Plumbing NC I common mistakes
Plumbing NC I · Understand the traps and how to avoid them.
Reading a plan and taking fixture counts or pipe lengths straight off the drawing without cross-checking the schedule, legend, or general notes, so quantities are short or duplicated.
Candidates treat the plan view as the complete source and skip the legend and schedules, which is where pipe sizes, fixture types, and material specs are actually defined. Plan reading is often practised in isolation from take-off.
What to do instead
Before counting anything, read the drawing set in a fixed order: title block, legend, notes, then plans, then schedules and details. Build the take-off from the schedule (fixture units, pipe sizes) and use the plan only for routing and lengths. Reconcile counts against the schedule at the end.
Calculating pipe lengths and fitting counts in millimetres on the drawing but ordering or reporting in metres without converting, producing answers off by a factor of 1000 or a decimal shift.
Plumbing drawings are dimensioned in millimetres while materials are bought in metres or lengths; candidates mix units mid-calculation, especially under time pressure.
What to do instead
Write the unit next to every number in the working. Convert all dimensions to a single working unit (metres) at the start of each calculation, and sanity-check the final answer against a rough physical estimate (e.g., a room 4 m long cannot need 400 m of pipe).
Selecting the wrong pipe diameter or gradient for a drainage run because the candidate applies a rule of thumb instead of the sizing table or the specified fall.
Rules of thumb (e.g., '100 mm everywhere', '1 in 60 is fine') are remembered from site talk and override the actual drainage-sizing and gradient requirements. Candidates also confuse fixture-unit loading with pipe diameter directly.
What to do instead
Use the drainage sizing table: total the fixture units served by the section, then read the required diameter and the minimum gradient for that diameter. State the gradient as a ratio (1:60, 1:40) and check that the run achieves it over its length.
Joining solvent-welded PVC/uPVC pipe without cleaning and priming, or applying cement to only one surface, then presenting the joint as complete.
Candidates rush the sequence or do not know the full procedure; primer and cleaner look optional next to the cement.
What to do instead
Follow the fixed sequence every time: cut square, deburr and chamfer, dry-fit and mark insertion depth, clean with cleaner, apply primer to both surfaces, apply cement to both, assemble with a quarter-turn and hold until set. Wipe the excess and do not disturb the joint.
Threading steel pipe with the die head set for the wrong nominal size or without adequate cutting oil, producing a tapered, galled, or undersized thread that leaks or will not seat.
Die heads and pipe sizes are easy to mismatch, and oil is often skipped to save time. Candidates also over-tighten the vice or feed the die too fast.
What to do instead
Confirm the die size against the pipe nominal size before starting, clamp the pipe so it cannot rotate, apply cutting oil continuously, and cut in stages with a slight back-off to clear chips. Check the thread with a thread gauge and test-fit a fitting by hand before final tightening.
Bending copper tube to the wrong radius or without an adequate bend allowance, so the developed length is short and the installed run does not reach the fitting.
Candidates measure centre-to-centre and cut to that length, forgetting that a bend consumes tube length and that the bend radius depends on the tube diameter and bender.
What to do instead
Calculate the developed length: measure the two straight legs to the tangent points, add the arc length for the chosen radius (arc = radius x bend angle in radians), and mark the tube at the tangent points. Use the correct former for the tube size and anneal only where the specification allows.
Installing a trap or fixture connection without the required vent, or with an incorrect trap seal depth, and treating the drain as adequate because water flows.
Venting is invisible in service and candidates focus on the visible drain path. Trap seal depth and vent distance are specific code values that are guessed rather than looked up.
What to do instead
For every fixture, confirm the trap seal depth required by the code, the maximum distance from the trap to its vent, and the vent size. Draw the vent path on the isometric before installing, and check that no fixture is wet-vented outside the permitted arrangement.
Ignoring isolation, lockout, or PPE because the task is short, e.g., working live on a circuit, cutting overhead without eye protection, or entering a trench without shoring.
Short tasks feel low-risk, and experienced candidates normalise hazards. Time pressure during a practical assessment reinforces the shortcut.
What to do instead
Apply the hierarchy of control before each task: isolate and lock out where required, verify isolation, wear the PPE the task demands (eye, hearing, hand, respiratory as applicable), and never enter an unsupported excavation. State the control out loud when planning the task so it becomes deliberate.
Choosing the wrong tool for the material or task, e.g., using a wood saw on plastic pipe, a pipe wrench on chrome or plastic fittings, or a hammer where a mallet is needed, causing damage or a poor joint.
Candidates reach for the nearest familiar tool and do not match jaw type, tooth pitch, or blade material to the workpiece.
What to do instead
Match the tool to the material before starting: fine-tooth or plastic-specific blades for plastic pipe, strap or adjustable wrenches on finished and plastic fittings, pipe wrenches only on steel. Inspect the tool for damage and confirm it is rated for the task.
Reporting a measurement or test result that does not match what was actually observed, e.g., recording a pressure test as passed before the hold period, or rounding a dimension to the nearest convenient figure.
Candidates want to finish, and a test that 'looks fine' is recorded as passed. Measurement rounding is a habit carried from rough work.
What to do instead
Record only what the instrument or tape shows, to the precision the specification requires. For pressure tests, complete the full hold period at the specified pressure, record start and end readings and any drop, and note the ambient conditions. If a result is marginal, report it as such rather than rounding it into a pass.