Cable Selection, Installation Methods and Voltage Drop
Cable selection is a chain of checks: current-carrying capacity (after derating for installation method, ambient temperature, grouping and thermal insulation), voltage drop, fault loop impedance, mechanical suitability and environmental rating (UV, oil, fire, corrosives). AS/NZS 3008 provides the current ratings by installation method and conductor configuration.
Installation method matters enormously: a cable in free air carries more than the same cable enclosed in conduit in a wall, and much more than one buried under insulation. Derating factors are applied multiplicatively. Voltage drop limits in AS/NZS 3000 are typically 5% from point of supply to any point of the installation (and tighter for some circuits); three-phase drop uses √3 × I × L × (R cosφ + X sinφ), single-phase uses 2 × I × L × (R cosφ + X sinφ) to account for the return path.
Traps: using the free-air rating for an enclosed cable; forgetting the return-path factor of 2 in single-phase drop; ignoring power factor in larger cables where reactance matters; and selecting a cable that passes voltage drop but fails fault loop impedance (or vice versa).
Worked example: A 4 mm² cable with a tabulated 32 A rating, derated by 0.87 (temperature) and 0.80 (grouping), gives 22.3 A - too small for a 25 A load. Step up to 6 mm² (tabulated ~41 A → 28.5 A derated), then confirm voltage drop over the run.