AS/NZS 3008.1 has two parts. Part 1.1 is for Australian conditions and Part 1.2 is for New Zealand conditions. Both parts use the same method. The reference air temperature is different, and that difference can change the cable size.
The reference conditions
Each part gives its current ratings for one set of site conditions. A rating applies without correction only at those conditions.
| Condition | AS/NZS 3008.1.1 (AU) | AS/NZS 3008.1.2 (NZ) |
|---|---|---|
| Air temperature | 40 °C | 30 °C |
| Soil temperature | 25 °C | 15 °C |
| Soil thermal resistivity | 1.2 K·m/W | 1.2 K·m/W |
| Depth of burial | 0.5 m | 0.5 m |
AS/NZS 3000:2018 Cl 3.3.2.1 sets the reference ambient for each country. It applies when the current rating comes from the AS/NZS 3008.1 series. Australia uses 40 °C air and 25 °C soil. New Zealand uses 30 °C air and 15 °C soil. The soil resistivity and the depth are the same in both parts.
Why one cable has two ratings
A current rating is the current a cable carries before its insulation reaches its temperature limit. Hot surroundings leave less margin for the heat that the current makes. So the same cable has a lower rating at 40 °C than at 30 °C.
For most circuits, that margin is the only difference between the two parts. The page on current-carrying capacity and derating explains the other conditions: installation method, grouping, soil and depth.
One circuit on both parts
The inputs:
- Supply: 230 V single phase, power factor 0.9
- Design current: 32 A
- Route length: 25 m
- Cable: multicore copper, PVC insulation
- Installation method: Unenclosed touching, with no other circuit near it
- Voltage drop limit: 5 %
New Zealand at 30 °C. The engine selects 4 mm². Table 13 of AS/NZS 3008.1.2:2017 rates 4 mm² at 33 A for this method. No correction applies at the reference ambient, so 33 A carries the 32 A load. The voltage drop is 3.54 %. Open the NZ case in the calculator.
Australia at 40 °C. The engine selects 6 mm². Table 13 of AS/NZS 3008.1.1:2017 rates 4 mm² at 29 A for this method, which is less than 32 A. The next size, 6 mm², carries 37 A. The voltage drop is 2.38 %. Open the AU case in the calculator.
Both answers are correct. Each answer is correct for the air temperature its part assumes.
A site that is not at the reference
Tables 27(1) and 27(2) of each part give a correction factor (derating factor in 2017) for other temperatures. The factor moves the rating from the reference to the real site.
Put the same circuit on the AU part at 30 °C. The correction factor is 1.14, so 4 mm² carries 29 × 1.14 = 33.06 A. The engine selects 4 mm², the same size as the NZ part gives. Open the AU part at 30 °C. On AS/NZS 3008.1.1:2025 the correction factor at 30 °C is 1.13, so 4 mm² carries 32.77 A. The engine still selects 4 mm². Open it on the 2025 edition.
Put the same circuit on the NZ part at 40 °C. The correction factor is 0.88, so 4 mm² carries 33 × 0.88 = 29.04 A. That is less than 32 A, and the engine selects 6 mm². That is the size the AU part gives.
The two parts agree when you correct each one to the real site temperature. The reference only sets the point where the correction starts.
How a calculator can select the wrong size
An error occurs when a tool reads the ratings of one part and the reference ambient of the other. Two cases are common.
- AU ratings at a New Zealand site, with no correction. At 30 °C the tool selects 6 mm² where 4 mm² is sufficient. The cable is one size too large. The circuit is safe, but it costs more.
- NZ ratings at a 40 °C site, with no correction. The tool selects 4 mm² at 33 A. At 40 °C that cable carries only 29.04 A, which is less than the 32 A load. The cable is too small.
The second case is the dangerous one. Before you use a tool, find the part it cites and the ambient it assumes. You cannot check a tool that states neither.
What AmpSize does
The Standard select on each cable tool sets the part. NZ reads AS/NZS 3008.1.2:2017 with a 30 °C air and 15 °C soil reference. AU reads AS/NZS 3008.1.1:2017 with a 40 °C air and 25 °C soil reference.
The ambient field starts at 30 °C for both parts. For an Australian site, enter the real ambient. The field shows the AU reference as a hint when you select AU.
Each step in the result names the part and the table it read. The final check compares the corrected rating with the design current. The page on Ib, In and Iz explains that check.
This page is a design aid. Verify every value against the current edition of the standard.