16 mm² copper multicore PVC cable carries 66 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 63 A.
The case
The circuit is single‑phase at 230 V and runs 20 m at the full 63 A of the breaker. Table 5.1 gives a 6 mm² earth. The voltage drop is 1.53 % and the earth fault loop impedance is 0.45 Ω, just inside the 0.49 Ω that Table 8.1 allows.
The circuit check reads Passes.
What changes the rating
Clipped direct to a surface, the cable carries 78 A.
At 40 °C ambient it carries 58.08 A, below a 63 A breaker. Bunched with two other circuits, it carries 46.2 A.
See also: Cable size for a 3‑phase 100 A submain and Current-carrying capacity and derating.