATURAL GAS" to distinguish them.
Do fuel oil lines and gas lines use the same color? Yes. Both natural gas and fuel oil use brown/white under ASME A13.1 since both are combustible fuels. The label text distinguishes what's in each pipe.
Do gas lines need flow direction arrows? Yes. ASME A13.1 requires flow arrows on all pipe systems. On gas lines, arrows confirm which direction gas flows from the meter/regulator toward the appliances — important for service work and emergency response.
What code governs gas pipe marking? ASME A13.1 sets the pipe identification standard. The IFGC (International Fuel Gas Code), NFPA 54, and NFPA 58 also include pipe identification requirements that are generally satisfied by ASME A13.1 compliance.
How often do gas lines need to be labeled? At minimum: at the meter/regulator, at every shutoff valve (both sides), at every branch, at every wall/floor penetration (both sides), at every equipment connection, and at 25–50 foot intervals on straight runs.
What about propane lines — same color as natural gas? Yes. Propane (LPG) pipe markers are also brown/white under ASME A13.1. Use label text "PROPANE" or "LP GAS" to distinguish propane lines from natural gas lines.
What size pipe marker for a ¾" gas branch line? A ¾" pipe OD falls in the ¾"–2" range, which requires an 8" × 1⅛" label under ASME A13.1.
