A 4 mm² copper cable enclosed in air at 30 °C takes a 25 A breaker. A 32 A breaker fails, because the cable carries only 29 A on that route. It depends on the route, though. This guide shows how, one change at a time, in the circuit check tool. The breaker rule is set out in IB ≤ IN ≤ IZ.
The circuit
The cable is 4 mm² multicore copper with PVC insulation, enclosed in air. It feeds a 25 A load at 230 V single‑phase, 20 m from the switchboard. The ambient is 30 °C, with no other circuit beside it. The breaker is a C curve, Ze is 0.35 Ω and the voltage drop limit is 5 %. The tool reads the New Zealand tables, AS/NZS 3008.1.2:2017. Where a step changes an input, it says so.
Step 1. Try a 32 A breaker
Check 4 mm² on a 32 A breaker. Table 13 gives 29 A for 4 mm² in this method. One circuit at 30 °C has a correction factor of 1, so Iz is 29 A.
The 25 A load is below 29 A, so the current check passes. The breaker check fails. The breaker rating must not exceed Iz (AS/NZS 3000:2018 Cl 2.5.3.1), and 32 A is 3 A above it. An overload between 29 A and 32 A can overheat the cable. The breaker may never trip on it.
The tool gives 25 A as the largest C curve breaker the cable allows. It also gives 6 mm² as the smallest size that passes on the 32 A breaker.
Step 2. Fit a 25 A breaker
Check 4 mm² on a 25 A breaker. The chain reads 25 ≤ 25 ≤ 29, and every check passes.
| Check | Result | Limit | State |
|---|---|---|---|
| Current capacity | Iz 29 A | Ib 25 A | Passes |
| Voltage drop | 2.21 % | 5 % | Passes |
| Breaker rating | In 25 A | Ib 25 A to Iz 29 A | Passes |
| Loop impedance | Zs 0.64 Ω | 1.23 Ω | Passes |
The tool sizes the earth conductor at 2.5 mm² from Table 5.1. The 1.23 Ω maximum is the Table 8.1 figure for a 25 A C curve breaker. Fault withstand runs only when you enter a fault current.
It depends on the route
The route changes Iz. It does not change the load or the breaker.
Grouped with other circuits. With one other circuit the grouping factor is 0.8 (Table 22), so Iz falls to 23.2 A. The 25 A breaker now fails. The 25 A load no longer fits, but a 20 A load on a 20 A breaker passes.
A hot ceiling. A roof space can reach 45 °C in summer. The ambient factor is 0.8 (Table 27(1)), so Iz is 23.2 A. The 25 A breaker fails and a 20 A breaker passes. Add one grouped circuit in the same roof space and Iz is 18.56 A. On the 25 A breaker 10 mm² is the smallest size that passes.
Thermal insulation. Insulation laid over the cable lowers the rating in Table 13. Partly surrounded, 4 mm² is rated 23 A, and a 20 A breaker passes. Fully surrounded with no wiring enclosure, the rating is 17 A. Only a 16 A breaker passes.
A long run fails on Zs
Now take a lighter load further away. The load is 10 A at 80 m, on the 25 A C curve breaker. Check the long run. The current check passes, and the voltage drop passes at 3.54 %. The loop impedance fails. Zs is 1.52 Ω against a maximum of 1.23 Ω, so the breaker may not clear an earth fault in time.
The tool points to three fixes. A 20 A C curve breaker is the largest the cable allows on this run. A 25 A B curve breaker passes, with a maximum Zs of 2.3 Ω. On the 25 A C curve breaker, 10 mm² is the smallest size that passes.
At 25 A over 80 m the voltage drop fails as well, at 8.86 %. Zs also depends on Ze, and 0.35 Ω is only an assumed figure. Measure Ze on site and enter the reading.
What size breaker for 6 mm² cable
Table 13 gives 38 A for 6 mm² in the same method. A 32 A breaker with a 32 A load passes. A 40 A breaker fails, because 40 A is above Iz. The largest C curve breaker the cable allows is 32 A. Grouped with one other circuit, Iz falls to 30.4 A. A 32 A breaker then fails and a 25 A breaker passes.
The result
| Route for 4 mm² | Iz | Largest C curve breaker that passes |
|---|---|---|
| Enclosed in air, 30 °C, one circuit | 29 A | 25 A |
| Two circuits grouped | 23.2 A | 20 A |
| Roof space at 45 °C | 23.2 A | 20 A |
| Partly in insulation | 23 A | 20 A |
| Fully in insulation, no enclosure | 17 A | 16 A |
| 80 m run, 10 A load | 29 A | 20 A, set by Zs |
In Australia, set the Standard to AS/NZS 3008.1.1. On the 2025 edition the tool reads Table 3.18, and Iz is 28.25 A at 30 °C. The 25 A breaker still passes.
The tool works from your inputs. It does not test the cable or check the RCD. Checking an existing circuit covers what to measure on site.
See also: AS/NZS 3008 Table 13 multicore current ratings, AS/NZS 3000 Table 8.1 maximum Zs explained and Cable size for 32 A single‑phase over 25 m.