A power point circuit on a 20 A breaker in New Zealand takes 2.5 mm² copper cable on a typical route. The route is 25 m of multicore PVC cable, enclosed in air at 30 °C, with no other circuit beside it. The tool gives 23 A of derated capacity and a voltage drop of 3.92 %.
The case
The circuit is single‑phase at 230 V. The design current is the full 20 A of the curve C breaker, at a power factor of 1. The voltage drop limit is 5 % and Ze is 0.35 Ω. Table 5.1 gives a 2.5 mm² earth, and the earth fault loop impedance is 0.8 Ω.
What changes the answer
Ceiling insulation changes it most. With the cable partly surrounded by insulation, the tool selects 4 mm² with 26 A of capacity.
Length is next. On a run of 40 m, voltage drop sets the size, and the tool selects 4 mm² with a drop of 3.9 %.
Before you install
Enter the real route: the installation method, the longest run and the circuits that share it. The tool prints each check with its clause or table.
See also: What size breaker for 4 mm² cable and Current-carrying capacity and derating.