Wiring Systems and Installation Practice
Wiring Systems and Installation Practice tests how you choose and install a wiring method that is safe, compliant and suited to the building. Questions cover conduit and trunking, cable routing and support, derating factors, segregation, special locations, accessories and workmanship standards.
Core rules
- Wiring systems: surface or concealed conduit, trunking, cable tray, PVC/PVC clipped direct, SWA for underground and exposed runs, mineral-insulated cable for fire-critical circuits. Choose by environment, mechanical protection, appearance and cost.
- Derating factors multiply the tabulated current rating: Ca for ambient temperature, Cg for grouping, Ci for thermal insulation, Cf for the protective device, Cs for soil. Required: Ib is less than or equal to In, which is less than or equal to Iz after derating.
- Conduit fill should not exceed about 40 percent of the internal cross-section; avoid tight bends so cables draw in without insulation damage.
- Segregation: band I (low voltage) and band II (for example telecoms and extra-low-voltage signalling) must be separated, or separated by a barrier within the same enclosure.
- Special locations: bathroom zones 0, 1 and 2 with IP-rated equipment; socket outlets are not permitted within 3 m of the zone 1 boundary unless they are shaver units.
- Good workmanship: grommets at metal boxes, no inaccessible joints, no joints inside conduit, correct polarity, secure accessories, and cables protected from nails and screws.
Worked example: derating
2.5 mm squared PVC twin-and-earth clipped direct has a tabulated rating of 27 A (reference method C). Site conditions: 35 degrees C ambient (Ca 0.87) and two circuits bunched together (Cg 0.80), no thermal insulation. Iz = 27 x 0.87 x 0.80 = 18.8 A. A 16 A MCB is acceptable because 16 A is below 18.8 A. A 20 A MCB is not, because 20 A exceeds the derated capacity. If the cable also passed through insulation, Ci might be 0.5, reducing capacity to about 11.7 A and forcing a 10 A device or a larger cable.
Traps
- Using the tabulated rating for the wrong reference method.
- Forgetting ambient and grouping correction in hot roof spaces.
- Making joints inside conduit or leaving joints inaccessible.
- Overfilling conduit and damaging insulation while drawing cables in.
- Ignoring bathroom zone rules and IP ratings.