South Carolina Residential Plumbing Contractor License Practice Exam

Description

Sample Questions

Question 1. A contractor is roughing in a clothes washer standpipe for a single-family dwelling. The standpipe connects to a 2-inch trap arm that will be vented by a branch vent. Assuming the trap arm slopes 1/4 inch per foot, what is the maximum developed length permitted between the trap and the vent fitting under the model residential plumbing code?

A. 6 feet
B. 5 feet
C. 8 feet
D. 12 feet

Correct Answer: C. 8 feet

Answer Explanation:
The code limits how far a fixture trap may sit from its vent so that draining water cannot siphon the trap seal. The permitted distance increases with trap arm diameter at a slope of 1/4 inch per foot: 5 feet for 1 1/4 inch, 6 feet for 1 1/2 inch, 8 feet for 2 inch, and 12 feet for 3 inch. A 2-inch trap arm may therefore run 8 feet before reaching the vent. Beyond that distance, flowing water can pull air through the trap and break the seal, allowing sewer gas into the dwelling. The installer must also keep the arm sloped within the allowed range, since too much drop can induce self-siphonage.

Why the Other Options Are Incorrect:
A. 6 feet — Incorrect. This is the limit for a 1 1/2-inch trap arm, so it understates the allowance for a 2-inch arm.
B. 5 feet — Incorrect. This applies to a 1 1/4-inch trap arm and is too restrictive here.
D. 12 feet — Incorrect. This is the limit for a 3-inch trap arm and would allow an unvented run that is too long for 2-inch pipe.

Quick Study Guide:
Trap arm distance depends on pipe diameter. Memorize the pairs: 1 1/4 inch at 5 feet, 1 1/2 inch at 6 feet, 2 inch at 8 feet, 3 inch at 12 feet, and 4 inch at 16 feet. Measure developed length from the trap to the vent fitting, and keep the slope at 1/4 inch per foot.

Question 2. A plumber is installing a gas-fired storage water heater in the garage of a single-family home. The inspector will check the temperature and pressure relief valve discharge piping. Which arrangement complies with the model residential plumbing code?

A. A 3/4-inch pipe reduced to 1/2 inch at the floor to simplify routing
B. A pipe with a shutoff valve near the end so the homeowner can stop any dripping
C. A pipe with a threaded end so a hose can be attached for draining
D. A pipe at least as large as the valve outlet that drains by gravity, with no valves or traps, to an approved location

Correct Answer: D. A pipe at least as large as the valve outlet that drains by gravity, with no valves or traps, to an approved location

Answer Explanation:
The relief valve is the water heater’s last safeguard against excess temperature and pressure, so its discharge path must never be restricted. The discharge pipe must be at least the size of the valve outlet, must not be reduced, and must drain by gravity to an approved termination. No valve, plug, cap, or trap may appear anywhere between the relief valve and the discharge end, and the end must not be threaded. Any of those features could block the flow, allowing pressure to build and potentially causing the tank to rupture. The installer should also use piping approved for the temperatures the valve can discharge.

Why the Other Options Are Incorrect:
A. A 3/4-inch pipe reduced to 1/2 inch at the floor — Incorrect. Reducing the pipe restricts discharge capacity and is not permitted.
B. A pipe with a shutoff valve near the end — Incorrect. A valve could be closed and would prevent the relief valve from discharging.
C. A pipe with a threaded end for a hose — Incorrect. A threaded end invites a cap or fitting that could block the discharge.

Quick Study Guide:
Relief valve discharge piping must never be restricted. Keep it full size, gravity-drained, and free of valves, traps, caps, and threaded ends. Terminate it in an approved location as the code directs. Know that the piping material must tolerate high-temperature discharge and that only one valve should connect to each discharge pipe.

Question 3. A plumbing contractor finishes the rough-in of drain, waste, and vent piping in a new single-family dwelling before any fixtures are set. The inspector accepts an air test as the method for checking the system. What test conditions does the model residential plumbing code require?

A. 5 pounds per square inch for 15 minutes
B. 15 pounds per square inch for 10 minutes
C. 50 pounds per square inch for 15 minutes
D. 5 pounds per square inch for 5 minutes

Correct Answer: A. 5 pounds per square inch for 15 minutes

Answer Explanation:
Rough plumbing for drain, waste, and vent systems may be tested with water or air. An air test requires a gauge pressure of 5 pounds per square inch, held for at least 15 minutes without a drop. Drain piping is designed for gravity flow, not high pressure, so a low-pressure test is enough to reveal leaks at joints while avoiding damage to fittings and plastic pipe. A water test, by comparison, requires filling the system to the overflow level of the highest opening with a head of at least 10 feet. The installer should seal all openings securely and use a gauge that is accurate for low pressures.

Why the Other Options Are Incorrect:
B. 15 pounds per square inch for 10 minutes — Incorrect. This pressure exceeds the code value and could stress joints, and the duration is also too short.
C. 50 pounds per square inch for 15 minutes — Incorrect. This resembles a water-supply pressure test and would risk damaging DWV piping and test plugs.
D. 5 pounds per square inch for 5 minutes — Incorrect. The pressure is correct, but the hold time is too short to detect slow leaks.

Quick Study Guide:
Remember the DWV rough-in test values: air at 5 psi for 15 minutes, or water to the highest overflow with at least a 10-foot head. Do not confuse these with water-supply pressure tests, which use much higher pressure. Seal openings carefully, and confirm the system holds for the full time before the inspector arrives.

Question 4. A contractor is installing a 30-foot horizontal run of 2-inch drain pipe beneath a slab in a residence. The code minimum slope for horizontal drainage piping of 2 1/2 inches or less is 1/4 inch per foot. What is the minimum total vertical drop between the upstream and downstream ends of this run?

A. 3.75 inches
B. 7.5 inches
C. 10 inches
D. 15 inches

Correct Answer: B. 7.5 inches

Answer Explanation:
Total drop equals the slope multiplied by the length of the run. At 1/4 inch per foot, a 30-foot run requires 30 × 0.25, which is 7.5 inches of fall. The slope keeps flow velocity high enough to carry solids along with the liquid and prevents stoppages. Slope that is too flat allows solids to settle, while slope that is too steep can let liquid outrun solids. Larger pipes, from 3 to 6 inches, may be installed at a minimum of 1/8 inch per foot, but this run uses 2-inch pipe, so the 1/4-inch rate governs. The contractor should set trench grade carefully before pouring the slab.

Why the Other Options Are Incorrect:
A. 3.75 inches — Incorrect. This applies a 1/8 inch per foot slope, which is allowed only for larger pipe sizes.
C. 10 inches — Incorrect. This applies about 1/3 inch per foot, which is not the code minimum and over-excavates the trench.
D. 15 inches — Incorrect. This applies 1/2 inch per foot, which is steeper than needed and could cause solids to separate from the flow.

Quick Study Guide:
Minimum drain slope is 1/4 inch per foot for pipe of 2 1/2 inches or smaller and 1/8 inch per foot for 3 to 6 inches. Multiply slope by length to find total fall. Verify grade before concealing the piping, since correcting slope after backfilling or pouring a slab is difficult and expensive.

Question 5. A homeowner complains of sewer odor from a rarely used basement floor drain. The plumber finds that the trap seal has evaporated. When the contractor replaces the trap, which water seal depth range does the model residential plumbing code require for fixture traps?

A. Not less than 1 inch and not more than 2 inches
B. Not less than 1 inch and not more than 3 inches
C. Not less than 2 inches and not more than 4 inches
D. Not less than 3 inches and not more than 6 inches

Correct Answer: C. Not less than 2 inches and not more than 4 inches

Answer Explanation:
A trap holds a water seal that blocks sewer gas from entering the building. The code requires that seal to be at least 2 inches deep and no more than 4 inches deep. A shallow seal can be lost easily to evaporation, siphonage, or back pressure, while an excessively deep seal slows self-cleansing and encourages solids to accumulate. A floor drain that is rarely used is especially prone to evaporation, so the contractor may recommend a trap primer or periodic water addition to keep the seal intact. The plumber should confirm that the trap is properly vented and installed to prevent siphonage after the repair.

Why the Other Options Are Incorrect:
A. Not less than 1 inch and not more than 2 inches — Incorrect. A seal this shallow would be vulnerable to evaporation and pressure changes.
B. Not less than 1 inch and not more than 3 inches — Incorrect. The minimum is too low, so it does not meet the 2-inch requirement.
D. Not less than 3 inches and not more than 6 inches — Incorrect. These depths exceed the code maximum and would reduce the trap’s self-cleansing action.

Quick Study Guide:
Trap seals must be 2 to 4 inches deep. Shallow seals fail by evaporation or siphonage, and deep seals clog more easily. Rarely used drains need a trap primer or regular water to keep the seal. When a trap dries out repeatedly, check venting and fixture use before assuming the trap itself is defective.