5.2 Order of Cable-Selection Checks

Order of Cable-Selection Checks

Quick Answer: Select cables in this order: (1) current-carrying capacity after derating so Ib ≤ In ≤ Iz, (2) voltage drop to the Clause 3.6 5% end-to-end limit, (3) short-circuit / earth-fault disconnection, (4) mechanical and installation constraints. Revisit earlier steps whenever a later step forces a size or type change.

Why Order Matters More Than “Biggest Cable Wins”

Under time pressure it is tempting to jump straight to a favourite cross-section. That habit fails both on site and on the capstone. Each check answers a different physical question:

Skipping ahead produces false confidence: a 16 mm² cable may crush voltage drop on a long run yet still be wrong if grouping derating drops Iz below In, or if earth-fault-loop impedance prevents 0.4 s disconnection on a final subcircuit.

Step 1 — Current After Derating (CCC First)

Start from load and protection, not from the cable aisle.

1. Determine design current Ib (circuit load or diversified demand for mains/submains).
2. Select protective device rating In so the device will carry Ib continuously and provide overload protection as required (Ib ≤ In).
3. Identify installation method (clipped direct, enclosed in conduit, underground, bunched, etc.).
4. Apply derating factors for grouping, ambient temperature, soil thermal resistivity, enclosure and any other AS/NZS 3008 factors that apply.
5. Choose a cable whose Iz (current-carrying capacity after derating) satisfies In ≤ Iz (hence Ib ≤ In ≤ Iz).

Chapter 4 of this guide develops CCC, insulation temperature ratings and grouping. For Chapter 5, lock the discipline: do not proceed to voltage drop until a candidate size passes CCC. If drop later forces an upsize, re-confirm Iz — larger cables usually help CCC, but a change of type or installation method may not.

Coordination reminder

• Ib — design current of the circuit.
• In — nominal current of the protective device.
• Iz — effective current-carrying capacity of the conductors for the installed conditions.

If Iz is less than In, the protective device may not protect the cable against overload. That is a coordination failure, not a paperwork nicety.

Step 2 — Voltage Drop (Clause 3.6)

With a CCC-compliant candidate:

1. Obtain Vc (mV/A·m) for that size and circuit arrangement from AS/NZS 3008.
2. Compute Vd = (L × I × Vc) / 1000 for each series segment.
3. Sum consumer mains + submains + final subcircuit to the critical point of consumption.
4. Compare with 5% of nominal voltage (11.5 V on 230 V; 20 V on 400 V).

If the total exceeds 5%, increase conductor size (or redesign length/arrangement), then return to Step 1 only as needed to confirm the new size still matches protective device coordination and installation method assumptions.

Design budgets such as “keep mains near 2.5%” are useful planning tools inside Step 2. They do not replace the total check.

Step 3 — Short-Circuit and Earth-Fault Disconnection

Electrically warm and voltage-compliant cables can still fail fault-performance checks.

Short-circuit withstand

Prospective fault current and protective device clearing time impose an adiabatic / withstand demand on active conductors (and sometimes on the cable as a whole). AS/NZS 3008 short-circuit capacity data and protective device energy let-through must be compatible. If the fault level is high and the device is slow, a larger conductor may be required even though CCC and Vd already passed.

Earth-fault disconnection

For MEN installations, earth-fault-loop impedance Zs must be low enough that the protective device disconnects within the required time (commonly discussed as 0.4 s for many final subcircuits and 5 s for distribution circuits — confirm the applicable AS/NZS 3000 tables/clauses for the circuit under assessment). Cable length and size affect R1 and R2. Upsizing actives for voltage drop often helps loop impedance; omitting a properly sized protective earthing conductor does not.

Treat Step 3 as two related questions:

• Will the cable survive the fault until the device clears?
• Will the device clear within the permitted disconnection time for the fault path you have created?

Step 4 — Mechanical and Installation Constraints

Only after the electrical checks should you clear practical installation limits:

• Minimum bending radius and drawing tension for the cable type.
• Suitability for underground, aerial, or exterior exposure.
• Enclosure fill, grouping already assumed in derating, and heat sources.
• Segregation from other services; mechanical protection where required.
• Termination capacity of switchboard lugs, breakers and accessories — a 70 mm² solution that will not land on the chosen breaker is not a solution.
• Fire, chemical or UV environment requiring a different sheath or insulation family (which then sends you back to CCC and Vc tables for that construction).

Mechanical rejection is common in practical assessments: the “correct mm²” on paper may be the wrong product for the run.

Iteration Loop (How Professionals Actually Work)

Real selection is iterative:

Ib / In → candidate size for Iz
        → Vd check (Clause 3.6)
        → fault / EFLI check
        → mechanical fit
        → if any fail, upsize or change type/method and repeat

On the written capstone, show that loop in your working. Markers can follow “Iz OK → Vd = … → total 4.1% → Zs estimate OK → clipped direct OK” far more easily than a lone final mm² with no path.

Capstone Calculation Mindset (Open Book)

• Name the step you are on before flipping tables.
• Cite the book: AS/NZS 3000 for the 5% rule and disconnection concepts; AS/NZS 3008 for CCC and Vc.
• Keep units: metres, amperes, mV/A·m, volts, percent.
• Never “average” conflicting checks — all four gates must pass.
• When upsizing for drop, glance back at In/Iz and terminations immediately.

Section 5.3 applies this exact order to a domestic run and a small commercial feeder so the sequence becomes automatic under exam timing.