Gas Pipe Sizing: A Complete Guide for Plumbers and Contractors

Understanding Gas Pipe Sizing Fundamentals

Proper gas pipe sizing is one of the most critical aspects of residential and commercial gas installations. Undersized piping starves appliances of fuel, leading to inefficient combustion, carbon monoxide production, and premature equipment failure. Oversized piping, while less dangerous, wastes materials and increases installation costs. This comprehensive guide covers everything plumbers and contractors need to know about sizing natural gas and LP gas piping systems according to the International Fuel Gas Code (IFGC) and NFPA 54.

Gas pipe sizing isn't guesswork—it's a precise engineering calculation based on BTU demand, pipe length, pressure drop, and gas characteristics. Whether you're installing a single water heater or designing a complex multi-appliance system, following code-approved sizing methods ensures safe, reliable operation for years to come.

Calculating Total BTU Load

The foundation of any gas pipe sizing project is accurately determining the total BTU input rating of all gas appliances on the system. This is NOT the output rating—you must use the input rating found on the appliance nameplate or specification sheet.

Common Appliance BTU Ratings

Here's what to expect for typical residential and light commercial appliances:

Heating Equipment:

• Residential furnaces: 40,000-120,000 BTU/hr (80,000 BTU/hr is common)
• Boilers: 50,000-200,000 BTU/hr depending on size
• Unit heaters (garage/workshop): 30,000-80,000 BTU/hr
• Radiant floor heating boilers: 80,000-150,000 BTU/hr

Water Heating:

• Tank-type water heaters (40-50 gal): 30,000-40,000 BTU/hr
• Tank-type water heaters (75-100 gal): 50,000-76,000 BTU/hr
• Tankless water heaters: 120,000-199,000 BTU/hr
• Commercial water heaters: 100,000-400,000 BTU/hr

Cooking Equipment:

• Residential ranges (4-6 burners): 45,000-65,000 BTU/hr
• Professional-style ranges: 60,000-90,000 BTU/hr
• Cooktops: 40,000-60,000 BTU/hr
• Wall ovens: 15,000-25,000 BTU/hr
• Commercial ranges: 100,000-200,000 BTU/hr per section

Other Appliances:

• Clothes dryers: 20,000-35,000 BTU/hr
• Fireplaces (vented log sets): 25,000-40,000 BTU/hr
• Fireplaces (direct vent): 20,000-60,000 BTU/hr
• Pool/spa heaters: 100,000-400,000 BTU/hr
• Standby generators: 30,000-100,000 BTU/hr (varies by kW rating)
• Outdoor grills: 30,000-80,000 BTU/hr

Calculating System Demand

For a typical home with a furnace (80,000 BTU/hr), water heater (40,000 BTU/hr), range (55,000 BTU/hr), and dryer (25,000 BTU/hr), the total connected load is:

Total BTU = 80,000 + 40,000 + 55,000 + 25,000 = 200,000 BTU/hr

Some jurisdictions allow diversity factors for systems where not all appliances run simultaneously, but IFGC and NFPA 54 sizing tables are based on maximum connected load. For residential applications, size the system for 100% of the total BTU load unless local code specifically permits diversity calculations.

When appliances aren't installed yet, use maximum anticipated values. It's better to size piping generously now than to discover undersized piping during a future kitchen remodel.

Natural Gas vs LP Gas: Critical Differences

Natural gas and LP (liquefied petroleum) gas have fundamentally different properties that affect pipe sizing. Never use natural gas sizing tables for LP installations or vice versa.

Specific Gravity

Natural gas has a specific gravity of approximately 0.60 (lighter than air). LP gas (primarily propane) has a specific gravity of approximately 1.50 (heavier than air). Specific gravity is the ratio of gas density compared to air.

Because LP gas is denser, it flows differently through piping and requires different sizing tables. For the same BTU load and pipe length, LP gas typically requires larger pipe sizes than natural gas.

Energy Content

Natural gas contains approximately 1,000-1,050 BTU per cubic foot, though this varies by region and supplier composition.

LP gas contains approximately 2,500 BTU per cubic foot of vapor, more than double natural gas.

This means a 100,000 BTU/hr appliance running on natural gas consumes about 100 cubic feet per hour, while the same appliance on LP gas consumes only 40 cubic feet per hour. However, the higher specific gravity of LP gas offsets this advantage in pipe sizing calculations.

Supply Pressure Requirements

Natural gas residential systems typically operate at:

• Inlet pressure: 7 inches water column (WC) or less at the meter
• Appliance manifold pressure: 3.5 inches WC (common for atmospheric burners)
• Allowable pressure drop: 0.5 inches WC from meter to appliance

LP gas systems operate at higher pressures:

• First-stage regulator output: 10-11 inches WC (common for two-stage systems)
• Second-stage regulator output: 11 inches WC (vapor withdrawal systems)
• Some appliances require 11 inches WC; others step down to 3.5 inches WC at appliance
• Allowable pressure drop: 0.5 inches WC (same as natural gas)

The higher operating pressure of LP systems allows smaller pipe sizes in some applications, but you must use LP-specific sizing tables that account for specific gravity.

IFGC and NFPA 54 Sizing Tables

The International Fuel Gas Code (IFGC) and NFPA 54 (National Fuel Gas Code) contain comprehensive sizing tables for all common pipe materials and gas types. These tables are based on the Spitzglass formula and empirical flow testing.

How to Use Sizing Tables

Each table specifies:

• Gas type: Natural gas or undiluted propane (LP)
• Inlet pressure: Usually 0.5 psi (14 inches WC) or less for natural gas
• Pressure drop: Typically 0.5 inches WC
• Specific gravity: 0.60 for natural gas, 1.50 for LP gas

Tables are organized with:

• Rows: Pipe length in feet (measured length of piping from point of delivery to most remote outlet)
• Columns: Pipe nominal diameter (½", ¾", 1", 1¼", etc.)
• Cell values: Maximum capacity in thousands of BTU per hour

To size a pipe:

1. Determine total BTU load for the section of pipe
2. Measure the longest pipe run from the meter/tank to the farthest appliance
3. Locate the row for your pipe length
4. Move across the row until you find a capacity equal to or greater than your BTU load
5. Read up to find the minimum pipe size

Example: You have a 150,000 BTU/hr load and 60 feet of pipe run on a natural gas system.

Looking at IFGC Table 402.4(2) for natural gas at 0.5 psig inlet pressure with 0.5-inch WC pressure drop:

• 60-foot row, ¾" column: 120,000 BTU/hr (too small)
• 60-foot row, 1" column: 245,000 BTU/hr (adequate)

You need 1-inch pipe for this run.

Pressure Drop: The 0.5-Inch Rule

Residential gas systems are designed for a maximum pressure drop of 0.5 inches water column from the point of delivery (meter or second-stage regulator) to the appliance inlet. This ensures adequate pressure reaches each appliance for proper burner operation.

Pressure drop is caused by friction as gas flows through pipes, fittings, and valves. Longer runs, smaller diameters, and more fittings increase pressure drop.

The formula for pressure drop in low-pressure gas systems is:

ΔP = (0.0004 × f × L × Q²) / (D⁵)

Where:

• ΔP = pressure drop in inches WC
• f = friction factor (depends on pipe material roughness)
• L = pipe length in feet
• Q = gas flow rate in cubic feet per hour
• D = inside diameter in inches

Fortunately, you don't need to calculate this manually—IFGC/NFPA 54 tables do it for you. The tables are pre-calculated for 0.5-inch WC drop.

Fitting Equivalent Length

Each fitting adds resistance equivalent to a certain length of straight pipe:

• 90-degree elbow: 5 feet equivalent length
• 45-degree elbow: 2.5 feet equivalent length
• Tee (flow through run): 1.5 feet
• Tee (flow through branch): 5 feet
• Gate valve (fully open): 1 foot

For precise calculations, add equivalent length to measured pipe length. However, for residential systems with typical fitting counts, adding 5-10% to measured length accounts for fittings adequately.

Longest Length Method vs Branch Length Method

There are two approaches to sizing gas piping systems: the longest length method and the branch length method. Understanding both is essential for complex installations.

Longest Length Method

This is the simpler, more conservative approach required by many local codes for residential applications.

Process:

1. Determine the longest pipe run from the meter to the farthest appliance
2. Add up the BTU load of ALL appliances on the system
3. Size every section of pipe using this longest length and total BTU load

This method oversizes some portions of the piping but provides generous capacity and simplifies calculations. It's ideal for simple residential systems.

Example: A home has a meter 10 feet from the main, the main runs 40 feet through the basement, and branches serve a furnace 10 feet away and a water heater 20 feet away.

• Longest run: 10 + 40 + 20 = 70 feet
• Total load: All appliances (200,000 BTU/hr)
• Size ALL piping based on 70 feet and 200,000 BTU/hr

Branch Length Method

This more sophisticated approach sizes each section of pipe based on actual length and cumulative load, resulting in smaller pipes for shorter runs.

Process:

1. Draw a schematic of the entire system
2. Label each section with its length and cumulative BTU load
3. Start at the farthest appliance and work backward to the meter
4. Size each section based on its specific length and cumulative BTU downstream

Example using the same house:

• Water heater branch (20 ft from main): 40,000 BTU/hr → Total length = 10 + 40 + 20 = 70 ft
• Furnace branch (10 ft from main): 80,000 BTU/hr → Total length = 10 + 40 + 10 = 60 ft
• Main line (40 ft): Serves both, so 120,000 BTU/hr → Total length = 10 + 40 = 50 ft
• Service line (10 ft): Serves everything, 200,000 BTU/hr → Length = 10 ft

Each section is sized independently. The service line carries 200,000 BTU/hr for only 10 feet, so it may be smaller than if you used the 70-foot longest length for everything.

The branch length method saves on materials for larger systems but requires more careful calculation. Some jurisdictions prohibit it for residential work to reduce errors.

Gas Pipe Materials: Options and Applications

Code-approved gas piping materials include black iron steel pipe, corrugated stainless steel tubing (CSST), copper, polyethylene (PE), and polyvinyl chloride (PVC) in limited applications. Each has advantages and limitations.

Black Iron Steel Pipe

Advantages:

• Traditional, widely accepted by all jurisdictions
• Rigid and durable
• Suitable for exposed and concealed installations
• Not subject to lightning strike concerns
• Proven 100+ year service life
• Readily available in all sizes
• Can be used indoors and outdoors above ground

Disadvantages:

• Heavy and labor-intensive to install
• Requires threading or welding
• Many fittings needed for directional changes
• Susceptible to corrosion in wet/humid environments
• Must be coated or wrapped for underground use

Best applications: Commercial installations, exposed piping, areas where mechanical protection is needed, installations where lightning bonding is problematic.

Sizing considerations: Use IFGC/NFPA 54 Table 402.4(1) or 402.4(2) for Schedule 40 metallic pipe.

Corrugated Stainless Steel Tubing (CSST)

Advantages:

• Lightweight and flexible
• Faster installation (60% less labor than black iron)
• Fewer fittings required
• Can be snaked through walls and joists
• Stainless steel resists corrosion
• Comes in coils up to 250 feet (fewer joints)

Disadvantages:

• More expensive material cost (offset by labor savings)
• Requires special cutting tools and fittings
• Must be bonded per NEC 250.104(B) to prevent lightning damage
• Cannot be directly buried (must be in conduit)
• Vulnerable to physical damage (requires protection in exposed areas)
• Some jurisdictions restrict or prohibit use

Best applications: New construction and retrofits where labor costs dominate, residential installations, complex routing situations.

Sizing considerations: CSST manufacturers provide their own sizing tables specific to their product. CounterStrike CSST, TracPipe, and Gastite all have different flow characteristics. You MUST use the manufacturer's table for the specific product being installed. Generic IFGC tables do not apply to CSST.

Critical bonding requirement: The 2008 NEC and newer require CSST bonding to the electrical grounding system to prevent perforation from lightning-induced current. Use a bonding clamp rated for the application and connect to the electrical service grounding electrode system with #6 AWG copper wire minimum. This is not optional—unbonded CSST has been involved in hundreds of house fires following nearby lightning strikes.

Copper and Copper Alloy

Advantages:

• Corrosion resistant
• Relatively lightweight
• Can be soldered or brazed (no threading)
• Good for tight spaces

Disadvantages:

• Prohibited by some jurisdictions (check local amendments)
• More expensive than steel
• Requires proper alloy selection (Type K or L copper, not M)
• Sulfur compounds in some gas supplies corrode copper
• Fewer installers familiar with gas-rated copper joining methods

Best applications: Areas where steel corrosion is problematic, short runs, specific code-approved applications.

Sizing considerations: Use IFGC/NFPA 54 tables specifically for copper tube. Copper has different internal diameter and roughness than steel.

Polyethylene (PE) Pipe

Advantages:

• Excellent for underground service lines
• Corrosion-proof
• Flexible (fewer fittings)
• Lower material cost for long underground runs
• Tracer wire can be embedded for locating

Disadvantages:

• Underground use ONLY (not permitted above ground or indoors)
• Requires special training and fusion equipment for joining
• Susceptible to damage from rocks and roots
• Must be yellow color-coded for gas service
• Requires proper burial depth and protection

Best applications: Underground service lines from street to building, underground runs between buildings.

Sizing considerations: Use manufacturer's tables specific to PE pipe material and SDR (standard dimension ratio). PE pipe has different pressure ratings and flow characteristics than metallic pipe.

Two-Pound Gas Systems

Most residential gas systems operate at 7 inches WC or less (low-pressure systems). However, two-pound systems (27.4 inches WC) are becoming more common for large homes, homes with high-BTU appliances, or installations where long runs make low-pressure sizing impractical.

Advantages of Two-Pound Systems

• Smaller pipe sizes: Higher pressure allows smaller diameter piping for the same BTU load
• Material savings: Significant cost reduction on large or complex installations
• Longer allowable runs: Can serve distant appliances without excessive pipe sizes
• Flexibility: Easier to add appliances later without re-piping

Requirements for Two-Pound Systems

• Line regulator: Second-stage regulator at or near each appliance steps down 2 psi to appliance pressure (typically 3.5" or 7" WC)
• Specialized piping: Must use pipe rated for higher pressure (Schedule 40 steel, CSST rated for 5 psi or greater)
• Pressure testing: Higher test pressure required (typically 3 psi for 30 minutes minimum)
• Professional design: Most jurisdictions require engineer or master plumber design
• Regulator venting: Each appliance regulator must be properly vented
• Pressure relief: Some codes require pressure relief valves

When to Consider Two-Pound Systems

• Total system BTU load exceeds 500,000 BTU/hr
• Pipe runs exceed 100 feet to some appliances
• High-BTU appliances (tankless water heaters, large generators, pool heaters)
• Multi-story buildings where vertical runs are significant
• Retrofit situations where existing piping is undersized

Example: A large home has a 200-foot run to a pool heater (400,000 BTU/hr). Using low-pressure natural gas tables, you'd need 2½-inch pipe. With a two-pound system, 1¼-inch pipe is adequate, saving hundreds of dollars in materials and labor.

Manifold Systems and Home-Run Design

Modern gas piping increasingly uses manifold systems similar to PEX plumbing—a central manifold with dedicated home-run lines to each appliance.

Manifold System Advantages

• Simplified sizing: Each appliance has a dedicated line sized for its individual load
• Reduced fittings: Straight runs from manifold to appliance
• Easy modifications: Add or remove appliances without affecting others
• Faster installation: Pre-cut CSST runs from manifold
• Clear labeling: Each port serves a specific appliance

Manifold Sizing

The manifold inlet must be sized for the total BTU load of all appliances. The inlet pipe length is measured from meter to manifold.

Each outlet is sized for its individual appliance BTU load. Outlet length is measured from manifold to appliance.

Example manifold system:

• Meter to manifold: 20 feet
• Manifold inlet: Sized for 200,000 BTU/hr total load at 20 feet → 1¼" pipe
• Furnace outlet: 80,000 BTU/hr, 35 feet from manifold → ¾" CSST
• Water heater outlet: 40,000 BTU/hr, 30 feet from manifold → ½" CSST
• Range outlet: 55,000 BTU/hr, 45 feet from manifold → ¾" CSST
• Dryer outlet: 25,000 BTU/hr, 40 feet from manifold → ½" CSST

Each run is sized independently based on its length and load. This is simpler than calculating branch sizing on a trunk-and-branch system.

Regulator Placement and Selection

Gas regulators reduce and stabilize pressure. Understanding proper placement is critical for safe, code-compliant installations.

Service Regulators

The service regulator (first-stage regulator) reduces utility supply pressure (which can be 60 psi or higher) to usable building pressure. This is typically installed:

• At the meter (utility-owned and maintained)
• At the tank (LP installations)
• At the building entrance (large commercial)

For natural gas, the utility usually provides and maintains the service regulator as part of the meter assembly.

For LP gas, the first-stage regulator is typically at the tank, reducing tank pressure (100-200 psi) to 10-11 inches WC for two-stage systems or directly to appliance pressure for single-stage systems.

Appliance Regulators

Each gas appliance has an internal or external regulator that reduces line pressure to the specific pressure required by the burners (typically 3.5 inches WC for atmospheric burners, 7 inches WC for some forced-air burners).

For two-pound systems, a line regulator (second-stage regulator) is installed at or near each appliance to step pressure down from 2 psi to appliance inlet pressure.

Regulator Venting

All regulators must be vented to atmosphere to operate correctly. The vent allows the diaphragm to sense atmospheric pressure.

• Outdoor regulators: Vent opening faces downward to prevent water entry
• Indoor regulators: Must vent to outdoors via dedicated vent piping (two-pound systems) or into the space if permitted by appliance listing
• Appliance regulators: Usually vent into the appliance combustion chamber or draft hood area

Never block, plug, or paint over regulator vents. A blocked vent causes malfunction and potential safety hazards.

Elevation and Temperature Effects

Gas density varies with elevation and temperature, affecting system performance.

Elevation Effects

At higher elevations, atmospheric pressure is lower, which affects:

• Regulator operation: Spring-loaded regulators may need adjustment
• Appliance combustion: Less oxygen available; some appliances require de-rating or re-jetting
• Pressure measurements: Inches of water column remain the same, but actual pressure differential decreases

For installations above 2,000 feet elevation, consult appliance manufacturers for de-rating requirements. Some furnaces and boilers lose 4% capacity per 1,000 feet above sea level.

Pipe sizing tables remain valid at altitude (the reduced density is offset by reduced pressure), but appliance performance changes.

Temperature Effects

Gas density increases as temperature decreases. Extremely cold LP tanks can have vaporization issues:

• LP vapor withdrawal systems may freeze in cold climates
• Tank pressure decreases as temperature drops
• Inadequate vaporization causes pressure drop and appliance starvation

For high-demand LP systems in cold climates (northern states, Canada), consider:

• Vaporizer systems
• Multiple tanks manifolded together
• Buried tanks for temperature stability
• Propane/air mixtures

Natural gas is less affected by temperature because it remains gaseous at all ambient temperatures.

Common Gas Pipe Sizing Mistakes

Avoid these frequent errors that lead to code violations, failed inspections, and dangerous conditions:

1. Using Main Line Size for All Branches

The trunk line might be 1¼ inches, but that doesn't mean every branch should be. Each branch must be sized based on its length and cumulative load. A branch serving only a dryer 15 feet from the trunk may only need ½-inch pipe.

2. Measuring to the Nearest Appliance Instead of Farthest

Pipe sizing length is the longest run in the system, not the average or nearest. If your meter is 20 feet from your main, your main runs 50 feet, and one branch goes another 30 feet, you size for 100 feet total, even if other appliances are closer.

3. Using Output Rating Instead of Input Rating

Appliance output rating is after efficiency losses. An 80% efficient furnace with 80,000 BTU/hr output has a 100,000 BTU/hr input rating. Always size piping for INPUT, found on the nameplate.

4. Forgetting Future Appliances

You're adding a dryer today, but the homeowner mentions wanting a gas fireplace next year. Size the main piping for both loads now. Adding capacity later often means replacing entire runs.

5. Mixing Gas Types

Natural gas and LP gas are not interchangeable. Using natural gas tables for an LP installation undersizes the piping. Always verify gas type and use corresponding tables.

6. Ignoring Elevation Changes

Multi-story buildings have vertical pipe runs. A furnace in the basement served from a meter 25 feet away horizontally and 10 feet below the meter has a total run of 35 feet, not 25 feet.

7. Using Wrong Pressure Drop Tables

IFGC has multiple tables for different inlet pressures and pressure drops. Table 402.4(1) is for 0.5" WC drop and 10" WC inlet. Table 402.4(2) is for 0.5" WC drop and 0.5 psi inlet. Using the wrong table can lead to 50% undersizing.

8. Not Accounting for CSST Manufacturer Differences

TracPipe, Gastite, and CounterStrike CSST are not interchangeable. Each manufacturer's sizing table is specific to their product's corrugation design. A ¾" TracPipe has different capacity than a ¾" Gastite.

9. Undersizing for Tankless Water Heaters

Tankless water heaters have high BTU input (150,000-199,000 BTU/hr is common). A ½-inch line that worked for a 40,000 BTU/hr tank heater won't come close for a tankless unit.

10. Inadequate Pressure Testing

Gas piping must be pressure tested per code before appliances are connected. Test pressure is typically 3 psi (83 inches WC) or 1.5 times the working pressure, whichever is greater, held for 30 minutes minimum with no pressure loss. Soapy water test at every joint and fitting.

When Professional Engineering Is Required

While many residential gas piping systems can be designed by qualified plumbers and contractors using IFGC tables, complex systems require professional engineering.

Situations requiring engineer involvement:

• Commercial and industrial installations: Buildings with BTU loads exceeding 1,000,000 BTU/hr
• Two-pound systems in some jurisdictions: Local code may mandate engineer design for 2 psi systems
• Unusual gases: Manufactured gas, biogas, or mixed gases with non-standard specific gravity
• High-rise buildings: Multi-story systems with complex pressure requirements
• Complex manifolding: Multiple meters, pressure zones, or mixing systems
• Critical facilities: Hospitals, laboratories, assisted living facilities
• Alternative calculations: Systems not using IFGC tables (computational fluid dynamics, custom formulas)

A professional engineer can perform detailed pressure drop calculations for complex systems, specify regulator sizing and placement, design custom manifold systems, and provide sealed drawings for permit submittal.

Cost: Engineering services for residential gas piping typically cost $500-2,000. Commercial systems run $2,000-10,000 depending on complexity. This is money well spent for complex installations—incorrect design can cost tens of thousands to fix and creates serious safety hazards.

Practical Sizing Example: Whole-House System

Let's work through a complete residential natural gas system design.

House details:

• Meter is outside at front of house
• Main gas line enters house and runs through basement
• 6 appliances total

Appliance locations and loads:

1. Furnace (basement, 15 feet from main): 100,000 BTU/hr
2. Water heater (basement, 20 feet from main): 40,000 BTU/hr
3. Range (kitchen, main runs 40 feet, then up to first floor and 10 feet to kitchen): 60,000 BTU/hr
4. Dryer (first floor laundry, main runs 25 feet, then up and 5 feet to laundry): 25,000 BTU/hr
5. Fireplace (living room, main runs 30 feet, then up and 8 feet to fireplace): 35,000 BTU/hr
6. Future generator (outside, main runs 45 feet to back of house): 50,000 BTU/hr

Total load: 310,000 BTU/hr

Longest run: Meter to main (10 ft) + through basement (40 ft) + vertical to first floor (10 ft) + to range (10 ft) = 70 feet

Using longest length method (conservative approach):

• All piping sized for 70 feet and 310,000 BTU/hr
• Using IFGC Table 402.4(2) (natural gas, 0.5 psi inlet, 0.5" WC drop)
• 70-foot row, scan across: 1¼" column shows 370,000 BTU/hr capacity
• Main line and all branches: 1¼ inches minimum

This wastes material on short runs but is simple and provides generous capacity.

Using branch length method (optimized):

Working backward from each appliance:

Generator branch:

• Load: 50,000 BTU/hr
• Length: 10 + 45 = 55 ft
• Size: ¾" (98,000 BTU/hr capacity at 50 ft table row)

Fireplace branch:

• Load: 35,000 BTU/hr
• Length: 10 + 30 + 10 + 8 = 58 ft
• Size: ¾" (98,000 BTU/hr at 60 ft row)

Range branch:

• Load: 60,000 BTU/hr
• Length: 10 + 40 + 10 + 10 = 70 ft
• Size: ¾" (85,000 BTU/hr at 70 ft row)

Dryer branch:

• Load: 25,000 BTU/hr
• Length: 10 + 25 + 10 + 5 = 50 ft
• Size: ½" (73,000 BTU/hr at 50 ft row)

Water heater branch:

• Load: 40,000 BTU/hr
• Length: 10 + 20 = 30 ft
• Size: ¾" (138,000 BTU/hr at 30 ft row)

Furnace branch:

• Load: 100,000 BTU/hr
• Length: 10 + 15 = 25 ft
• Size: 1" (278,000 BTU/hr at 25 ft row)

Main basement trunk:

• Serves: Generator (50k) + fireplace (35k) + range (60k) + dryer (25k) + water heater (40k) + furnace (100k) = 310,000 BTU/hr
• Length: From meter inlet to farthest branch takeoff = 10 + 40 = 50 ft
• Size: 1¼" (399,000 BTU/hr at 50 ft row)

Meter to main (service entrance):

• Full load: 310,000 BTU/hr
• Length: 10 ft
• Size: 1" (1,510,000 BTU/hr at 10 ft row) – 1¼" would also work and provides future capacity

The branch length method uses smaller pipes for shorter runs (½" for dryer, ¾" for several branches) while the longest length method uses 1¼" everywhere. Material savings are significant on a house this size.

Actionable Takeaways

Gas pipe sizing is a critical safety issue that requires careful calculation and adherence to code. Here's what to remember:

1. Always use appliance input ratings from the nameplate, never output ratings or guesses
2. Identify your gas type (natural gas vs LP) and use the corresponding IFGC/NFPA 54 tables
3. Measure the longest pipe run from meter/regulator to the farthest appliance for system sizing
4. Add up total BTU load of all appliances unless code specifically allows diversity factors
5. Choose appropriate pipe material based on application, budget, and code requirements
6. Size conservatively – when capacity falls between two sizes, choose the larger pipe
7. Account for future appliances during initial design to avoid expensive re-piping
8. Follow manufacturer's tables for CSST – generic tables don't apply
9. Bond all CSST installations per NEC requirements to prevent lightning damage
10. Consider two-pound systems for large loads or long runs to reduce pipe sizes
11. Pressure test all installations at 3 psi for 30 minutes minimum before connection
12. Consult local code for amendments to IFGC/NFPA 54 and specific requirements

Undersized gas piping creates dangerous conditions: incomplete combustion, carbon monoxide production, yellow flames, soot buildup, and premature appliance failure. Taking the time to properly size gas piping using code-approved methods protects occupants, ensures reliable appliance operation, and prevents costly callbacks and re-work.

When in doubt, consult the current edition of IFGC or NFPA 54, reach out to your local code official, or engage a professional engineer for complex installations. The small additional cost of doing it right the first time is insignificant compared to the cost of doing it wrong.