Cable size calculator (AS/NZS 3008)
Minimum copper conductor size for a load, run length and protective device — current-carrying capacity and voltage drop to AS/NZS 3008.1.1, device coordination to AS/NZS 3000.
AS/NZS 3008.1.1:2025 · AS/NZS 3000 cl 3.6
Optional — checks device ≤ cable capacity.
Optional — needed for the voltage-drop check.
Enter the load current to calculate.
How this is worked out
- Walks the standard copper sizes from 1 mm² to 120 mm² and returns the smallest that passes every applicable check.
- Check 1 — current-carrying capacity from AS/NZS 3008.1.1 for the installation method, insulation and cores, must be at least the load current.
- Check 2 — voltage drop, when a run length is entered, must not exceed 5% of 230 V (single-phase) or 400 V (three-phase) per AS/NZS 3000 cl 3.6.
- Check 3 — when a protective device rating is entered, it must not exceed the cable's current-carrying capacity.
- Assumptions: single circuit, 40 °C ambient air / 25 °C soil, no derating for grouping, ambient or burial depth, copper only, sizes up to 120 mm². Minimum earth from AS/NZS 3000 Table 5.1.
Common questions
Why does this give a bigger size than another calculator?+
Two reasons. This tool also checks that the protective device rating does not exceed the cable's capacity, so a 32 A breaker on a 27 A cable is refused even if the load is small. And it uses full-load-temperature tables, which are conservative at light load. Both are valid AS/NZS readings; the difference is usually one size.
Does it handle derating for grouping or high ambient?+
No. It assumes a single circuit at 40 °C in air or 25 °C in soil. Grouped cables, hot roof spaces and deep burial need derating factors from AS/NZS 3008.1.1 — apply them before relying on the size.
Can I use it for aluminium or above 120 mm²?+
No. Copper only, 1 mm² to 120 mm². Larger or aluminium conductors need the full tables.
Is this a design tool?+
It is a verification aid: it tells you the minimum size that passes the three checks so you can sanity-check what is installed. Design work — derating, fault-loop length limits, short-circuit withstand — needs the standard and a design calculator.
Related guide
Field Efficient vs electrical calculator apps: different tools, different jobsCalculator apps check the maths before the work; Field Efficient proves the work after it — capture, AS/NZS validation, reporting and state certificate prep.
Recording this test?
The same calculator sits under every cable-size field in Field Efficient, pre-filled from the breaker you recorded — one tap applies the size to the report.
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Indicative only. Verify against the current edition of the standard before relying on a result for design, certification or a compliance decision. This tool is not a substitute for the standard or for a licensed electrician's judgement.