1.5 mm² copper multicore PVC cable carries 16 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, with no derating. The largest curve C breaker it allows is 16 A.
The case
The circuit is single‑phase at 230 V and runs 20 m. The design current is the full 16 A of the breaker. Over that run the voltage drop is 4.59 %, close to its limit, and the earth fault loop impedance is 1.01 Ω. Table 8.1 allows up to 1.92 Ω for the breaker.
The circuit check reads Passes.
What changes the rating
The installation method sets the column the tool reads. Clipped direct to a surface, the cable carries 17 A.
Heat lowers it. At 40 °C ambient the factor is 0.88 and the cable carries 14.08 A.
Grouping lowers it further. Bunched with two other circuits, it carries 11.2 A.
See also: What size cable for a lighting circuit in NZ? and Current-carrying capacity and derating.