7.2 Gas Pipe Sizing Calculations, Longest Length & Branch Method

7.2 Gas Pipe Sizing Calculations, Longest Length & Branch Method

Proper sizing of fuel gas piping is essential to ensure that every connected gas appliance receives adequate volumetric gas flow at the required manifold working pressure during peak simultaneous operation. Undersized gas piping causes severe pressure drops, resulting in incomplete fuel combustion, yellow flame soot deposition, elevated carbon monoxide ($CO$) generation, pilot light outages, and premature appliance failure.

Both the International Fuel Gas Code (IFGC Chapter 4) and the Uniform Plumbing Code (UPC Chapter 12) prescribe rigorous mathematical procedures and tabular lookup methods to size fuel gas piping. Plumbers must master the primary code methodology known as the Longest Length Method.

---

1. Hydraulic Factors & Pipe Sizing Variables

Calculating fuel gas pipe capacity depends on five fundamental physical variables:

1. Total Volumetric Demand (CFH): The combined maximum gas consumption of all downstream appliances operating at full fire.
2. Total Developed Length (Feet): The measured distance from the point of delivery (utility gas meter outlet or LP second-stage regulator) along the pipe path to the outlet being evaluated, including equivalent length additions for fittings.
3. Allowable Pressure Drop (in. w.c. or psi): The allowable pressure loss between the point of delivery and the appliance inlet. For standard low-pressure natural gas systems ($0.5 \text{ psi}$ supply), maximum pressure drop is typically set at 0.5 inches w.c. (or 0.3 inches w.c. under conservative utility rules).
4. Specific Gravity of Gas ($SG$): Gas density relative to air ($1.00$). Baseline code capacity tables assume natural gas at $SG = 0.60$.
5. Pipe Material Smoothness: Interior wall roughness factors (e.g., Schedule 40 black steel vs. semi-rigid copper vs. corrugated CSST).

Specific Gravity Multiplier for Propane Sizing

Because propane is denser than natural gas ($SG = 1.50$ vs. $0.60$), it experiences greater friction drag inside pipe walls. When utilizing standard natural gas code tables to size piping for propane, the tabular capacity values must be multiplied by a Specific Gravity Correction Multiplier ($F$): $F = \sqrt{\frac{SG_{\text{Base}}}{SG_{\text{Actual}}}} = \sqrt{\frac{0.60}{1.50}} = \sqrt{0.40} \approx 0.632$

Rule: To size a propane pipe using natural gas capacity tables, divide the required propane CFH demand by 0.632 to determine the equivalent natural gas table capacity required.

---

2. Step-by-Step Procedure: The Longest Length Method

The Longest Length Method is the standard procedure required by IFGC Section 402.4 and UPC Section 1208.4. It guarantees adequate terminal pressure by sizing every segment of the piping network against the maximum friction loss pathway in the building.

[ Gas Meter / Point of Delivery ]
              |
       (Main Trunk Section 1)
              |
       +------+------+ 
       |             |
  (Branch A)    (Sub-Main Section 2)
  Furnace            |
  (45 ft)      +-----+----+
               |          |
          (Branch B)  (Branch C: Furthest Appliance)
          Water Htr    Patio Grill
          (60 ft)      (95 ft  <--- SYSTEM LONGEST LENGTH)

Procedure Rules:

1. Step 1: Determine Appliance Load: Calculate the maximum hourly input rating (BTU/hr) for each appliance. Divide by 1,000 for Natural Gas (or 2,500 for Propane) to express demand in CFH.
2. Step 2: Measure the System Longest Length: Measure the linear piping distance from the Point of Delivery (meter outlet or second-stage LP regulator) to the single furthest appliance outlet in the entire piping network. This single maximum measurement is the System Longest Length.
3. Step 3: Select Code Sizing Table: Choose the correct sizing table corresponding to the pipe material (Schedule 40 metallic pipe, CSST, etc.), operating pressure (e.g., $<0.5 \text{ psi}$), and design pressure drop (e.g., $0.5 \text{ in. w.c.}$).
4. Step 4: Establish Table Length Column: Locate the column in the sizing table corresponding to the System Longest Length (rounding up to the next tabular length increment, e.g., 90 ft or 100 ft). CRITICAL RULE: This exact same length column MUST be used to size every single section of pipe throughout the entire system—including the main trunk, sub-mains, and short individual branch lines.
5. Step 5: Determine Section Gas Loads: For each pipe section being sized, calculate the cumulative total CFH load passing through that specific section (i.e., the sum of all downstream appliances served by that pipe segment).
6. Step 6: Select Pipe Diameter: Look down the established Longest Length column to find a gas capacity equal to or greater than the cumulative CFH load required for that section. Select the corresponding nominal pipe size.

---

3. Worked Sizing Calculation Example

Consider a residential Natural Gas system ($0.5 \text{ psi}$ supply, $0.5 \text{ in. w.c.}$ pressure drop, $SG = 0.60$, Schedule 40 metallic pipe) with the following appliance schedule:

Step-by-Step Execution:

• Longest Length Determination: The furthest appliance is Outlet D at 85 feet. In the standard Schedule 40 pipe table (0.5 psi, 0.5" drop), round up to the 90-Foot Column.
• Sizing Main Trunk Section 1 (Meter to Junction A):
  • Carries load for Appliances A + B + C + D = $100 + 40 + 65 + 35 = \mathbf{240 \text{ CFH}}$.
  • Check 90-ft column: 3/4" pipe carries 161 CFH (too small); 1" pipe carries 304 CFH (sufficient).
  • Selection: 1-Inch Pipe.
• Sizing Sub-Main Section 2 (Junction A to Junction B):
  • Carries load for Appliances B + C + D = $40 + 65 + 35 = \mathbf{140 \text{ CFH}}$.
  • Check 90-ft column: 3/4" pipe carries 161 CFH (sufficient).
  • Selection: 3/4-Inch Pipe.
• Sizing Individual Branch Lines (evaluated strictly against the 90-ft column):
  • Branch A (Furnace, 100 CFH): 3/4" carries 161 CFH $\rightarrow$ 3/4-Inch Pipe.
  • Branch B (Water Heater, 40 CFH): 1/2" carries 73 CFH $\rightarrow$ 1/2-Inch Pipe.
  • Branch C (Range, 65 CFH): 1/2" carries 73 CFH $\rightarrow$ 1/2-Inch Pipe.
  • Branch D (Dryer, 35 CFH): 1/2" carries 73 CFH $\rightarrow$ 1/2-Inch Pipe.

---

4. CSST Equivalent Hydraulic Diameter (EHD) Sizing

Corrugated Stainless Steel Tubing (CSST) features a helical or annular corrugated internal wall structure. Internal corrugation generates turbulent boundary layer eddies, causing higher flow resistance and pressure loss than smooth black iron pipe.

EHD System Principles:

• CSST is designated by an Equivalent Hydraulic Diameter (EHD) number (e.g., EHD 19, EHD 23, EHD 30, EHD 31) rather than standard fractional inch sizes.
• EHD values represent a normalized measure of hydraulic flow capacity under ANSI LC 1 testing.
• Because internal corrugation geometry varies between manufacturers (Gastite, TracPipe, WardFlex), CSST MUST BE SIZED EXCLUSIVELY USING THE SPECIFIC MANUFACTURER’S CAPACITY TABLES or the dedicated CSST tables in IFGC Chapter 4 / UPC Chapter 12. Interchanging brand capacity tables is prohibited.