CableHero

Cable Derating Calculator

Combined derating factor for grouping, temperature, depth and soil from the AS/NZS 3008.1.1:2025 tables

How to use

The factor a cable's tabulated current rating is multiplied by for grouping, ambient temperature, depth and soil, from the AS/NZS 3008.1.1:2025 tables.

Grouping

Table 3.33

The table prints up to 20; more than 20 reads the 20 column

Temperature

°C

Reference 40 °C (factor 1.00)

Derating Factor

Enter the temperature to work out the factor.

How it's calculated

How derating factors combine

Each condition that differs from the table's reference has a factor; the derated current rating is the tabulated rating times their product.

Derated current rating

tabulated current-carrying capacity (Tables 3.4 to 3.32)
combined derating factor

Cables in air

Table 3.33, 3.34 or 3.35
Table 3.44

Cables underground

Tables 3.36 to 3.43
Table 3.45
Table 3.46 or 3.47
Table 3.48

Ambient temperature factor

maximum conductor temperature, °C
actual ambient, °C
reference ambient: 40 °C in air, 25 °C in the ground

Every factor Tables 3.44 and 3.45 print follows this formula, to two decimals.

Standards referenced

  • AS/NZS 3008.1.1:2025 Clause 3.5 (derating factors)
  • AS/NZS 3008.1.1:2025 Tables 3.33 to 3.43 (grouping)
  • AS/NZS 3008.1.1:2025 Tables 3.44 and 3.45 (air and soil temperature)
  • AS/NZS 3008.1.1:2025 Tables 3.46 and 3.47 (depth of laying) and 3.48 (soil thermal resistivity)

Worked example: 4 circuits bunched at 45 °C

4 circuits of multicore X-90 cable bunched on a surface in a plant room where the air reaches 45 °C.

  1. 1. Grouping
  2. 2. Air temperature
  3. 3. Combined factor

Each cable carries 62% of its tabulated rating: a cable rated 100 A in Table 3.4 conditions is good for 61.8 A here.

Frequently Asked Questions

Derating Questions Answered

What is a cable derating factor?

The current-carrying capacities in AS/NZS 3008.1.1:2025 Tables 3.4 to 3.32 assume one circuit at a reference ambient (40 °C in air, 25 °C in the ground), laid 0.5 m deep in soil of 1.2 K·m/W. When the installation differs, each difference has a factor, and the tabulated rating is multiplied by all of them.

How are derating factors combined?

They are multiplied together. In air: grouping × air temperature. Underground: grouping × soil temperature × depth of laying × soil thermal resistivity. The calculator rounds the product to four decimal places, as the cable size calculator does.

Which grouping table applies?

Table 3.33 for cables bunched, on a surface or enclosed; 3.34 (single-core) and 3.35 (multicore) on trays, ladders, racks and cleats; 3.36 and 3.38 for cables buried direct; 3.40 and 3.42 for underground ducts. Choose the installation and the cable type and the calculator picks the table.

Why does the calculator warn about underground grouping at other depths?

AS/NZS 3008.1.1:2025 Clause 3.5.2.6 Note 4 and Clause 3.5.2.7 Note 3 say the underground grouping factors apply only at the standard depth and soil resistivity. Grouped cables in other conditions should be rated to IEC 60287, so the multiplied factor is then an approximation.

Can a derating factor be above 1?

Yes. A cooler ambient than the reference, shallower burial or lower soil resistivity raises the rating, for example 1.07 for a 90 °C cable at 30 °C in air.

What if the ambient is hotter than the table prints?

Beyond the hottest printed column the calculator uses the formula the 2025 tables follow, √((θc − θa) / (θc − θref)), at the actual ambient, which is lower than reading the last column.

Reviewed by Ahmed Tayeh, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

Guides

Learn the method

How to use this calculator

Find the grouping, temperature, depth and soil factors and the overall derating factor they multiply to.

Read the guide: How to use the cable derating calculator

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Apply It to a Cable

The cable size calculator applies the derating factors to the current rating and picks the smallest cable that carries the load, within voltage drop and fault limits.

Open the Cable Size Calculator