6 mm² copper multicore PVC cable carries 38 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 32 A, the size of a cooktop or EV charger circuit.
The case
The circuit is single‑phase at 230 V and runs 20 m at the full 32 A of the breaker. The voltage drop is 2.09 % and the earth fault loop impedance is 0.61 Ω, inside the 0.96 Ω that Table 8.1 allows.
The circuit check reads Passes.
What changes the rating
Clipped direct to a surface, the cable carries 42 A.
A hot garage ceiling lowers it. At 40 °C the cable carries 33.44 A, still above a 32 A breaker.
Bunched with two other circuits, it carries 26.6 A, and a 32 A circuit fails.
See also: What size cable for a 22 kW EV charger? and Current-carrying capacity and derating.