Journeyman Plumbers Isometric Drawings and Math Questions and Answers — Questions and Answers
Question 1: A plumber is installing a drain line with a simple horizontal offset of 24 inches using 45-degree fittings. What is the required center-to-center length of the travel piece?
- 24.00 inches
- 48.00 inches
- 16.97 inches
- 33.94 inches (Correct answer)
Correct answer: 33.94 inches
To calculate the length of the travel piece in a 45-degree offset, the formula is Travel = Run × 1.414. Given a run (the offset distance) of 24 inches, the calculation is 24" × 1.414 = 33.936", which rounds to 33.94 inches.
Question 2: An isometric drawing shows a rolling offset where a vent pipe rises 15 inches vertically and offsets 20 inches horizontally, using 45-degree fittings. What is the center-to-center length of the travel pipe required for this offset?
- 25.00 inches
- 35.35 inches (Correct answer)
- 49.49 inches
- 28.28 inches
Correct answer: 35.35 inches
This requires a two-step calculation. First, find the 'true offset' using the Pythagorean theorem (a² + b² = c²), where 'a' is the rise and 'b' is the horizontal offset. True Offset = √(15² + 20²) = √(225 + 400) = √625 = 25 inches. Second, calculate the travel piece by multiplying the true offset by the 45-degree constant: 25" × 1.414 = 35.35 inches.
Question 3: A journeyman plumber is installing a 3-inch horizontal sanitary drain that is 42 feet long. If the required minimum slope is 1/4 inch per foot, what is the total fall of the pipe over its entire length?
- 8.5 inches
- 14 inches
- 10.5 inches (Correct answer)
- 21 inches
Correct answer: 10.5 inches
To determine the total fall (or drop) of the pipe, multiply the length of the pipe run by the required slope per foot. The calculation is: 42 feet × 0.25 inches/foot = 10.5 inches.
Question 4: When interpreting a plumbing isometric drawing, a vertical pipe run is always represented by which of the following?
- A line drawn at a 45-degree angle.
- A true horizontal line on the page.
- A circle with a dot in the center.
- A true vertical line on the page. (Correct answer)
Correct answer: A true vertical line on the page.
In standard isometric drawing conventions, vertical pipes are always drawn as true vertical lines on the paper. Horizontal pipes are drawn at a 30-degree angle to the horizontal plane to give the illusion of three dimensions.
Question 5: A plumber needs to cut a piece of copper pipe to connect two 90-degree wrought copper elbows. The center-to-center measurement between where the fittings will be is 36 inches. The fitting allowance (take-off) for each elbow is 3/4 inch. What is the required end-to-end cut length of the pipe?
- 37 1/2 inches
- 36 inches
- 34 1/2 inches (Correct answer)
- 35 1/4 inches
Correct answer: 34 1/2 inches
To find the cut length of the pipe, the take-off for both fittings must be subtracted from the center-to-center measurement. The pipe fits into the sockets of the fittings. Calculation: 36" - (3/4" + 3/4") = 36" - 1 1/2" = 34 1/2 inches.
Question 6: In a plumbing isometric drawing, a vertical drain stack changes to a horizontal run. Which of the following fittings is typically used for this transition to ensure proper flow?
- A sanitary tee laid on its back
- A vent tee
- A long sweep 90-degree elbow or a combination wye and 1/8 bend (Correct answer)
- A standard short-radius 90-degree elbow
Correct answer: A long sweep 90-degree elbow or a combination wye and 1/8 bend
For a change of direction from vertical to horizontal in a drainage system, codes require a fitting that has a longer radius to prevent solids from stopping and causing a blockage. A long sweep 90-degree elbow or a combination wye and 1/8 bend provides a smoother transition for the waste flow compared to a standard short-radius elbow or a sanitary tee on its back.
