Electrical Installation Technology and Wiring Systems
This domain tests your ability to choose the right wiring system and cable, and to prove it is protected and within voltage drop limits for the duty it serves. It links the protective device, the installation method and the environment.
The selection rule
Every circuit must satisfy Ib ≤ In ≤ Iz, where Ib is design current, In the device rating and Iz the cable's capacity after correction. Voltage drop from the origin must stay within about 3% for lighting and 5% for other uses.
Correction factors
Iz = It × Ca × Cg × Ci × Cc, where It is the tabulated value, Ca corrects for ambient temperature, Cg for grouping with other loaded circuits, Ci for cable surrounded by thermal insulation, and Cc for burial or soil conditions. Ignoring Ci is the most common route to an overloaded cable, because a cable buried in loft insulation may be reduced to roughly half its tabulated rating.
Earthing systems and typical systems
TN-S uses a separate earth from the supply; TN-C-S uses a combined neutral and earth (PEN) conductor split at the origin; TT relies on a local electrode and therefore on RCD protection. Typical wiring: PVC singles in conduit for dry protected runs, PVC/PVC twin and earth for domestic circuits, SWA for underground and exposed work, MICC where fire resistance is needed, and LSZH where smoke emission in escape routes matters.
Worked example
A 20 A radial socket circuit is wired in 2.5 mm² over 22 m with a tabulated drop of 18 mV/A/m. Drop = 18 × 20 × 22 / 1000 = 7.9 V, which is 3.4% of 230 V - acceptable against the 5% limit. If the run were 40 m, the same cable would give 6.2% and a larger conductor would be needed.
Traps
Using tabulated Iz without correction factors; forgetting grouping when several circuits share trunking; applying 400 V to a single-phase drop calculation; and assuming a 32 A ring final can serve any floor area - its practical limit is about 100 m².