Domestic Wiring Systems
Domestic Wiring Systems
Domestic Wiring Systems tests the wiring methods used in houses, how circuits are divided, and how conductor sizes and sub-circuits are selected. Expect comparison and load-estimation questions.
Core rules and concepts
- Common systems: cleat wiring (temporary, cheap, exposed), casing-and-capping (surface PVC channel), batten wiring (wooden batten with TRS or CTS cable), conduit wiring (surface or concealed), and concealed conduit (neat, costly, difficult to modify).
- A domestic installation has a service connection, main switch, energy meter, distribution board, sub-circuits and points. Lighting and power circuits are kept separate.
- Typical teaching values: a lighting sub-circuit should not exceed 800 W or about 8 points; a power sub-circuit is commonly limited to 3000 W. Final circuits use 1.0 or 1.5 mm² for lighting and 2.5 or 4 mm² for power, with a separate earth.
- Conduit fill should not exceed about 40%; avoid sharp bends; use junction boxes; keep runs accessible.
- Load estimation applies diversity: not every appliance runs at once.
Worked example
A flat has 10 lamps × 40 W = 400 W, 4 fans × 75 W = 300 W, two 1.5 kW air conditioners = 3000 W, and 2 kW of general sockets. Connected load = 5.7 kW, current = 5700/230 = 24.8 A. Applying diversity, assume 4.5 kW actual demand: 4500/230 = 19.6 A. Choose a 32 A main MCB and 6 mm² incoming copper cable. Reasoning: size for diversified current, then check cable and MCB coordination.
Traps that cost marks
- Adding all loads without diversity.
- Using 1.0 mm² cable for a 16 A socket circuit.
- Sharing the earth conductor between circuits incorrectly.
- Ignoring derating for cables enclosed in conduit.