2.5 mm² copper multicore PVC cable carries 23 A enclosed in air at 30 °C in New Zealand. That is its corrected rating Iz from AS/NZS 3008.1.2:2017 Table 13. The largest curve C breaker it allows is 20 A, the usual power point circuit.
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 25 A.
Grouping is the common trap. Bunched with two other circuits, the factor is 0.7 and the cable carries 16.1 A. That is below a 20 A breaker, so the circuit fails.
See also: What size cable for power points in NZ? and Ib, In and Iz.