ICC G24 National Standard Master Plumber Exam — Questions and Answers

Question 1: What is 'rough-in plumbing'?

Temporary plumbing installed during construction
Plumbing installed without permits
The installation of supply and drain pipes before walls and floors are finished (Correct answer)
The initial flushing of new pipe systems

Correct answer: The installation of supply and drain pipes before walls and floors are finished

Rough-in plumbing refers to the installation of supply and drain/vent pipes during the framing stage, before walls, ceilings, and floors are closed in.

Question 2: What is the purpose of a lead-free requirement in US plumbing codes?

To meet recycling standards
To protect potable water from lead contamination in fittings and solder (Correct answer)
To reduce installation costs
To reduce pipe weight

Correct answer: To protect potable water from lead contamination in fittings and solder

Since 2014, the Safe Drinking Water Act requires all plumbing fittings, fixtures, and solder in contact with potable water to be lead-free (less than 0.25% weighted average lead content).

Question 3: What type of trap is prohibited in most US plumbing codes?

S-trap (Correct answer)
Running trap
Drum trap
P-trap

Correct answer: S-trap

S-traps are prohibited because they are prone to self-siphonage, which can empty the water seal and allow sewer gases into the building.

Question 4: What is the required minimum distance between a water supply line and a drain line when running parallel in a wall?

12 inches
No minimum required (Correct answer)
1 inch
6 inches

Correct answer: No minimum required

The IPC does not specify a minimum horizontal separation distance between supply and drain lines running parallel, but they must not come into contact.

Question 5: What fitting is typically installed at each gas appliance to allow individual shutoff without interrupting the entire system?

Ball check valve
Union fitting
Tee fitting
Appliance shutoff valve (gas cock) (Correct answer)

Correct answer: Appliance shutoff valve (gas cock)

An appliance shutoff valve (gas cock or stop valve) is required at each appliance connection to allow safe individual isolation for service or emergencies.

Question 6: Which model plumbing code is most widely adopted in the United States?

International Plumbing Code (IPC)
National Standard Plumbing Code (NSPC)
Uniform Plumbing Code (UPC)
Both UPC and IPC are widely adopted (Correct answer)

Correct answer: Both UPC and IPC are widely adopted

Both the Uniform Plumbing Code (UPC) and the International Plumbing Code (IPC) are widely adopted in the US, with western states generally using UPC and eastern states generally using IPC.

Question 7: What is the maximum flow rate for a residential showerhead under current federal EPAct standards?

2.5 GPM (Correct answer)
1.5 GPM
2.0 GPM
3.0 GPM

Correct answer: 2.5 GPM

Federal EPAct standards set the maximum flow rate for residential showerheads at 2.5 gallons per minute (GPM) at 80 psi.

Question 8: Where must a manual gas shutoff valve be installed for a new gas appliance?

In the main electrical panel room
Outside the building near the foundation
At the gas meter only
Within 6 feet of the appliance in the same room, in an accessible location (Correct answer)

Correct answer: Within 6 feet of the appliance in the same room, in an accessible location

Code requires an accessible manual shutoff valve within 6 feet of each gas appliance in the same room so it can be quickly reached in an emergency.

Question 9: What type of valve is used as the main shutoff valve for a residential water supply?

Globe valve
Ball valve or gate valve (Correct answer)
Butterfly valve
Needle valve

Correct answer: Ball valve or gate valve

Ball valves or gate valves are used as main shutoff valves because they provide full-bore flow when open and a complete shutoff when closed.

Question 10: What is the purpose of a check valve in a water supply system?

To measure water consumption
To allow water flow in only one direction (Correct answer)
To reduce water pressure
To filter debris from water

Correct answer: To allow water flow in only one direction

A check valve allows water to flow in only one direction, preventing backflow that could contaminate the supply system or damage equipment.

Question 11: What is the purpose of a sediment trap (dirt leg) on a gas line?

To measure gas flow
To prevent gas line freezing
To collect debris and moisture before it reaches the gas appliance valve (Correct answer)
To reduce gas pressure

Correct answer: To collect debris and moisture before it reaches the gas appliance valve

A sediment trap (dirt leg) is a downward-pointing nipple and cap installed near gas appliances that collects debris and condensate before they can clog or damage the appliance's gas valve.

Question 12: What is the maximum flush volume for a water closet (toilet) under federal standards?

2.0 GPF
1.6 GPF (Correct answer)
1.28 GPF
1.0 GPF

Correct answer: 1.6 GPF

Federal plumbing standards require water closets to use no more than 1.6 gallons per flush (GPF), with WaterSense certified models at 1.28 GPF.

Question 13: What is a basin wrench used for?

Reaching and tightening faucet mounting nuts in tight spaces under sinks (Correct answer)
Tightening supply line connections
Tightening large pipe fittings under sinks
Removing drain basket strainers

Correct answer: Reaching and tightening faucet mounting nuts in tight spaces under sinks

A basin wrench has a long handle with a pivoting jaw that allows tightening or loosening the faucet mounting nuts in the cramped space between the sink basin and the wall.

Question 14: What is the purpose of bonding a metal water pipe system to the electrical ground?

To prevent water hammer
To reduce pipe corrosion from chemical reactions
To improve water pressure
To equalize electrical potential and prevent electric shock from stray currents (Correct answer)

Correct answer: To equalize electrical potential and prevent electric shock from stray currents

Bonding metal pipes to the electrical grounding system equalizes electrical potential across the plumbing system, protecting occupants from electric shock due to stray currents or ground faults.

Question 15: What is 'potable water' and why is it important in plumbing codes?

Heated water above 120°F
Water treated with chlorine only
Recycled greywater
Water that is safe for human consumption and meets EPA standards (Correct answer)

Correct answer: Water that is safe for human consumption and meets EPA standards

Potable water is water that is safe for human consumption, cooking, and personal hygiene, and plumbing codes contain extensive provisions to protect it from contamination.

Question 16: What is a flapper valve in a toilet and what does it do?

A float that controls water level
A rubber seal that opens during flushing to release tank water into the bowl (Correct answer)
A valve that controls water entering the tank
A check valve in the supply line

Correct answer: A rubber seal that opens during flushing to release tank water into the bowl

The flapper is a rubber seal at the bottom of the toilet tank that lifts when the toilet is flushed, allowing tank water to flow into the bowl, then reseals the tank.

Question 17: What type of test is performed on a new DWV system to verify it is watertight?

Water test or air test at specified pressures per code (Correct answer)
Dye test only
Air pressure test at 100 psi
Visual inspection only

Correct answer: Water test or air test at specified pressures per code

New DWV systems are tested with either a water test (filled to the highest point) or an air test at pressures specified by the applicable plumbing code to verify no leaks.

Question 18: What tool is used to cut large-diameter cast iron pipe in the field?

Snap cutter (soil pipe cutter) or angle grinder with cutting disc (Correct answer)
Reciprocating saw only
Pipe cutter wheel only
Hacksaw

Correct answer: Snap cutter (soil pipe cutter) or angle grinder with cutting disc

A snap cutter (soil pipe cutter) uses hardened cutting wheels on a chain to score around large cast iron pipe and snap it cleanly, or an angle grinder with a cutting disc can be used.

Question 19: What safety device is required on a gas water heater flue to prevent backdrafting?

A barometric damper or draft hood (Correct answer)
An expansion tank
A sediment trap
A pressure relief valve

Correct answer: A barometric damper or draft hood

A draft hood (barometric damper) is required on atmospherically vented gas water heaters to prevent flue gases from backdrafting into the living space.

Question 20: What type of sink mounting method sits on top of the countertop with the basin above the surface?

Undermount sink
Drop-in (self-rimming) sink
Vessel sink (Correct answer)
Integrated sink

Correct answer: Vessel sink

A vessel sink sits entirely on top of the countertop as a bowl, requiring a taller faucet and providing a design-forward appearance.

Question 21: What is the maximum distance a lavatory trap can be from its vent?

6 feet
2 feet
8 feet
4 feet (Correct answer)

Correct answer: 4 feet

Under the IPC, a lavatory (sink) with a 1.25-inch pipe has a maximum trap-to-vent distance of 3.5 feet, but the common code limit for lavatories is 5 feet depending on jurisdiction — 4 feet is the most commonly tested standard answer.

Question 22: What is the purpose of a pressure reducing valve (PRV) in a residential plumbing system?

To reduce high incoming water pressure to a safe level (Correct answer)
To boost low water pressure
To filter sediment from water
To prevent water hammer

Correct answer: To reduce high incoming water pressure to a safe level

A PRV reduces high municipal water pressure to a safe operating pressure (typically 50-60 psi) to protect fixtures, appliances, and pipes.

Question 23: What is the maximum typical low-pressure natural gas system operating pressure for residential appliances?

2 psi
0.5 psi (14 in. WC) (Correct answer)
10 psi
5 psi

Correct answer: 0.5 psi (14 in. WC)

Residential natural gas appliances typically operate at low pressure — commonly 7 in. WC for most appliances, with a maximum of about 14 in. WC (0.5 psi) at the appliance.

Question 24: What is the term for a fitting that connects a branch drain to a vertical stack?

Wye fitting
Sanitary tee (Correct answer)
P-trap
Cleanout

Correct answer: Sanitary tee

A sanitary tee is used to connect branch drain lines to a vertical stack, with its swept inlet directing flow downward to prevent turbulence.

Question 25: What type of wax ring alternative provides a better seal for uneven floors when installing a toilet?

Silicone caulk only
Double-stacked wax ring or foam gasket seal (Correct answer)
Teflon tape wrap
Plumber's putty

Correct answer: Double-stacked wax ring or foam gasket seal

For uneven floors or extra-thick flooring, a double-stacked wax ring or a foam gasket seal (like Fluidmaster) provides a better, adjustable seal than a single wax ring.

Question 26: What is the primary purpose of a vent pipe in a plumbing system?

To drain condensation
To maintain air pressure and prevent siphoning of trap seals (Correct answer)
To supply fresh water to fixtures
To increase water pressure

Correct answer: To maintain air pressure and prevent siphoning of trap seals

Vent pipes maintain atmospheric pressure in the drain system, preventing the siphoning of water from trap seals that would allow sewer gases to enter the building.

Question 27: Which type of water heater heats water only when it is needed, without a storage tank?

Storage tank water heater
Heat pump water heater
Tankless (on-demand) water heater (Correct answer)
Solar water heater

Correct answer: Tankless (on-demand) water heater

A tankless (on-demand) water heater heats water directly as it flows through the unit, eliminating standby heat loss associated with storage tanks.

Question 28: What is the minimum burial depth for a residential water service line in a climate with a 36-inch frost depth?

12 inches below grade
24 inches below grade
48 inches below grade always
At or below the local frost line (36 inches) (Correct answer)

Correct answer: At or below the local frost line (36 inches)

Water service lines must be buried at or below the local frost line depth to prevent freezing; if the frost depth is 36 inches, the pipe must be buried at least 36 inches deep.

Question 29: What is the minimum required separation between a gas meter and electrical service equipment?

No minimum separation required
3 feet (Correct answer)
10 feet
5 feet

Correct answer: 3 feet

Most codes require at least 3 feet of clearance between a gas meter and electrical service equipment to reduce the risk of a spark igniting a gas leak.

Question 30: Which type of valve provides the best flow control for plumbing systems because it operates fully open or fully closed?

Gate valve (Correct answer)
Needle valve
Globe valve
Butterfly valve

Correct answer: Gate valve

Gate valves are designed to operate fully open or fully closed, providing minimal flow restriction when open and are ideal as isolation valves.

Question 31: What is the term for the main vertical drain pipe that carries waste from upper floors to the building drain?

Building sewer
Waste arm
Branch drain
Soil stack (Correct answer)

Correct answer: Soil stack

A soil stack is the large vertical pipe that collects waste from fixtures on multiple floors and carries it down to the building drain.

ICC G24 National Standard Master Plumber Exam

The ICC G24 National Standard Master Plumber exam tests knowledge of the International Plumbing Code (IPC), covering plumbing systems installation, safety regulations, drainage, venting, water supply, and fixture requirements for master plumber licensure.

Questions

35 min

Time Limit

70%

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