A tabulated current rating holds only under its reference conditions. A real route is hotter or cooler, shares space with other circuits, or runs underground. A correction factor adjusts the rating for each of those differences. This page explains each factor and what changed in 2025.
Why one cable has many ratings is covered in Current-carrying capacity and derating. The table by table walk through the 2017 New Zealand part is in AS/NZS 3008.1.2 Tables 22 to 29: derating.
The factors
Ambient. The air or soil temperature on the route. AS/NZS 3008.1.2 rates New Zealand cables for 30 °C air and 15 °C soil. AS/NZS 3008.1.1 rates Australian cables for 40 °C air and 25 °C soil. A hotter route gives a factor below 1, and a cooler route gives a factor above 1. For PVC in 40 °C air, the New Zealand factor is 0.88. In Australia it is 1.
Grouping. Circuits that run together heat each other. The factor depends on how many circuits there are and how they sit. Three circuits bunched in one enclosure give 0.7. Six circuits touching on a surface give 0.57. AmpSize counts each set of parallel cables as one circuit.
Tray rows. On a tray, ladder, rack or cleats, a separate table gives the grouping factor. It depends on the support, the circuits on one tier, and the number of tiers. Three circuits spaced on two tiers of perforated tray give 0.96 from Table 24. Table 3.35 is the 2025 version.
Soil. A buried cable sheds heat through the ground. Dry or sandy soil holds the heat in. The factor is 1 at 1.2 K·m/W and falls as the soil thermal resistivity rises.
Depth. A deeper cable sits further from the cooler surface. The factor is 1 at 0.5 m and falls as the depth increases. For cable buried direct, the factor also depends on the conductor size.
Harmonics. On a three-phase circuit, third harmonic current adds up in the neutral. The neutral then heats the cable. At 20 % third harmonic, the reduction factor from Table 2 is 0.86. Above 33 %, the neutral carries more current than the phases. The tool then sizes the cable on the neutral current.
How the factors multiply
The tool multiplies every factor that applies into one combined correction factor. It divides the design current by that factor. The result is the tabulated current the cable needs (AS/NZS 3000 Cl 3.4).
Three circuits enclosed in a 40 °C ceiling in New Zealand give 0.88 × 0.7. The tool shows 0.62. At 32 A the cable needs a tabulated 51.95 A. The tool divides by the unrounded product, so do not check it against the rounded 0.62.
A buried route can have four factors. Take a 35 mm² single-core circuit buried direct in Australia, on the 2025 edition. The tool reads 1.05 for 20 °C soil, 0.8 for 2 K·m/W and 0.95 for 1 m. The combined factor is 0.8.
Factors above 1 also multiply. In the buried example, the cool soil partly offsets the dry soil.
From derating factors to correction factors
The 2017 editions call these derating factors. The 2025 edition calls them correction factors. The new name fits better, because a factor can raise a rating as well as lower it. AmpSize uses the 2025 name for both editions.
The 2025 edition also numbers its tables by section. Table 27(1) is now Table 3.44, Table 22 is now Table 3.33, and Table 29 is now Table 3.48. The tool cites the table of the edition you pick.
Which values changed in 2025
Most correction factor values carried over from 2017. Some did not.
| Factor | Route | 2017 | 2025 |
|---|---|---|---|
| Ambient, PVC in air | 30 °C | 1.14 | 1.13 |
| Ambient, PVC in air | 45 °C | 0.91 | 0.93 |
| Ambient, PVC in air | 50 °C | 0.82 | 0.85 |
| Grouping, one layer touching on a ceiling | 8 circuits | 0.62 | 0.61 |
| Depth, 95 mm² buried direct | 1 m | 0.94 | 0.93 |
Each row comes from the tool, run once on each edition. To see a change, pick Australia and switch the Edition select between 2025 and 2017. Step 5 of the worked guide does this for 50 °C.
In the encoded tables, the soil resistivity, tray and harmonic factors are the same in both editions.
New Zealand and Australia
New Zealand uses AS/NZS 3008.1.2:2017. It has no 2025 edition, so the tool shows no Edition select for New Zealand. Australia uses AS/NZS 3008.1.1, and the tool offers 2025 and the superseded 2017 edition.
The base temperature differs, so the factors differ for the same route. At 40 °C, the New Zealand ambient factor is 0.88 and the Australian factor is 1. The Australian tabulated ratings are lower to match. The corrected rating Iz can come out almost the same. The worked guide Three circuits in a hot ceiling shows this on one route.
A saved link with no edition reads 2017. Pick 2025 for new Australian work, and keep 2017 only to reproduce an older design.
Using the factors
The correction factors tool shows every factor and the corrected rating of one size. The cable size calculator applies the same factors and picks the size. Both read the factors through the same code.
This page is a design aid. Verify every value against the current edition of the standard.