Cables

Updated Wiring Regulations and Implications for Cable Current-Carrying Capacity Calculations

The 2018 edition of AS/NZS 3000 introduces several amendments that influence the determination of cable current ratings. These changes necessitate careful selection of installation conditions, as specified in AS/NZS 3008.1, to ensure a safe and compliant design. Key clauses affecting cable capacity are summarized below.

3.3.2.13 Cables Passing Through Thermal Insulation

For thermosetting insulated cables (V75 and V90 types) with a cross-sectional area less than 10 mm² that traverse bulk thermal insulation, the applicable current-carrying capacity must be selected based on the length of cable within the insulation:

  • Less than 150 mm: Use the rating for “in air touching a surface.”
  • 150 mm to 400 mm: Use the rating for “partially surrounded” by thermal insulation.
  • Greater than 400 mm: Use the rating for “completely surrounded” by thermal insulation.

Implication for Design: The installation condition used in the current rating calculation (e.g., “Thermal insulation – partially surrounded”) must be selected based on the cable’s path length through the insulating material.

3.9.3.3 Wiring Systems in Areas Liable to Disturbance

Cables installed in locations where they are likely to be disturbed require supplementary mechanical protection. Approved methods include enclosure within steel conduits, metallic tubing, or rigid/flexible insulating conduit.

Implication for Design: This protective enclosure alters the cable’s thermal environment. The appropriate AS/NZS 3008.1 installation condition, such as “Enclosed – wiring enclosure in air,” must be applied for rating calculations.

3.9.4.4 Cables Subject to Mechanical Damage

Wiring systems in positions susceptible to mechanical damage must be adequately protected by one of the following methods:
(a) Provision of mechanical protection compliant with AS/NZS 3013.
(b) Incorporation of an earthed metallic armouring or screen capable of operating the circuit’s short-circuit protection device.
(c) Protection by a Residual Current Device (RCD) with a maximum rated operating current of 30 mA.

Implication for Design: Where mechanical protection via enclosure is used, the corresponding AS/NZS 3008.1 condition (e.g., “Enclosed – wiring enclosure in air”) governs the current rating. Furthermore, the installation of an RCD positively affects the earth fault loop impedance, which may permit a reduction in the required protective earth conductor size.

3.9.8.2.1 Conductors Within a Common Enclosure

To prevent operational interference and ensure safety, conductors from different electrical installations or individual occupancies within a single installation shall not be installed in the same conduit, tube, pipe, or multi-core cable. An exception is made for conductors terminated within a standard switchboard.

Implication for Design: Where separate wiring systems are installed in proximity (e.g., within the same trench or chase), the installation condition “Buried enclosure – Separate” from AS/NZS 3008.1 may be applicable for derating calculations.

Appendix C – Circuit Arrangements and Maximum Demand

Appendix C has been updated to include the growing prevalence of electric vehicle (EV) charging infrastructure in maximum demand calculations:

  • Table C1, Load group (j)(iv): Now includes “Charging equipment associated with electric vehicles.”
  • Table C2, Load group (c)(ii): Also amended to include “Charging equipment associated with electric vehicles.”

This formal recognition requires designers to account for the load of EV charging equipment when calculating the total maximum demand for an installation, thereby affecting supply and circuit sizing.

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