What is a cable correction factor?

A correction factor is a multiplier applied to a cable's base current-carrying capacity to account for installation conditions that differ from the reference conditions in AS/NZS 3008.1.1 — typically ambient temperature, grouping, soil thermal resistivity and depth of burial.

How do I combine multiple correction factors?

Multiply the factors together: derated capacity = base capacity × k1 × k2 × k3 × k4. The result must remain at or above the design current.

When does grouping derating apply?

Grouping derating from AS/NZS 3008.1.1 Tables 22–24 applies when multiple loaded cables are installed touching or in close proximity (in trays, conduits, or enclosed runs). Spacing of at least one cable diameter between cables can avoid the derating.

When do I need to apply derating factors to AS/NZS 3008 cable ratings?

Whenever the installation deviates from the reference conditions of the Section 3 base-rating tables: more than one circuit grouped together, ambient above the 40 °C air / 25 °C soil reference, soil resistivity above 1.2 K·m/W, or direct buried with closely spaced circuits. The applicable factors from Tables 3.33–3.48 are multiplied together.

What is the AS/NZS 3008 derating factor for 4 cables in a conduit?

For 4 multicore circuits enclosed in a single conduit, AS/NZS 3008.1.1:2025 Table 22 gives a grouping factor of 0.65 (for 3 or more conductors per circuit). For single-core circuits installed in trefoil the factor is different — refer to Table 23.

How does ambient temperature affect cable rating?

For ambient temperatures above the 30 °C reference, the Table 25–26 factor reduces. At 40 °C the V-90 PVC factor is 0.87; at 45 °C it is 0.79; at 50 °C it is 0.71. X-90 XLPE factors are higher because the insulation temperature limit is higher.

Do I apply derating before or after the cable selection check?

Derating is applied to the tabulated current-carrying capacity FIRST, then the derated capacity is compared to the design current. Applying derating after a cable selection (i.e., sizing on the unrated capacity and then "checking" derating) is the most common AS/NZS 3000 compliance non-conformance.

ElecAS

Cable Correction Factor Calculator

Apply ambient, grouping, soil and installation condition factors to cable current-carrying calculations.

Why this page matters

Apply ambient, grouping, soil and installation condition factors to cable current-carrying calculations. This static content is published so the canonical route has meaningful crawlable HTML even before the interactive application hydrates.

Who this page is for

Users adjusting cable current-carrying capacity for temperature, grouping and installation environment.

Relevant standards

AS/NZS 3008

What this tool helps with

Review ambient, grouping and soil-related derating factors.
Support cable selection decisions with environment-specific adjustments.
Move directly to cable sizing once correction factors are understood.

How to apply cable derating factors under AS/NZS 3008.1.1:2025

Set the installation method — Pick the AS/NZS 3008.1.1 installation method (A, B, C, D, E, F or G — corresponding to enclosed in conduit, on a wall, in a tray, direct buried, in free air, etc.). The installation method selects which Table 3–13 base rating to use.
Enter the number of circuits in the grouping — Enter the number of cable circuits in the same enclosure / on the same tray / in the same trench. The calculator picks the Table 22–24 / 27 / 29 grouping factor.
Enter the ambient temperature — Enter the design ambient air temperature in °C. The calculator picks the Table 25–26 temperature factor for the matching insulation type.
For buried installations, enter the soil thermal resistivity — Enter the soil thermal resistivity in K·m/W (reference is 1.2 for moist clay; sandy / dry soils are higher). The calculator picks the Table 3.48 factor.
Review the composite factor and derated rating — The calculator displays each individual factor with its table reference, the composite (multiplied) factor, and the derated current-carrying capacity. Export the branded PDF.

Cable current-carrying-capacity derating under AS/NZS 3008.1.1:2025

Why current-carrying capacity must be derated

The tabulated current-carrying capacities in AS/NZS 3008.1.1:2025 Tables 3–13 are based on a single circuit in free air or in a reference installation method at 30 °C ambient. Real installations almost always deviate from those reference conditions: cables are grouped, ambient air temperature differs, soil thermal resistivity differs, and direct-buried installations need a separate suite of corrections.

AS/NZS 3008.1.1:2025 Tables 3.33–3.48 publish derating factors for each deviation. The final derated current-carrying capacity is the tabulated value multiplied by the product of all applicable derating factors. Failure to apply derating is one of the most common AS/NZS 3000 compliance non-conformances flagged during certification.

The four derating dimensions

Grouping (Table 22–24): cables installed in the same enclosure, on the same tray, or in the same buried duct bank derate each other. Factors range from 1.00 for a single circuit to about 0.40 for 20+ grouped circuits.

Ambient air temperature (Table 25–26): for ambient temperatures above 30 °C (typical engine room, ceiling void, switchboard cabinet) the factor reduces. For V-90 PVC the 45 °C factor is 0.79; for X-90 XLPE the factor is 0.85.

Soil thermal resistivity (Table 3.48): the reference soil resistivity is 1.2 K·m/W (moist clay). Dry or sandy soils have higher resistivity (2.5–3.0 K·m/W) and the corresponding factor is 0.80–0.65.

Direct-buried grouping (Table 27, 29): cables in the same trench or duct bank derate each other; the spacing between cables drives the factor.

How the ElecAS derating calculator combines factors

All applicable factors are multiplied together to give a composite derating factor. The derated current-carrying capacity is then the Table 3–13 base rating multiplied by this composite. The branded PDF report lists every factor applied with its table reference so the calculation can be audited row-by-row.

The calculator integrates with the ElecAS cable selection calculator: when sizing a cable, the derating factors apply before the design current is checked against the rating. This avoids the common error of selecting a cable on its undertated tabulated rating only to find it non-compliant once grouping is applied.

Reviewed by

Wisam Tozah — Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). LinkedIn.

Frequently asked questions

What is a cable correction factor?: A correction factor is a multiplier applied to a cable's base current-carrying capacity to account for installation conditions that differ from the reference conditions in AS/NZS 3008.1.1 — typically ambient temperature, grouping, soil thermal resistivity and depth of burial.
How do I combine multiple correction factors?: Multiply the factors together: derated capacity = base capacity × k1 × k2 × k3 × k4. The result must remain at or above the design current.
When does grouping derating apply?: Grouping derating from AS/NZS 3008.1.1 Tables 22–24 applies when multiple loaded cables are installed touching or in close proximity (in trays, conduits, or enclosed runs). Spacing of at least one cable diameter between cables can avoid the derating.
When do I need to apply derating factors to AS/NZS 3008 cable ratings?: Whenever the installation deviates from the reference conditions of the Section 3 base-rating tables: more than one circuit grouped together, ambient above the 40 °C air / 25 °C soil reference, soil resistivity above 1.2 K·m/W, or direct buried with closely spaced circuits. The applicable factors from Tables 3.33–3.48 are multiplied together.
What is the AS/NZS 3008 derating factor for 4 cables in a conduit?: For 4 multicore circuits enclosed in a single conduit, AS/NZS 3008.1.1:2025 Table 22 gives a grouping factor of 0.65 (for 3 or more conductors per circuit). For single-core circuits installed in trefoil the factor is different — refer to Table 23.
How does ambient temperature affect cable rating?: For ambient temperatures above the 30 °C reference, the Table 25–26 factor reduces. At 40 °C the V-90 PVC factor is 0.87; at 45 °C it is 0.79; at 50 °C it is 0.71. X-90 XLPE factors are higher because the insulation temperature limit is higher.
Do I apply derating before or after the cable selection check?: Derating is applied to the tabulated current-carrying capacity FIRST, then the derated capacity is compared to the design current. Applying derating after a cable selection (i.e., sizing on the unrated capacity and then "checking" derating) is the most common AS/NZS 3000 compliance non-conformance.