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Master Plumber practice questions

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Question 1Plumbing Code and Regulations

What is the minimum uniform slope required for a 3-inch horizontal drainage pipe?

  1. 1/4 inch per foot Correct answer.
  2. 1/8 inch per foot
  3. 1/2 inch per foot
  4. 1/16 inch per foot

Explanation

Drainage pipe 3 inches and smaller must slope at least 1/4 inch per foot, while 4-inch and larger pipe may slope at least 1/8 inch per foot.

Question 2Plumbing Code and Regulations

What is the minimum depth of a liquid trap seal?

  1. 6 inches
  2. 2 inches Correct answer.
  3. 4 inches
  4. 1 inch

Explanation

A trap seal must be at least 2 inches and not more than 4 inches deep to prevent sewer gas while allowing flow.

Question 3Plumbing Code and Regulations

Which backflow prevention method provides the highest degree of protection at a plumbing fixture?

  1. Pressure vacuum breaker
  2. Double check valve assembly
  3. Reduced pressure zone assembly
  4. Air gap Correct answer.

Explanation

An air gap is an unobstructed vertical separation between the supply outlet and the flood-level rim, giving the highest protection against backflow.

Question 4Plumbing Code and Regulations

What is the minimum clearance required in front of a water closet?

  1. 21 inches
  2. 27 inches
  3. 24 inches
  4. 30 inches Correct answer.

Explanation

A water closet must have at least 30 inches of clear space from the front of the bowl to any wall or obstruction, and 15 inches from centerline to sidewall.

Question 5Plumbing Code and Regulations

How far must a vent pipe extend above a roof that is not used for any purpose other than weather protection?

  1. 12 inches
  2. 24 inches
  3. 6 inches Correct answer.
  4. 18 inches

Explanation

Vent terminals must extend at least 6 inches above the roof surface; if the roof is used for other purposes, the vent must extend at least 7 feet.

Question 6Plumbing Code and Regulations

Individual shower valves must be equipped with which feature to limit scalding?

  1. A check valve
  2. A vacuum breaker
  3. A tempering tank
  4. A pressure-balancing or thermostatic mixing valve Correct answer.

Explanation

Shower valves must be pressure-balancing, thermostatic, or combination type to limit temperature fluctuations and prevent scalding.

Question 7Plumbing Code and Regulations

Which type of trap is prohibited because it can self-siphon its seal?

  1. P-trap
  2. S-trap Correct answer.
  3. Crown-vented trap
  4. U-trap

Explanation

An S-trap has two vertical legs and can self-siphon; codes require a P-trap with a vent to protect the seal.

Question 8Plumbing Code and Regulations

What is the maximum flush volume for a water closet manufactured today under federal standards?

  1. 1.6 gallons per flush Correct answer.
  2. 2.5 gallons per flush
  3. 1.28 gallons per flush
  4. 1.0 gallons per flush

Explanation

Federal law limits water closets to 1.6 gallons per flush; many modern models use 1.28 gpf but 1.6 gpf remains the maximum.

Question 9Plumbing Code and Regulations

How must a water heater temperature and pressure relief valve discharge pipe terminate?

  1. Threaded into a floor drain for a tight seal
  2. Not more than 6 inches above the floor or drain, with an unthreaded end Correct answer.
  3. Directly connected to the sanitary drainage system
  4. Terminated outside without any drain receptor

Explanation

The discharge must be gravity-flowed to a safe receptor, terminate not more than 6 inches above the floor or drain, and have no threaded end or trap.

Question 10Plumbing Code and Regulations

Cleanouts on horizontal drainage piping must be installed at intervals not exceeding what distance?

  1. 50 feet
  2. 200 feet
  3. 150 feet
  4. 100 feet Correct answer.

Explanation

Codes require cleanouts at intervals not exceeding 100 feet on horizontal drainage piping, and at each aggregate horizontal change of direction exceeding 135 degrees.

Question 11Plumbing Code and Regulations

What is the maximum support spacing for horizontal PVC Schedule 40 drainage pipe?

  1. 2 feet
  2. 6 feet
  3. 4 feet Correct answer.
  4. 3 feet

Explanation

PVC Schedule 40 horizontal drainage pipe must be supported at intervals not exceeding 4 feet, with supports near joints as required.

Question 12Plumbing Code and Regulations

At what temperature or pressure is a water heater T&P relief valve typically set to open?

  1. 100 psi or 180°F
  2. 150 psi or 210°F Correct answer.
  3. 175 psi or 220°F
  4. 125 psi or 200°F

Explanation

Standard T&P relief valves are set to open at 150 psi or 210°F, whichever occurs first.

Question 13Plumbing Code and Regulations

Where is a backwater valve required on a drainage system?

  1. On every fixture in a building
  2. Only on water closets
  3. Only on floor drains in basements
  4. On fixtures whose flood-level rim is below the elevation of the next upstream manhole Correct answer.

Explanation

Backwater valves are required where fixtures are subject to backflow from a public sewer, typically when the fixture flood rim is below the next upstream manhole.

Question 14Plumbing Code and Regulations

What is the standard hydrostatic test requirement for a potable water distribution system?

  1. 50 psi for 30 minutes
  2. 100 psi for 60 minutes Correct answer.
  3. 150 psi for 2 hours
  4. 75 psi for 15 minutes

Explanation

The water distribution system must be tested at 100 psi or at the working pressure if higher, for at least 60 minutes.

Question 15Plumbing Code and Regulations

What is the minimum water test for a new drainage system before fixtures are installed?

  1. 15-foot head for 30 minutes
  2. 10-foot head for 15 minutes Correct answer.
  3. 5-foot head for 10 minutes
  4. 10-foot head for 5 minutes

Explanation

Drainage piping is tested with a 10-foot head of water for at least 15 minutes, or with air at 5 psi for 15 minutes where approved.

Question 16Plumbing Code and Regulations

A grease interceptor is typically required for which type of establishment?

  1. A photo processing lab
  2. A laundromat
  3. A commercial kitchen Correct answer.
  4. An auto repair garage

Explanation

Commercial kitchens and food service establishments discharge fats, oils, and grease and require a grease interceptor before the sanitary sewer.

Question 17Plumbing Code and Regulations

Which fixture discharge must be connected through an indirect waste receptor?

  1. A water closet
  2. A commercial dishwasher Correct answer.
  3. A lavatory
  4. A bathtub

Explanation

Commercial dishwashers and similar food-service equipment must discharge through an air gap or indirect waste receptor to prevent contamination of the drainage system.

Question 18Plumbing Code and Regulations

What is the minimum size of a vent pipe relative to the drain it serves?

  1. At least 2 inches in all cases
  2. At least one-quarter the drain diameter
  3. The same diameter as the drain
  4. At least one-half the drain diameter but not less than 1-1/4 inches Correct answer.

Explanation

A vent must be at least one-half the diameter of the drain it serves and never smaller than 1-1/4 inches.

Question 19Plumbing Code and Regulations

What is the maximum length of a 2-inch trap arm from the trap weir to the vent?

  1. 6 feet
  2. 10 feet
  3. 4 feet
  4. 8 feet Correct answer.

Explanation

The allowable trap arm length for a 2-inch trap is 8 feet, based on the developed length from the trap weir to the vent.

Question 20Plumbing Code and Regulations

When is a thermal expansion tank required on a water heater installation?

  1. Only on gas-fired water heaters
  2. Only on electric water heaters
  3. Never, if a T&P valve is installed
  4. On a closed system with a check valve or backflow preventer Correct answer.

Explanation

A closed water system prevents thermal expansion from pushing water back into the main, so an expansion tank is required to protect the system.

Question 21Plumbing Code and Regulations

Where must a sediment trap be installed in a gas piping system serving an appliance?

  1. Downstream of the appliance shutoff valve and as close to the appliance as practicable Correct answer.
  2. Upstream of the appliance shutoff valve
  3. At the gas meter before any branch
  4. At the street side of the gas service regulator

Explanation

The sediment trap must be installed downstream of the appliance shutoff valve and as close to the appliance as practicable so dirt and debris do not reach the controls.

Question 22Plumbing Code and Regulations

Which statement correctly distinguishes a stack vent from a vent stack?

  1. A stack vent serves only one fixture, while a vent stack serves a whole building
  2. A stack vent is the extension of a soil or waste stack above the highest fixture drain, while a vent stack is a separate vertical vent for a stack Correct answer.
  3. A stack vent is installed outdoors, while a vent stack is installed indoors
  4. A stack vent carries sewage, while a vent stack carries only air

Explanation

A stack vent is the continuation of the soil or waste stack above the highest horizontal drain; a vent stack is a separate vertical vent pipe connected to a soil or waste stack.

Question 23Plumbing Code and Regulations

What drainage fixture unit value is assigned to a water closet with a flush tank?

  1. 6 fixture units
  2. 1 fixture unit
  3. 2 fixture units
  4. 3 fixture units Correct answer.

Explanation

A flush-tank water closet is rated at 3 drainage fixture units, while a flushometer-valve water closet is rated at 6.

Question 24Plumbing Code and Regulations

A 4-inch horizontal drainage pipe is installed at its minimum slope. What is the minimum total fall over a 40-foot run?

  1. 3.3 inches
  2. 2.5 inches
  3. 5 inches Correct answer.
  4. 4 inches

Explanation

Four-inch pipe may slope at 1/8 inch per foot; 40 feet times 1/8 inch per foot equals 5 inches of fall.

Question 25Plumbing Code and Regulations

A vent terminal must be kept at least what distance from an openable window or door?

  1. 3 feet
  2. 10 feet Correct answer.
  3. 20 feet
  4. 5 feet

Explanation

Vent terminals must be at least 10 feet horizontally from or 2 feet above any openable window, door, or air intake.

Question 26Plumbing Code and Regulations

A fixture discharges indirectly through an air gap into a receptor, and the fixture outlet has an effective opening of 1/2 inch. What is the minimum required air gap?

  1. 2 inches
  2. 1/2 inch
  3. 3/4 inch
  4. 1 inch Correct answer.

Explanation

An air gap must be at least twice the diameter of the effective opening of the discharge outlet, and in no case less than 1 inch. Twice a 1/2-inch opening equals 1 inch, which also matches the absolute minimum.

Question 27Plumbing Code and Regulations

What is the minimum permitted distance from the centerline of a water closet to an adjacent side wall or partition?

  1. 24 inches
  2. 18 inches
  3. 15 inches Correct answer.
  4. 12 inches

Explanation

Plumbing code requires a water closet to be centered at least 15 inches from any side wall or partition, with a minimum 30-inch-wide clear space provided for the fixture. The clearance in front of the bowl must also be maintained.

Question 28Plumbing Code and Regulations

When the static pressure in a potable water distribution system exceeds a certain value, a pressure-reducing valve must be installed. What is that value?

  1. 60 psi
  2. 80 psi Correct answer.
  3. 50 psi
  4. 100 psi

Explanation

Code requires a pressure-reducing valve or regulator where static water pressure in the distribution system exceeds 80 psi. On a closed system, a thermal expansion control device is also typically needed once a regulator is installed.

Question 29Plumbing Code and Regulations

A vent pipe terminates through a flat roof that is used as an occupied roof deck. What is the minimum height the vent must extend above the roof surface?

  1. 4 feet
  2. 6 inches
  3. 7 feet Correct answer.
  4. 12 inches

Explanation

A vent must extend at least 6 inches above a roof surface, but where the roof is used for purposes other than weather protection, the vent must extend at least 7 feet above the roof. This keeps the terminal clear of occupants and snow.

Question 30Plumbing Code and Regulations

A lavatory is vented by an air admittance valve installed inside the cabinet beneath the fixture. At what minimum height above the fixture drain being vented must the valve be located?

  1. 1 inch
  2. 6 inches
  3. 4 inches Correct answer.
  4. 12 inches

Explanation

An air admittance valve must be installed not less than 4 inches above the horizontal branch or fixture drain it vents, and it must remain accessible. The valve also must be protected from debris and located where it will not be submerged.

Question 31Water Supply and Distribution Systems

To limit noise, erosion and excessive friction loss, building water distribution piping is generally sized so that the flow velocity does not exceed approximately:

  1. 20 feet per second
  2. 2 feet per second
  3. 10 feet per second Correct answer.
  4. 1 foot per second

Explanation

Standard design practice limits velocity in building water distribution piping to roughly 10 feet per second, and many designers use 5 to 8 feet per second for branch piping. Above that range, noise, erosion-corrosion of the pipe wall and high friction loss become significant.

Question 32Water Supply and Distribution Systems

In water supply fixture unit sizing, one fixture unit corresponds to a demand of approximately:

  1. 0.5 gallon per minute
  2. 2.5 gallons per minute
  3. 5 gallons per minute
  4. 1 gallon per minute Correct answer.

Explanation

One water supply fixture unit is defined as roughly 1 gallon per minute of demand, which allows fixture unit counts taken from code tables to be converted into a system flow in gallons per minute. The conversion is the basis for sizing services, meters and mains.

Question 33Water Supply and Distribution Systems

Which of the following provides the highest degree of protection against backflow at a cross-connection?

  1. A double check valve assembly
  2. A hose-bibb vacuum breaker
  3. An air gap Correct answer.
  4. A reduced pressure principle assembly

Explanation

An air gap is a physical separation between the potable supply and the receiving vessel, so it cannot fail mechanically and is considered complete protection. Mechanical devices such as check assemblies and vacuum breakers depend on functioning internal parts and can fail or be defeated.

Question 34Water Supply and Distribution Systems

Which device is installed to absorb the pressure surge caused by rapid valve closure in a water distribution system?

  1. A thermal expansion tank
  2. An air release valve
  3. A water hammer arrestor Correct answer.
  4. A pressure reducing valve

Explanation

Rapid closure of a valve or fixture stop creates a shock wave in the piping, and a water hammer arrestor absorbs it with a gas cushion or a spring-loaded piston. Arrestors are placed near quick-closing fixtures such as flushometer valves and solenoid-operated appliances.

Question 35Water Supply and Distribution Systems

What static pressure is produced by a column of water 100 feet high?

  1. 43.3 psi Correct answer.
  2. 23.1 psi
  3. 62.4 psi
  4. 100 psi

Explanation

One foot of water column equals 0.433 psi, so a 100-foot column produces 100 x 0.433, or 43.3 psi. This conversion is used constantly to relate elevation differences to available pressure.

Question 36Water Supply and Distribution Systems

Which valve type is best suited for throttling flow in a water line?

  1. Check valve
  2. Gate valve
  3. Globe valve Correct answer.
  4. Dielectric union

Explanation

A globe valve has a curved body and a plug or disc that moves against the flow, allowing gradual, controlled throttling without damaging the seating surfaces. Gate valves are intended to be fully open or fully closed, and their seats erode when used to throttle.

Question 37Water Supply and Distribution Systems

The minimum nominal size of the water service pipe to a single-family dwelling is generally:

  1. 1/2 inch
  2. 1-1/4 inches
  3. 3/4 inch Correct answer.
  4. 5/8 inch

Explanation

Plumbing codes set 3/4 inch as the minimum nominal service pipe size for a one- or two-family dwelling, because smaller sizes cannot deliver the fixture demand with acceptable pressure loss. Larger sizes are used when the computed demand or run length requires them.

Question 38Water Supply and Distribution Systems

A pressure reducing valve is required on a building water service when the static pressure at the building exceeds:

  1. 80 psi Correct answer.
  2. 70 psi
  3. 60 psi
  4. 50 psi

Explanation

Plumbing codes require a pressure reducing valve where the static pressure exceeds 80 psi, protecting fixtures, appliances and piping rated for lower pressures. Normal household fixtures are rated to operate well below that threshold.

Question 39Water Supply and Distribution Systems

A dwelling has a pressure reducing valve and a check valve at the meter and a tank-type water heater. Why must a thermal expansion tank be installed?

  1. Because the check valve prevents hot water from reaching the fixtures
  2. Because the water heater's temperature and pressure relief valve is undersized
  3. Because the reducing valve lowers the flow rate to the heater
  4. Because the closed system gives heated water no path to expand back into the main Correct answer.

Explanation

A check valve, backflow preventer or pressure reducing valve can isolate the building piping from the public main, creating a closed system. As the heater warms water, the expanding volume has nowhere to go and pressure can climb until the relief valve discharges, so an expansion tank provides an air cushion.

Question 40Water Supply and Distribution Systems

A testable reduced pressure principle backflow assembly in service must be:

  1. Rebuilt every two years regardless of test results
  2. Tested only when it is first installed
  3. Replaced every five years
  4. Tested at least annually by a certified tester Correct answer.

Explanation

Codes and water purveyors require testable backflow assemblies to be field tested at installation and at least annually afterward by a certified tester, with a retest after any repair. Routine rebuilding on a fixed cycle and outright replacement are done only when testing shows the assembly has failed.

Question 41Water Supply and Distribution Systems

If the flow through a given length of pipe is doubled, the friction loss increases by a factor of approximately:

  1. 1.5 times
  2. 2 times
  3. 10 times
  4. 3.5 times Correct answer.

Explanation

In the Hazen-Williams relationship, head loss varies with roughly the 1.85 power of flow, so doubling the flow multiplies the loss by about 2 raised to the 1.85, or 3.5. This is why upsizing pipe is far more effective than adding pressure when demand grows.

Question 42Water Supply and Distribution Systems

The street main pressure is 60 psi. A fixture is 30 feet above the main, and friction loss through the service and piping totals 8 psi. What pressure is available at the fixture?

  1. 47 psi
  2. 39 psi Correct answer.
  3. 52 psi
  4. 26 psi

Explanation

Elevation costs 0.433 psi per foot, so the 30-foot rise consumes about 13 psi. Subtracting both the elevation loss and the 8 psi of friction from 60 psi leaves 60 - 13 - 8 = 39 psi at the fixture.

Question 43Water Supply and Distribution Systems

The primary purpose of a fire hydrant flow test is to determine:

  1. The maximum static pressure the main can withstand
  2. The chlorine residual in the distribution system
  3. The flow available from the main at a specified residual pressure Correct answer.
  4. The minimum service line size required by code

Explanation

A flow test records static pressure, then residual pressure while a measured quantity of water is discharged, allowing the available flow at a chosen residual pressure to be estimated for that location. The results guide fire protection design and service sizing.

Question 44Water Supply and Distribution Systems

The minimum free chlorine residual typically maintained throughout a public water distribution system is about:

  1. 0.2 mg/L Correct answer.
  2. 0.02 mg/L
  3. 10 mg/L
  4. 2.0 mg/L

Explanation

Regulations require a measurable disinfectant residual to be carried throughout the distribution system, commonly cited as at least 0.2 mg/L free chlorine in the great majority of samples. This residual protects against microbial regrowth in the mains after treatment.

Question 45Water Supply and Distribution Systems

Why are dead-end water mains flushed periodically?

  1. To increase the pressure in the main
  2. To lower the pipe roughness coefficient
  3. To remove stagnant water and restore the disinfectant residual Correct answer.
  4. To raise the pH of the water

Explanation

Water in a dead end sits without flow, so the disinfectant residual decays and sediment accumulates. Planned flushing replaces that stagnant water with fresh treated water and removes deposits, protecting water quality.

Question 46Water Supply and Distribution Systems

Automatic air release valves are installed at high points of a water main because:

  1. They prevent the main from freezing in winter
  2. They vent trapped air that would restrict flow and cause erratic pressure Correct answer.
  3. They regulate pressure downstream of a pump
  4. They increase the velocity of water in the main

Explanation

Air released from solution collects at high points and reduces the effective cross-section of the pipe, raising head loss and causing surging, noisy operation and inaccurate metering. Air release valves vent the pocket so the main flows full.

Question 47Water Supply and Distribution Systems

A buried water service pipe should be installed:

  1. As shallow as possible to simplify future repairs
  2. Below the local frost depth Correct answer.
  3. At the same depth as the sewer line regardless of climate
  4. At least 12 inches deep in every climate

Explanation

Water services must be buried below the frost line established for the locality so that soil heat keeps the water above freezing, and the required depth ranges from a few inches in mild climates to several feet in cold ones. Shallow installation risks freezing and bursting during cold snaps.

Question 48Water Supply and Distribution Systems

A hose bibb on the outside of a house should be protected against backflow by:

  1. An atmospheric vacuum breaker attached to the bibb outlet Correct answer.
  2. A double check valve assembly
  3. A pressure reducing valve
  4. An air gap at the water heater

Explanation

A hose with a nozzle can be submerged in a bucket, pool or chemical sprayer, so a hose-bibb vacuum breaker screwed onto the outlet admits air and breaks suction whenever supply pressure drops. This prevents the hose contents from being drawn into the potable system.

Question 49Water Supply and Distribution Systems

Water distribution piping is commonly hydrostatically tested at:

  1. 250 psi for 24 hours
  2. 50 psi for 5 minutes
  3. 25 psi for 1 hour
  4. 100 psi or the working pressure, whichever is greater, for 15 minutes Correct answer.

Explanation

Standard plumbing code practice is to test the water distribution system with water at 100 psi, or the working pressure if that is higher, held for at least 15 minutes without a pressure drop. This confirms that joints and fittings will hold under service conditions.

Question 50Water Supply and Distribution Systems

If the diameter of a pipe is doubled while carrying the same flow, the friction loss per unit length is reduced to approximately:

  1. One eighth
  2. One sixteenth
  3. One thirty-second Correct answer.
  4. One quarter

Explanation

In the Hazen-Williams equation, head loss varies inversely with roughly the 4.87 power of diameter, so doubling the diameter cuts the loss to about 1 divided by 2 to the 4.87, or roughly one thirty-second. This steep relationship is why a small increase in pipe size produces a large gain in capacity.