Wiring Rules and installation standards (AS/NZS 3000)
What the Wiring Rules do
AS/NZS 3000 is not optional in Queensland: it is called up by the electrical safety legislation, so its requirements are legal duties. Know the companion standards it depends on - AS/NZS 3008 for cable selection, AS/NZS 3017 for verification, AS/NZS 3012 for construction sites, AS/NZS 3760 for in-service testing and AS/NZS 4836 for safe working.
The design principles
The aim is protection of people, livestock and property from shock and fire. The mechanisms are: the multiple earthed neutral (MEN) system, with the neutral earthed at the point of supply only; protective earthing and equipotential bonding of conductive parts; automatic disconnection of supply within the maximum time appropriate to the circuit; RCD protection (30 mA for final subcircuits in domestic installations); correct selection of cable and switchgear for maximum demand, voltage drop (not more than 5% from the point of supply), ambient temperature and installation method; and protection against mechanical damage, ingress (IP rating), heat and other external influences. Equipment must be identified, accessible for maintenance, and not adversely affected by other services.
Traps
The 5% voltage drop limit applies from the point of supply to the furthest point of the installation, not separately to each cable run. Additions and alterations must comply with the current edition, not the edition in force when the building was wired. There is only one MEN link in an installation. Domestic RCD requirements are stricter than many candidates assume. A deemed-to-comply solution is not the only compliant solution.
Worked example
A 16 A single-phase subcircuit uses 20 m of 2.5 mm² TPS with a tabulated voltage drop of about 18 mV/A·m. V_d = 18 × 16 × 20 / 1000 = 5.8 V, roughly 2.5% of 230 V, leaving about 2.5% of the 5% allowance for consumer mains and other sections.