Cables, Conductors and Installation Methods
Cables, Conductors and Installation Methods tests whether you can select a cable and prove it is safe: correct cross-section for current, acceptable voltage drop, correct identification, and correct termination. Most marks are lost on derating and on the difference between single-phase and three-phase voltage drop.
Cable selection factors
- Conductor material and class: copper, aluminium, solid, stranded or flexible; aluminium needs correct terminations and oxide-inhibiting compound.
- Insulation: thermoplastic PVC (70 °C) or thermosetting XLPE (90 °C), low-smoke zero-halogen for escape routes, armoured or screened where mechanical or EMC protection is needed.
- Reference installation method, number of loaded conductors, ambient temperature, grouping, thermal insulation and, if buried, depth and soil thermal resistivity.
- Tabulated current-carrying capacity Iz, then derated: It = Iz × Ca × Cg × Ci, and the device rating must satisfy Ib ≤ In ≤ It.
Identification and voltage drop
Hong Kong new installations use harmonised colours: brown, black and grey for phases, blue for neutral, green-yellow for the protective conductor; older installations use red, yellow, blue, black and green. Voltage drop is checked from the mV/A/m tables: single-phase vd = mV/A/m × Ib × L / 1000; three-phase tables already include the √3 factor, so do not multiply again. The total drop from the point of supply to the furthest load should not exceed 4% of nominal voltage, shared between submain and final circuit.
Worked example
A 20 A single-phase load is 35 m from the board in 4 mm² cable, tabulated at 11 mV/A/m. vd = 11 × 20 × 35 / 1000 = 7.7 V, which is 3.5% of 220 V. The final circuit alone uses almost all the allowance, leaving 0.5% for the submain. Moving to 6 mm² at 7.3 mV/A/m gives 5.1 V, or 2.3%, restoring a workable design.
Traps that cost marks
Multiplying a three-phase mV/A/m value by √3 a second time; using the tabulated Iz without derating; assuming a reduced CPC is acceptable without the adiabatic check; ignoring motor starting volt drop; and mixing 70 °C and 90 °C ratings when applying correction factors.