A 32 A single‑phase circuit sounds simple. The size still depends on the route. Two circuits in the same enclosure, a warm ceiling space and a 25 m run all pull in different directions. This guide works one case from end to end.
The circuit
A sub‑board feeds a small workshop. The supply is 230 V single phase. The design current is 32 A at a power factor of 0.9. The route is 25 m of multicore copper PVC cable, enclosed in air. The ceiling space runs warm, so the ambient temperature is 35 °C. One other circuit shares the same enclosure, so the grouping count is two. The protective device is a 32 A curve C circuit breaker. The supply impedance Ze is 0.35 Ω. The voltage drop limit is 5 %.
What the tool does with it
The tool tests every standard size in turn. A size has to pass five checks, and the answer is the smallest size that passes all of them.
Derating. The ambient factor at 35 °C for PVC is 0.94. The grouping factor for two circuits is 0.8. Nothing is buried, so the soil and depth factors are both 1. The product is 0.75. The tool divides the design current by that factor, so the cable has to carry 42.55 A in the table before any factor is applied. That figure comes from AS/NZS 3000:2018 Cl 3.4.
Current-carrying capacity. The tool then reads the tabulated rating for each size. The first size that reaches 42.55 A tabulated is 10 mm². Its derated capacity is 37.6 A, which is above the 32 A design current with a small margin.
Voltage drop. At 10 mm² the drop over 25 m is 1.42 %. The limit is 5 %, so the check passes with plenty of room. Cl 3.6.2 sets that limit.
The protective device. The 32 A breaker sits below the 37.6 A derated capacity, so the device protects the cable. Cl 2.5.3.1 covers that comparison.
Earthing and the loop. Table 5.1 gives a 4 mm² earth for a 10 mm² active. The loop impedance works out at 0.55 Ω, inside the limit for a 32 A curve C device from Table 8.1.
The result
The tool reports 10 mm². The 6 mm² size fails on current capacity once the grouping factor is applied: it cannot reach the 42.55 A the table has to show. That single factor is the difference between a 6 mm² and a 10 mm² cable on this route.
What moves the answer
Change the grouping and the answer changes with it. Run this circuit alone and the grouping factor goes back to 1. The required tabulated current drops to 34.04 A, and 6 mm² passes with a derated capacity of 35.72 A. Two more circuits in the same enclosure pushes the factor lower again.
Length matters less here than most people expect. At 10 mm² the drop is well inside the limit, so the route could be far longer before voltage drop, rather than current capacity, sets the size. On a smaller cable that flips: a long 2.5 mm² run is nearly always limited by drop.
Temperature is the quiet one. A cable in a roof space in Northland sees a very different ambient from the same cable in a plant room. The factor for that difference is small in isolation and decisive when it is stacked with grouping.
Before you install
Check the installation method matches what is really being built. Enclosed in air, clipped direct and buried are three different ratings for the same cable. Check the number of circuits that actually share the route, not the number on the drawing. Check the ambient the cable will see in summer, not the day you measured.
The tool shows each step and the clause or table behind it, so the reasoning can be checked line by line.
See also: AS/NZS 3000 Clause 3.6.2: voltage drop limit, Voltage drop with the mV/A·m method and Current-carrying capacity and derating.