Cable Sizing Calculator per AS/NZS 3008.1.1:2017 for Commercial Buildings
Cable sizing for AS/NZS 3008 commercial buildings involves selecting sub-main and final circuit conductors using Table 3 current-carrying capacities for installation methods typical of commercial construction — cable tray (Column 3), enclosed in conduit (Column 6), or direct buried (Columns 14-16) — with Table 13 grouping factors and Table 22 soil thermal resistivity corrections for any underground sections.
Quick Reference Table
| AS/NZS 3008 Commercial Cable Sizing References — AS/NZS 3008.1.1:2017 (2017) | ||
|---|---|---|
| Parameter | Value / Requirement | Clause Reference |
| Cable tray ratings | Touching & spaced single and multicore cables | Table 3, Columns 3-5 |
| Grouping factors | Derating for multiple cables per tray/conduit | Table 13 |
| Soil thermal resistivity | Correction for buried cable sections | Table 22 |
| Voltage drop | Impedance values per conductor size and arrangement | Table 25 |
| Short circuit capacity | Maximum fault current withstand duration | Clause 3.7, Table 34 |
How to Calculate Cable Sizing for Commercial Buildings
- 1
Determine the maximum demand per distribution board
Calculate the diversified maximum demand for each distribution board using AS/NZS 3000:2018 Part 2 methods for commercial loads including lighting, power, HVAC, and lifts.
- 2
Select the installation method
Identify cable routing — perforated tray in risers (Columns 3-5), enclosed conduit in concrete (Column 6/7), or underground between buildings (Columns 14-16 with Table 22).
- 3
Apply derating for grouping and temperature
Apply Table 13 grouping factors for cables on shared trays. For underground sections, apply Table 22 soil thermal resistivity corrections based on the local soil conditions.
- 4
Size sub-main and final cables
Select cable cross-sections from Table 3 where the derated current rating exceeds the circuit design current. Consider future load growth by selecting one size above minimum.
- 5
Verify voltage drop across the distribution tier
Calculate cumulative voltage drop from main switchboard to final outlets using Table 25 impedance values. Apportion the 5% total budget between sub-mains and final circuits.
- 6
Check short circuit withstand
Verify cable thermal short circuit capacity per Clause 3.7 using Table 34 constants. Ensure the cable can withstand the prospective fault current for the protective device clearing time.
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Calculate Cable Sizing NowBS 7671 vs AS/NZS 3008 Cable Sizing Comparison
| Parameter | BS 7671 | AS/NZS 3008 |
|---|---|---|
| Region | United Kingdom | Australia & New Zealand |
| Voltage drop limit | 3% lighting / 5% other | 5% general (AS/NZS 3000 Table C7) |
| Reference ambient temp | 30°C air | 40°C air (Table 22) |
| Ampacity tables | Appendix 4 Tables 4D1A–4J4A | Tables 13–20 (by method) |
| Derating tables | Ca, Cg, Ci, Cc factors | Tables 22, 24, 25, 26 factors |
| Soil temp reference | 20°C | 25°C (Table 25) |