Wiring Systems
Wiring Systems is about choosing and installing the containment and cable route. The exam tests whether you can match the system to the environment, apply the installation rules on support, separation, filling and fire stopping, and recognise where a system must be protected or earthed.
Selecting the system
Match the system to the environment: mechanical protection where impact is possible, IP rating in wet or dusty areas, corrosion resistance in plant rooms and car parks, temperature rating near heaters, fire resistance for life-safety circuits, and accessibility for maintenance. Typical choices are PVC conduit concealed in slab or plaster, surface metal conduit or trunking, cable tray and basket, busbar trunking for high-rise risers, armoured cable for buried or external runs, and mineral-insulated or fire-resistant cable for fire alarm and emergency lighting.
Installation rules that are examined
- Conduit fill should not exceed about 40% of the internal cross-section, and trunking about 45%, so cables can be drawn in without damage.
- Respect the manufacturer's minimum bending radius; armoured and mineral cables need a much larger radius than flexible cords.
- Support at the spacing required for the system, with additional support at boxes and changes of direction.
- Keep band I (SELV/PELV) circuits separated from band II circuits by at least 50 mm or a barrier; keep fire alarm and emergency lighting circuits separate from normal wiring.
- Earth metal conduit, trunking and tray, bond across expansion couplers and flexible connections, and fit earthing nuts at boxes.
- Do not run cables inside thermal insulation unless the current rating is derated, and keep concealed cables within recognised safe zones.
- Fire-stop every penetration to the fire resistance of the element breached.
Worked example
A 4 mm² PVC single-core circuit, tabulated at 30 A for an enclosed method, runs in surface trunking in a plant room at 45 °C shared with four other loaded circuits. Apply 0.87 for ambient temperature and 0.60 for grouping of five circuits: 30 × 0.87 × 0.60 = 15.7 A. That will not carry a 20 A load, so the answer is to increase to 10 mm² or improve the route and reduce grouping, not to keep the device at 20 A.
Traps that cost marks
Not applying ambient and grouping factors; treating trunking as a protective conductor without continuity checks; leaving a cable outside the safe zone in a stud wall; and forgetting that a fire-rated wall needs an equally rated seal.