CO Master Plumber Exam Questions
And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A| Instant Download Pdf
1. What is the minimum slope required for a horizontal drainage pipe?
a) 1/16 inch per foot
b) 1/8 inch per foot
c) 1/4 inch per foot
d) 1/2 inch per foot
Rationale: A minimum slope of 1/4 inch per foot ensures proper
gravity drainage and prevents standing water in horizontal drainage
pipes.
2. Which material is commonly used for underground potable water
piping?
a) PVC Schedule 40
b) Copper Type K
c) Cast iron
d) Galvanized steel
Rationale: Copper Type K is commonly used underground for potable
water because it has a thick wall suitable for burial and is corrosion
resistant.
3. What is the standard spacing for plumbing vent pipes on a 4-inch
diameter drain line?
a) 10 feet
b) 20 feet
c) 30 feet
d) 40 feet
Rationale: Plumbing codes typically require a maximum horizontal
distance of 20 feet between vent pipes for 4-inch drain lines to prevent
siphoning of traps.
,4. What type of valve is typically used to shut off water to an individual
fixture?
a) Ball valve
b) Gate valve
c) Angle stop valve
d) Check valve
Rationale: Angle stop valves are installed at fixtures to allow
individual shut-off without affecting the entire system.
5. What is the maximum allowable lead content for potable water pipes in
the U.S.?
a) 4%
b) 2%
c) 0.25%
d) 1%
Rationale: The Safe Drinking Water Act limits lead content in
potable water pipes to 0.25% to prevent lead poisoning.
6. Which fixture requires an air gap in its drain connection?
a) Water closet
b) Lavatory
c) Dishwasher
d) Shower
Rationale: Dishwashers require an air gap to prevent backflow
contamination into the potable water supply.
7. What is the minimum diameter for a building sewer connecting to the
public sewer?
a) 3 inches
b) 4 inches
c) 6 inches
d) 8 inches
Rationale: Building sewers must be at least 6 inches in diameter to
handle typical wastewater flow and prevent blockages.
8. What is the purpose of a trap in plumbing systems?
a) Reduce water pressure
b) Prevent sewer gases from entering the building
c) Filter solids
d) Increase flow rate
, Rationale: A trap holds water to create a barrier that blocks sewer
gases from entering the interior space.
9. Which type of pipe is most resistant to corrosion in acidic soils?
a) Galvanized steel
b) PVC
c) Cast iron
d) Copper
Rationale: PVC pipes are resistant to chemical corrosion and are
commonly used in acidic or aggressive soil conditions.
10. What is the minimum distance a water heater relief valve
discharge must be from the floor?
a) 6 inches
b) 6 inches to 12 inches
c) 12 inches
d) 24 inches
Rationale: The valve discharge must terminate 6–12 inches above the
floor to prevent scalding and ensure proper drainage.
11. Which fixture requires a minimum 2-inch drain?
a) Lavatory
b) Shower
c) Dishwasher
d) Water closet
Rationale: Showers require at least a 2-inch drain to handle expected
wastewater flow efficiently.
12. What is the minimum distance a cleanout should be from a
change of direction?
a) 1 foot
b) 2 feet
c) 3 feet
d) 4 feet
Rationale: Cleanouts must be located within 2 feet of changes of
direction to allow proper access for clearing blockages.
13. What type of joint is used for joining copper water pipes?
a) Threaded
b) Solvent weld
c) Soldered
d) Flanged
And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A| Instant Download Pdf
1. What is the minimum slope required for a horizontal drainage pipe?
a) 1/16 inch per foot
b) 1/8 inch per foot
c) 1/4 inch per foot
d) 1/2 inch per foot
Rationale: A minimum slope of 1/4 inch per foot ensures proper
gravity drainage and prevents standing water in horizontal drainage
pipes.
2. Which material is commonly used for underground potable water
piping?
a) PVC Schedule 40
b) Copper Type K
c) Cast iron
d) Galvanized steel
Rationale: Copper Type K is commonly used underground for potable
water because it has a thick wall suitable for burial and is corrosion
resistant.
3. What is the standard spacing for plumbing vent pipes on a 4-inch
diameter drain line?
a) 10 feet
b) 20 feet
c) 30 feet
d) 40 feet
Rationale: Plumbing codes typically require a maximum horizontal
distance of 20 feet between vent pipes for 4-inch drain lines to prevent
siphoning of traps.
,4. What type of valve is typically used to shut off water to an individual
fixture?
a) Ball valve
b) Gate valve
c) Angle stop valve
d) Check valve
Rationale: Angle stop valves are installed at fixtures to allow
individual shut-off without affecting the entire system.
5. What is the maximum allowable lead content for potable water pipes in
the U.S.?
a) 4%
b) 2%
c) 0.25%
d) 1%
Rationale: The Safe Drinking Water Act limits lead content in
potable water pipes to 0.25% to prevent lead poisoning.
6. Which fixture requires an air gap in its drain connection?
a) Water closet
b) Lavatory
c) Dishwasher
d) Shower
Rationale: Dishwashers require an air gap to prevent backflow
contamination into the potable water supply.
7. What is the minimum diameter for a building sewer connecting to the
public sewer?
a) 3 inches
b) 4 inches
c) 6 inches
d) 8 inches
Rationale: Building sewers must be at least 6 inches in diameter to
handle typical wastewater flow and prevent blockages.
8. What is the purpose of a trap in plumbing systems?
a) Reduce water pressure
b) Prevent sewer gases from entering the building
c) Filter solids
d) Increase flow rate
, Rationale: A trap holds water to create a barrier that blocks sewer
gases from entering the interior space.
9. Which type of pipe is most resistant to corrosion in acidic soils?
a) Galvanized steel
b) PVC
c) Cast iron
d) Copper
Rationale: PVC pipes are resistant to chemical corrosion and are
commonly used in acidic or aggressive soil conditions.
10. What is the minimum distance a water heater relief valve
discharge must be from the floor?
a) 6 inches
b) 6 inches to 12 inches
c) 12 inches
d) 24 inches
Rationale: The valve discharge must terminate 6–12 inches above the
floor to prevent scalding and ensure proper drainage.
11. Which fixture requires a minimum 2-inch drain?
a) Lavatory
b) Shower
c) Dishwasher
d) Water closet
Rationale: Showers require at least a 2-inch drain to handle expected
wastewater flow efficiently.
12. What is the minimum distance a cleanout should be from a
change of direction?
a) 1 foot
b) 2 feet
c) 3 feet
d) 4 feet
Rationale: Cleanouts must be located within 2 feet of changes of
direction to allow proper access for clearing blockages.
13. What type of joint is used for joining copper water pipes?
a) Threaded
b) Solvent weld
c) Soldered
d) Flanged
