Technical Drawing, Sketching and Trade Calculations
This domain tests whether you can read and produce working drawings, sketch a pipe route so another plumber can follow it, and use trade arithmetic for lengths, falls, volumes and pressures. The exam sets short calculation items and asks you to interpret line types, projections and plumbing symbols.
Drawing conventions
- First-angle projection is the standard in Ghanaian and British practice: the plan sits below the front view, and the view from the left is drawn on the right. Third angle reverses this.
- Line types: continuous thick for visible outlines, continuous thin for dimensions and leaders, short dashes for hidden detail, chain-dotted for centre lines, wavy for break lines.
- Scales: 1:20 for details, 1:50 for room layouts, 1:100 for site plans. At 1:50, 20 mm on paper equals 1 m on site.
- Schematics are single-line diagrams showing pipe size, material, flow direction, valves and fittings; isometrics show the three axes at 30° to the horizontal.
Calculations you must show
- Area, perimeter and volume; litres = cubic metres × 1000.
- Fall = length × gradient, with the gradient written as a fraction (1:60 = 1/60).
- Static pressure: 1 m head of water ≈ 9.81 kPa ≈ 0.098 bar.
- Pipe volume = πr²L, used for chlorination dosing and test quantities.
- Bend gain for 90° = 0.43 × radius; developed length of an arc = radius × angle in radians.
Worked example
A 110 mm drain runs 18 m at 1:60 from a building. Fall = 18 ÷ 60 = 0.30 m = 300 mm. If the upstream invert is 600 mm below ground, the downstream invert is 900 mm below the same datum. Reason: convert the gradient to a fraction, multiply by the length in matching units, then add to the upstream invert. Multiplying 18 × 60 is the classic error and gives a nonsense figure.
Traps
- Drawing the left-hand view on the left because you are used to third-angle textbooks.
- Reading 1:60 as 60 mm per metre.
- Adding fall to the wrong invert, or mixing millimetres and metres in one line.
- Forgetting to scale off the drawing before ordering material.
- Sketching a route with no sizes, no fall and no flow direction - it cannot be installed from.