A 25 A single‑phase circuit takes 4 mm² copper cable on a typical New Zealand route. For 25 m of multicore PVC cable enclosed in air at 30 °C, the tool gives 29 A of derated capacity and a voltage drop of 3.05 %.
Why not 2.5 mm²
On the same route, 2.5 mm² has 23 A of derated capacity. That is below the 25 A breaker, so the breaker would not protect the cable. Cl 2.5.3.1 sets that comparison: the device rating must not exceed the cable's capacity.
The case
The circuit runs at 230 V with a curve C breaker and a design current of 25 A. 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 0.72 Ω.
Before you install
Grouping, a hot ceiling and thermal insulation all lower the capacity. Enter the real route and read the check the tool prints for each candidate size.
See also: What size breaker for 4 mm² cable and Ib, In and Iz.