What is 2024 International Plumbing Code (IPC) Appendix E & Hunter's Curve Standards?
Sizing water distribution and service lines based on simple linear fixture summation leads to massive oversizing, stagnant water risks, biofilm formation, and high construction costs. Sizing lines too small causes dangerous pressure drops, cavitation, fixture starvation, and water hammer. IPC Appendix E implements Dr. Roy B. Hunter's binomial probability curve to convert Water Supply Fixture Units (WSFU) into probable simultaneous peak demand (GPM) under strict fluid velocity constraints.
Mathematical Foundation
Hunter's Curve Binomial Probability Model
= Total number of fixtures of a given type connected to the water distribution system.
= Number of identical fixtures operating concurrently at any given instant.
= Probability that a single fixture is in operation (ratio of fixture run time to average cycle interval between uses, typically 0.01 to 0.03 for residential).
Continuity Equation & Pipe Fluid Velocity
= Mean fluid flow velocity through the pipe in feet per second (FPS).
= Probable peak volumetric flow rate in gallons per minute (GPM) derived from Hunter's Curve.
= Actual internal diameter of the pipe in inches (0.785" for 3/4" Type L Copper; 0.671" for 3/4" PEX).
= Volumetric flow dimensional conversion constant (0.1337 ft³/gal ÷ 60 s/min × 4/π × 144 in²/ft²).
Laws & Principles
- IPC Section 604.4 Maximum Velocity: Water distribution systems must be designed so that fluid velocity does not exceed 8.0 feet per second for cold water lines and 5.0 feet per second for hot water lines to prevent pipe erosion, excessive pressure loss, and disruptive hydraulic noise.
- Hunter's Curve Probability Factor: Dr. Roy B. Hunter (NBS Report BMS65) proved that fixtures operate intermittently for brief durations (e.g. 10 to 60 seconds per flush). The probability of simultaneous discharge diminishes exponentially as the total number of connected fixtures increases.
- Flushometer vs. Flush Tank Divergence: Flushometer valves require high instantaneous flow (25–35 GPM for 3–5 seconds), demanding a much higher initial peak design curve than gravity flush tanks (which fill slowly through ballcocks at 3 GPM).
- Minimum Service Size (IPC Table 604.5): Under no circumstances may a building water service entrance pipe be sized smaller than 3/4-inch nominal diameter, regardless of calculated fixture units.
Step-by-Step Example Walkthrough
" A new 3-bedroom, 2.5-bathroom single-family dwelling is built with 3 gravity tank toilets, 3 lavatories, 2 bathtub/shower combinations, 1 kitchen sink, 1 automatic dishwasher, 1 clothes washer, and 2 hose bibbs using Type L copper. "
- 1. Calculate WSFU: 3 toilets @ 2.5 = 7.5; 3 lavs @ 1.0 = 3.0; 2 tubs @ 2.0 = 4.0; 1 kitchen sink = 1.5; 1 dishwasher = 1.5; 1 washer = 2.0; 1st hose bibb = 2.5; 2nd hose bibb = 1.0. Total = 23.0 WSFU.
- 2. Hunter's Curve Peak Flow: For 23.0 WSFU on a flush-tank system per IPC Table E103.3(3), peak demand interpolates to 15.5 GPM.
- 3. Evaluate Pipe Sizing at 8.0 FPS: For 3/4" Type L copper (ID = 0.785"), velocity = (0.4085 × 15.5) / 0.785² = 10.27 FPS (Exceeds 8.0 FPS limit!).
- 4. Upsize to 1" Type L Copper (ID = 1.025"): Velocity = (0.4085 × 15.5) / 1.025² = 6.03 FPS (Complies with IPC 604.4, well below 8.0 FPS).
- 5. Meter Recommendation: Standard 3/4" AWWA disc meter handles 15.5 GPM continuous peak flow with under 5 psi pressure loss.
Final Result: Specify 1" Type L copper building service line with a 3/4" water meter. A 3/4" line would have failed code due to 10.27 FPS velocity.