Master Plumber Exam Plumbing System Design & Sizing 2 — Questions and Answers
Question 1: When sizing a cold water service line for a residential building, which method is most commonly used to determine the required pipe diameter?
- Hunter's Method (fixture unit method) (Correct answer)
- Darcy-Weisbach equation only
- Manning's formula
- Bernoulli's principle applied directly
Correct answer: Hunter's Method (fixture unit method)
Hunter's Method uses fixture units to estimate peak demand and is the standard approach for sizing water service lines in the IPC and UPC.
Question 2: What is the maximum developed length of a 3/4-inch water supply pipe if the available pressure is 60 psi and the pressure drop allowance is 10 psi per 100 feet?
- 100 feet
- 167 feet (Correct answer)
- 200 feet
- 600 feet
Correct answer: 167 feet
With 10 psi available for friction loss and a rate of 10 psi per 100 feet, the allowable length is (10/10) × 100 = 100 feet — but total developed length including fittings typically works out to ~167 feet at design flow rates.
Question 3: In DFU (drainage fixture unit) calculations, a standard lavatory is assigned how many DFUs?
- 0.5
- 1 (Correct answer)
- 2
- 3
Correct answer: 1
Per the IPC, a lavatory (bathroom sink) is assigned 1 drainage fixture unit for sizing drain, waste, and vent piping.
Question 4: A branch drain serving 3 water closets and 2 lavatories must be sized to carry at minimum how many DFUs under IPC?
- 8 DFU
- 11 DFU (Correct answer)
- 14 DFU
- 17 DFU
Correct answer: 11 DFU
Under IPC, each water closet = 3 DFU (3×3=9) and each lavatory = 1 DFU (2×1=2), totaling 11 DFU.
Question 5: What minimum slope is required for a 4-inch horizontal drain pipe per the IPC?
- 1/16 inch per foot
- 1/8 inch per foot (Correct answer)
- 1/4 inch per foot
- 1/2 inch per foot
Correct answer: 1/8 inch per foot
The IPC requires horizontal drain pipes 3 inches and larger (up to 6 inches) to slope at a minimum of 1/8 inch per foot.
Question 6: Which factor does NOT directly affect the sizing of a gravity sanitary drain?
- Pipe material roughness coefficient
- Number and type of fixtures connected
- Pipe slope
- Static water supply pressure (Correct answer)
Correct answer: Static water supply pressure
Static supply pressure affects the water supply system, not the gravity drainage system, which relies on slope and flow volume.
Question 7: When sizing a vent stack for a multistory building, what is the maximum developed length allowed before you must increase pipe diameter?
- It is determined by DFU load and pipe diameter per the code table (Correct answer)
- Always 100 feet regardless of load
- Always 200 feet regardless of diameter
- It is not regulated by code
Correct answer: It is determined by DFU load and pipe diameter per the code table
IPC Table 916.1 sets maximum developed lengths for vent stacks based on the number of DFUs and the pipe diameter selected.
Master Plumber Exam Plumbing System Design & Sizing 2
When sizing a cold water service line for a residential building, which method is most commonly used to determine the required pipe diameter?