General Characteristics and Assessment of General Characteristics
The design sequence
Before selecting cables you must characterise the installation. The classic sequence is: (1) maximum demand and diversity; (2) earthing arrangement and supply characteristics; (3) division into circuits; (4) protective measures; (5) external influences; (6) cross-sectional area selection; (7) protective device selection; (8) voltage drop; (9) protective conductor sizing. Questions often present a scenario and ask which characteristic must be assessed first - the answer is normally maximum demand.
Maximum demand and diversity
Maximum demand is the greatest load the installation is expected to draw; diversity recognises that not all loads run simultaneously. You apply diversity factors to arrive at design current (Ib) per circuit. A common trap: applying diversity to a load that is genuinely continuous (e.g. a dedicated 24-hour refrigeration circuit) or forgetting to check the supply capacity against total after diversity.
Earthing arrangement (system type)
Know TN-S, TN-C-S, TT and IT. TN systems use a metallic path back to the source; TT relies on a local earth electrode and therefore needs an RCD for fault protection; IT is unearthed or high-impedance earthed. Questions frequently test which arrangement requires an RCD regardless of loop impedance - for TT, yes.
External influences and division into circuits
External influences (water, dust, impact, temperature, corrosion) are coded and drive IP ratings and cable choice. Division into circuits should limit inconvenience and danger: separate lighting and power, avoid a single fault disabling everything, and keep circuits that need isolation together.
Worked example
A kitchen has a 9 kW range (40 A at 230 V) and a 3 kW water heater (13 A). They are unlikely to run fully together; apply 0.6 diversity to the range: 24 A + 13 A = 37 A design current - then verify the supply and cable can carry it. Don't apply diversity to both blindly.