2.5 mm² copper multicore PVC cable carries 20 A enclosed in air in Australia. The tool reads AS/NZS 3008.1.1:2025 Table 3.18 at its 40 °C air reference, with no derating. The largest curve C breaker it allows is 20 A, with no margin to spare.
The case
The circuit is single‑phase at 230 V and runs 20 m at the full 20 A of the breaker. The voltage drop is 3.13 % and the earth fault loop impedance is 0.71 Ω, inside the 1.53 Ω that Table 8.1 allows.
The circuit check reads Passes.
What changes the rating
Clipped direct to a surface, the cable carries 22 A.
The enclosed rating equals the breaker, so any derating fails a 20 A circuit on enclosed 2.5 mm². A grouped or insulated run needs the next size, or a smaller breaker.
See also: What size cable for power points in Australia? and AS/NZS 3008.1.1:2025 overview.