5.3 Worked Cable Selection Examples
Key Takeaways
Work every cable example in order: CCC after derating → voltage drop → fault/disconnection thinking → mechanical fit, showing units at each step
Domestic example: long single-phase final subcircuit often passes Iz on a modest size but fails Clause 3.6 until the conductor is upsized
When upsizing for drop, re-state Ib ≤ In ≤ Iz and recompute Vd with the new Vc — do not assume the old drop figure still applies
Small commercial example: three-phase submain drop must use three-phase Vc and the 400 V (20 V) 5% limit, while still leaving budget for final circuits if checking end-to-end
Capstone answers score marks for transparent working and the correct limit (5% total), not for an unexplained final mm²
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.
Example A — Domestic: Long Single-Phase Final Subcircuit
Scenario
A Queensland dwelling needs a dedicated single-phase circuit to a detached workshop:
| Given | Value |
|---|---|
| Supply | 230 V a.c., MEN |
| Load / design current Ib | 20 A continuous (workshop outlets / small tools diversified to 20 A for this circuit) |
| Protective device In | 20 A circuit breaker |
| Route length L (MSB to workshop DB/outlet) | 45 m |
| Installation | Two-core + earth, enclosed in conduit for part of the run; after derating, treat required Iz ≥ 20 A |
| Consumer mains drop already calculated | 3.0 V (≈ 1.3%) on the path feeding this MSB |
| No submain | Final subcircuit leaves the main switchboard |
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:
| Conductor | Iz after your derating factors | CCC result |
|---|---|---|
| 2.5 mm² | 18 A | Fail — Iz < In |
| 4 mm² | 24 A | Pass — Ib ≤ In ≤ Iz |
Candidate after Step 1: 4 mm².
Step 2 — Voltage drop
Illustrative single-phase Vc:
| Size | Vc (mV/A·m) |
|---|---|
| 4 mm² | 11.2 |
| 6 mm² | 7.5 |
| 10 mm² | 4.5 |
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.
Example B — Small Commercial: Three-Phase Submain
Scenario
A small warehouse distribution board is fed from the main switchboard:
| Given | Value |
|---|---|
| Submain | Three-phase + neutral + earth |
| Nominal voltage | 400 V phase–phase (230 V phase–neutral) |
| Design current Ib | 63 A (after diversity for that DB) |
| Protective device In | 63 A MCCB |
| Route length L | 55 m |
| Installation | Multicore copper, cable tray; after grouping/ambient derating, need Iz ≥ 63 A |
| Consumer mains drop POS → MSB | 4.0 V on the 400 V basis already assessed for three-phase |
| Longest final subcircuit from warehouse DB (separate later check) | Design target leave ~6 V on 400 V basis for finals |
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:
| Conductor | Iz | Result |
|---|---|---|
| 10 mm² | 55 A | Fail |
| 16 mm² | 72 A | Pass |
| 25 mm² | 95 A | Pass |
Candidate: 16 mm².
Step 2 — Voltage drop (three-phase Vc)
Illustrative three-phase Vc:
| Size | Vc (mV/A·m) |
|---|---|
| 16 mm² | 2.4 |
| 25 mm² | 1.55 |
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.
Capstone Template (Copy This Structure Into Your Working)
- Ib, In, installation method, derating → Iz (state Ib ≤ In ≤ Iz).
- List series segments from POS; convert 5% → volts.
- Vd = (L × I × Vc)/1000 per segment; sum; compare.
- Fault / EFLI / withstand note.
- Mechanical / termination note.
- 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.
In Example A, why was 4 mm² rejected even though Iz after derating was adequate for a 20 A breaker?
Because AS/NZS 3000 forbids any final subcircuit longer than 30 m
Because 4 mm² copper cannot be used on MEN systems
Because voltage drop on the 45 m run, added to existing consumer-mains drop, exceeded the Clause 3.6 5% end-to-end limit
Because short-circuit capacity always requires a minimum of 10 mm² on domestic circuits
For the three-phase warehouse submain, which voltage should be used when converting the Clause 3.6 5% limit into volts for a phase-to-phase drop check?
400 V, giving a 20 V maximum end-to-end drop
230 V, giving an 11.5 V maximum for all three-phase calculations
415 V only, because 400 V systems are never assessed in Queensland
12 V ELV, because submains are always treated as SELV for drop
After upsizing a cable solely to pass voltage drop, what must you do next in the selection order?
Discard all derating factors because larger cables never overheat
Change the point of supply definition so the 5% limit no longer applies
Remove the protective earthing conductor to reduce Vc
Re-confirm Ib ≤ In ≤ Iz with the new size/type, recompute Vd with the new Vc, and continue through fault and mechanical checks
Sections you finish are checked off in the contents.