A power point circuit on a 20 A breaker in Australia takes 2.5 mm² copper cable on a typical route. The tool reads AS/NZS 3008.1.1:2025 at its 40 °C air reference. For 25 m of multicore PVC cable enclosed in air, it gives 20 A of derated capacity and a voltage drop of 3.92 %.
The case
The circuit is single‑phase at 230 V, with a design current of 20 A on a curve C breaker. No other circuit shares the route. The voltage drop limit is 5 %. Table 5.1 gives a 2.5 mm² earth, and the earth fault loop impedance is 0.8 Ω.
The capacity equals the breaker rating here, so the cable has no spare margin. Any derating moves the size up.
What changes the answer
Grouping changes it. With three circuits bunched together, the tool selects 6 mm² with 23.1 A of capacity.
The same circuit in New Zealand reads AS/NZS 3008.1.2:2017 at its own reference temperature. There the cable has 23 A of capacity.
See also: What size cable for power points in NZ? and AS/NZS 3008.1.1:2025 overview.