5.3 Worked Cable Selection Examples

Worked Cable Selection Examples

Quick Answer: Treat every sizing problem as a four-gate workflow. The domestic example shows a long 230 V final subcircuit that needs upsizing for Clause 3.6 even after CCC passes. The commercial example shows a three-phase submain checked with three-phase Vc against a 20 V (5% of 400 V) limit while preserving end-to-end thinking.

How to Read These Examples

Figures for Vc and some Iz values below are illustrative teaching numbers chosen so arithmetic is clear in an open-book setting. On assessment day you must read the actual AS/NZS 3008 tables in the edition your RTO permits. The method, order, and 5% Clause 3.6 rule are what transfer — not the sample millivolt figures.

Assume copper PVC/PVC multicore cables, clipped or enclosed as stated, and MEN 230/400 V supply unless noted.

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Example A — Domestic: Long Single-Phase Final Subcircuit

Scenario

A Queensland dwelling needs a dedicated single-phase circuit to a detached workshop:

Clause 3.6 allowance to the workshop point of consumption: 5% of 230 V = 11.5 V total from POS. With 3.0 V already used on consumer mains, the final subcircuit may use at most 11.5 − 3.0 = 8.5 V.

Step 1 — Current after derating

Ib = 20 A, In = 20 A → need Iz ≥ 20 A after derating.

Suppose AS/NZS 3008 (illustrative) shows:

Candidate after Step 1: 4 mm².

Step 2 — Voltage drop

Illustrative single-phase Vc:

For 4 mm²:

Vd = (45 × 20 × 11.2) / 1000 = 10.08 V

Final subcircuit alone is 10.08 V, but only 8.5 V remains after mains. Total POS → load ≈ 3.0 + 10.08 = 13.08 V (5.7%) → fails Clause 3.6.

Upsize to 6 mm²:

Vd = (45 × 20 × 7.5) / 1000 = 6.75 V

Remaining budget 8.5 V → OK for this segment. Total ≈ 3.0 + 6.75 = 9.75 V (4.2%) → passes 5%.

Re-check Step 1: 6 mm² Iz will be higher than 4 mm² → still Ib ≤ In ≤ Iz.

Step 3 — Short-circuit / earth-fault disconnection (summary check)

• Confirm the 20 A breaker and 6 mm² actives (with correctly sized earth) give a plausible Zs for 0.4 s disconnection on a final subcircuit — long 45 m runs are exactly where undersized earths fail EFLI even if actives were upsized for drop.
• Confirm short-circuit withstand for the prospective fault level at the MSB is within cable/device limits (usually comfortable at domestic fault levels for these sizes, but still state the check).

Step 4 — Mechanical / installation

• Conduit diameter and bend radius accept 6 mm² 2C+E.
• Underground section (if any) uses a cable type permitted for burial / in conduit underground.
• Terminations at 20 A breaker and workshop accessories suit 6 mm².

Domestic result

Select 6 mm² (not 4 mm²): CCC passed at 4 mm², but voltage drop against the remaining 8.5 V budget forced the upsize. Document mains drop + subcircuit drop = total ≤ 11.5 V.

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Example B — Small Commercial: Three-Phase Submain

Scenario

A small warehouse distribution board is fed from the main switchboard:

End-to-end three-phase limit: 5% of 400 V = 20 V. After 4.0 V mains, 16 V remains for submain + finals. If finals may use 6 V, submain budget ≈ 10 V (design allocation inside the mandatory 20 V total).

Step 1 — Current after derating

Need Iz ≥ 63 A.

Illustrative CCC after derating:

Candidate: 16 mm².

Step 2 — Voltage drop (three-phase Vc)

Illustrative three-phase Vc:

For 16 mm²:

Vd = (55 × 63 × 2.4) / 1000 = 8.32 V

Against a 10 V submain design budget → passes the allocation. End-to-end so far: 4.0 + 8.32 = 12.32 V, leaving 7.68 V for finals (near the 6 V planning figure — finals must be checked explicitly on the longest run).

If a question instead asked only “does the submain alone stay within 5%?”, 8.32 V ≪ 20 V — but that weaker question is not the full Clause 3.6 story when mains and finals also exist. Capstone-quality answers keep the POS → load narrative.

Suppose grouping was worse and only 16 mm² barely passed Iz, but the longest final later consumed 9 V. Then total 4.0 + 8.32 + 9.0 = 21.32 V > 20 V → fail. Remedy: upsize submain to 25 mm²:

Vd = (55 × 63 × 1.55) / 1000 = 5.37 V

Total with 9 V final ≈ 18.37 V → passes 5%.

Step 3 — Fault performance

• Check MCCB breaking capacity ≥ prospective fault current at the MSB.
• Confirm cable short-circuit capacity for the clearing time.
• Confirm earth size and length keep Zs within disconnection limits for the warehouse DB protection scheme.

Step 4 — Mechanical

• Tray fill and support spacing for 16 mm² or 25 mm² multicore.
• Gland and MCCB lug capacity.
• Neutral and earth continuity through the submain consistent with MEN rules (MEN link remains at the main switchboard — do not create parallel MEN links at the warehouse DB).

Commercial result

Start at 16 mm² for CCC; confirm three-phase Vd; if end-to-end finals push the installation over 20 V, upsize to 25 mm² and re-check. Always use three-phase Vc and the 400 V / 20 V limit for the three-phase path.

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Capstone Template (Copy This Structure Into Your Working)

1. Ib, In, installation method, derating → Iz (state Ib ≤ In ≤ Iz).
2. List series segments from POS; convert 5% → volts.
3. Vd = (L × I × Vc)/1000 per segment; sum; compare.
4. Fault / EFLI / withstand note.
5. Mechanical / termination note.
6. Final selected size with one sentence why (usually “CCC OK at X mm²; Vd required Y mm²”).

That template is the calculation mindset: open book, ordered gates, no mystery millimetres.