13.3 Cable Selection (AS/NZS 3008.1.2 NZ Conditions)

13.3 Cable Selection (AS/NZS 3008.1.2 NZ Conditions)

AS/NZS 3008.1.2—cable selection for New Zealand conditions—is B-level knowledge on the EWRB electrician regulations prescription. You are not expected to memorise every ampacity row, but you are expected to know what drives selection, how 3008 relates to AS/NZS 3000, and how to use tables when a calculation stem supplies them.

Where 3008 sits relative to AS/NZS 3000

Open-book method: AS/NZS 3000 tells you rules and limits; 3008 supplies capacity tables and factors for conductors under those methods. Do not answer a capacity stem from maximum-demand alone, and do not answer a MEN stem from a 3008 table.

Current-carrying capacity drivers

A cable’s current-carrying capacity (Iz / tabulated rating as presented) depends on heat leaving the conductor insulation system. Key factors:

1. Conductor material and cross-section — copper vs aluminium; larger CSA → higher capacity (same conditions).
2. Insulation type / temperature rating — higher-temperature insulation systems may allow higher tabulated ratings (only if terminations and environment allow that temperature class).
3. Installation method — clipped direct, conduit, thermal insulation, underground, bunched, in free air, etc. Method codes in the tables matter.
4. Ambient temperature — hotter ambient reduces capacity; cooler may increase it (use the factor tables).
5. Grouping — multiple circuits close together share heat → derating.
6. Soil thermal resistivity / burial depth (underground methods) — buried ratings are not the same as free air.

Selection logic (teachable algorithm):

1. Determine design current Ib (from load / demand method as applicable).
2. Select protective device In such that coordination rules are satisfied (In ≥ Ib typically, with diversity/demand context).
3. Ensure cable capacity Iz ≥ In (or the AS/NZS 3000 coordination form required for the protection type)—cable not smaller than the protective device requires.
4. Choose installation method that matches the real route.
5. Apply derating factors (ambient, grouping, etc.) from 3008.
6. Check voltage drop (AS/NZS 3000 limits; length and load).
7. Consider fault / short-circuit withstand at high level (cable and joints must tolerate prospective fault energy until protection clears).
8. Confirm mechanical protection and environment (Chapter 13.2).

Voltage drop interaction

Even if thermal capacity is fine, voltage drop on long runs can force a larger CSA. Voltage drop depends on:

• Conductor size and material.
• Length.
• Load current.
• Circuit type (single-phase vs three-phase factors in the formulae/tables used).

Exam pairing: a submain that “passes 3008 amps” can still fail AS/NZS 3000 voltage-drop limits (Chapter 8). Always run both checks in method questions. Prefer answers that say size for the more onerous of capacity (after derating) and voltage drop, coordinated with protection.

Short-circuit withstand (high level)

Under fault, conductors heat rapidly. High-level exam awareness:

• Prospective fault current and protective device clearing time impose an energy (I²t) demand on the cable.
• Very small conductors on high fault-level boards can fail thermally under short-circuit even if continuous load was modest.
• Coordination is a system problem: cable, protective device, and fault level belong together.

You rarely need a full adiabatic calculation memorised; you need to recognise when a stem is about fault withstand / coordination, not only continuous amps.

Tables may be supplied in the paper

Teaching-guidelines style exams often provide excerpts of tables for calculation items. Treat them as authoritative for that question:

• Use only the table/method given.
• Read column headers: method, insulation, copper/aluminium.
• Apply stated derating factors in the order the question expects.
• Do not override the supplied table with a remembered number from another edition.

If no table is supplied, questions usually test principles (what increases/decreases capacity; coordination with In; voltage drop interaction), not obscure milliamp trivia.

Coordinate with protective device and installation method

Classic non-compliance pattern:

• Protective device In = 32 A.
• Cable selected from a free-air table at 32 A capability.
• Actually installed bunched in thermal insulation.
• True Iz after derating < 32 A → unprotected cable under continuous load.

Exam correctives:

• Match method to site.
• Derate first, then pick CSA.
• Ensure Iz supports the protective device arrangement required by AS/NZS 3000.
• Do not “upsize the breaker” to fix an undersized cable—that worsens protection.

Common traps

• Selecting on load current alone and ignoring In and Iz coordination.
• Ignoring grouping and ambient.
• Using AU-only tables or wrong 3008 part—prescription points to 3008.1.2 NZ conditions.
• Passing capacity but failing voltage drop.
• Memorising one “house wiring size” for all submains.

Study drill

On every cable stem, write five words: Ib → In → method → derate → Vd. If short-circuit or mechanical protection appears, add those checks. B-level means competent navigation and method, not blind memorisation of entire capacity books.