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How this is calculated#

The tool reads the simplified tables of AS/NZS 3000 Appendix C. They cover copper cable from 1 mm² to 25 mm². They give the largest device rating and the voltage drop in one pass.

1. Maximum device rating. The tool reads the rating for your size, phases and installation condition from Tables C6 and C7.
2. Ampere metres. It multiplies the load current by the one-way length.
3. Ampere metres per percent. It reads the value for your size from Table C8.
4. Voltage drop. It divides the ampere metres by that value. The result is the drop in percent.
5. Size for the limit. It divides the ampere metres by your limit. It then picks the smallest size from Table C8 that meets the figure.

Worked example#

• 2.5 mm² on one phase over 25 m at 20 A. The cable is in unenclosed air. The maximum device rating is 25 A. The run is 500 ampere metres. The drop is 3.91 %, inside the 5 % limit. The size for the limit is 2.5 mm².

Limits#

The tables assume one circuit on its own. Where circuits are grouped, use the cable size calculator. The tool does not apply correction factors. It does not read AS/NZS 3008.1 tables. It stops at 25 mm².

See also: Cable size for a 32 A single‑phase run of 25 m, AS/NZS 3000 Clause 3.6.2 voltage drop limit and Voltage drop basics.