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 3008.1.1:2025 Tables 3.33 to 3.43 applies when multiple loaded cables are installed touching or in close proximity (in trays, conduits, or enclosed runs). Spacing the cables apart lifts the factor but does not remove it: a spaced single layer on a wall or floor still sits at 0.90 in Table 3.33, and six circuits spaced on a ladder are 0.87 rather than the 0.73 they take touching.

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, burial deeper than 0.5 m, or direct buried with closely spaced circuits. The applicable factors from Tables 3.33 to 3.48 are multiplied together.

What if AS/NZS 3008 has no factor for my arrangement?

Then there is no correction factor to apply, and that is a design answer rather than a calculation failure. The tables stop where the Standard stops: Table 3.33 tabulates bunched-in-air groups up to six circuits, the single-core tray table (3.34) up to three circuits per row, and the multi-row buried and enclosure tables up to six or twelve depending on which one applies. Beyond those the arrangement has to change, or the factor has to come from a manufacturer or a thermal calculation. ElecAS only offers the combinations the Standard publishes, so it will not hand you a number the tables do not support.

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

For 4 circuits bunched on a surface or enclosed in a conduit, AS/NZS 3008.1.1:2025 Table 3.33 gives a grouping factor of 0.65. Single-core circuits in trefoil use a different set of tables (3.34 to 3.43): pick the one matching your arrangement.

How does ambient temperature affect cable rating?

AS/NZS 3008.1.1 rates cables in air at a 40 °C ambient, so no correction applies at 40 °C. Above that the factor drops: for a 75 °C PVC cable it is 0.93 at 45 °C and 0.85 at 50 °C. XLPE factors are higher (0.95 and 0.89), because the insulation tolerates a higher conductor temperature. Below 40 °C the factor goes above 1.0, so a cool location can actually gain you capacity.

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: AS/NZS 3008.1.1 Derating Factors k1 to k4

Free Australian cable derating calculator. Work out the AS/NZS 3008.1.1:2025 correction factors for ambient temperature, grouping, soil thermal resistivity and burial depth, see the table and row each factor was read from, and export a branded PDF that records the installation arrangement behind the number.

What a correction factor does to a cable rating

A cable’s tabulated current-carrying capacity assumes conditions the installation rarely matches, so AS/NZS 3008.1.1 derates it. This correction factor calculator applies the four factors that decide how much of the rating survives: k1 for ambient temperature, k2 for grouping with other circuits, k3 for soil thermal resistivity and k4 for depth of burial. It returns each factor with the table it came off and the combined factor they multiply to, so the gap between the table value and what the cable can actually carry where it is installed is on the page rather than assumed. Multiply that factor by the cable’s tabulated rating, or hand the whole job to the cable selection calculator, which applies it for you.

The four AS/NZS 3008.1.1 correction factors at a glance

The four factors multiply: derated capacity = tabulated capacity × k1 × k2 × k3 × k4.
The reference conditions are 40 °C in air and 25 °C in soil, so a factor of 1.00 is a real answer, not an unset one.
Grouped circuits share heat, so each cable's current rating falls as the group grows: four circuits bunched on a surface is 0.65.
Spacing cables apart lifts the grouping factor but does not remove it: a spaced single layer on a wall still sits at 0.90.
Only the arrangements the Standard tabulates give an answer. Where AS/NZS 3008.1.1 publishes no row, there is no factor to quote.
The corrected current rating must still exceed the protective device rating for the circuit to comply.

Who this page is for

Electrical engineers, designers, contractors and electricians correcting cable current-carrying capacity for ambient temperature, grouping, soil thermal resistivity and burial depth on AS/NZS 3000:2018 installations in Australia and New Zealand.

Relevant standards

AS/NZS 3008.1.1:2025: Electrical installations, selection of cables, Tables 3.33 to 3.43 (grouping factors in air, on trays and supports, buried direct and in underground enclosures)
AS/NZS 3008.1.1:2025 Table 3.44 (ambient air temperature) and Table 3.45 (soil temperature), keyed to the conductor operating temperature of the insulation class
AS/NZS 3008.1.1:2025 Tables 3.46 and 3.47 (depth of burial, direct buried and in an underground enclosure)
AS/NZS 3008.1.1:2025 Table 3.48 (soil thermal resistivity, referenced to 1.2 K·m/W)
AS/NZS 3000:2018 Clause 2.5.3.1: the derated capacity Iz is what the protective device rating has to sit inside

What this tool helps with

All four AS/NZS 3008.1.1:2025 correction factors in one place: k1 ambient temperature, k2 grouping, k3 soil thermal resistivity and k4 burial depth, multiplied to a single combined factor.
Only the arrangements the Standard tabulates are offered, so every factor on screen is a published table value: the calculator will not invent one where AS/NZS 3008.1.1 has no row.
Every factor is shown with the table it came from (3.33 to 3.48), and the trace names the row, so the result can be checked against the printed Standard line by line.
Covers cables bunched or in a single layer in air, on ladders, racks and trays, buried direct in trefoil or laid flat, and in underground enclosures, single-core and multicore.
Grouping runs to twenty circuits where the table does, with touching and spaced columns and the cable-to-cable spacings the Standard publishes for each arrangement.
Branded PDF report recording the combined factor, each factor with its table reference, and the installation arrangement the grouping factor was read for.
Feeds straight into the ElecAS cable selection calculator, where the same engine applies the factor before the design current is checked against the rating.

How to work out a cable correction factor

Set the installation environment: Pick where the cable runs: unenclosed air on a surface or wall, unenclosed air on a tray, ladder or rack, buried direct, or in an underground enclosure. The environment decides which grouping table applies, and whether the soil and burial-depth factors apply at all.
Set the cable temperature rating and core configuration: Pick 75 °C PVC, 90 °C XLPE/EPR or 110 °C, and whether the run is multicore or single core. Both change which table row is read: single-core and multicore have separate grouping tables on trays and underground.
Enter the ambient temperature: Set the design ambient air temperature, or the soil temperature for a buried run. The calculator reads the Table 3.44 (air) or Table 3.45 (soil) factor for the matching insulation class. Only temperatures the table publishes for that class can be selected.
Set the grouping arrangement and the number of circuits: Pick how the circuits sit together (bunched, a single layer, touching or spaced, one or more rows) and how many there are. The calculator reads the matching factor from Tables 3.33 to 3.43, and offers only the circuit counts that table publishes.
For buried installations, enter the soil resistivity and burial depth: Enter the soil thermal resistivity in K·m/W (the reference is 1.2 for moist clay; sandy or dry soils are higher) and the burial depth in metres. The calculator reads the Table 3.48 factor and the Table 3.46 or 3.47 depth factor.
Review the combined factor: The calculator shows each factor with the table it came from and the combined factor they multiply to. Multiply your cable's tabulated capacity by that factor to get its derated rating, and export the branded PDF, which records the arrangement each factor was read for.

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 the AS/NZS 3008.1.1:2025 Section 3 rating tables are based on a single circuit in a reference installation, at 40 °C ambient in air and 25 °C in soil. 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 in air (Tables 3.33 to 3.35): cables bunched in the same enclosure, or laid on the same tray or ladder, derate each other. Table 3.33 runs from 1.00 for a single circuit down to 0.38 at twenty circuits bunched on a surface or enclosed.

Ambient temperature (Table 3.44 in air, Table 3.45 in soil): the factor is 1.00 at the reference condition, which is 40 °C in air and 25 °C in soil, and falls above it. At 45 °C in air a 75 °C PVC cable takes 0.93 and a 90 °C XLPE cable takes 0.95; at 50 °C they are 0.85 and 0.89.

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, and for a direct-buried multicore cable 2.5 K·m/W gives 0.74 and 3.0 K·m/W gives 0.69.

Underground grouping and burial depth (Tables 3.36 to 3.43, and 3.46 to 3.47): cables in the same trench or duct bank derate each other, with the spacing between them driving the factor, and burial below the 0.5 m reference depth derates further.

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 Section 3 base rating multiplied by this composite. The branded PDF report lists every factor applied with its table reference, and the grouping arrangement the k2 figure was read for, so the calculation can be checked row-by-row against the printed table.

The page only offers the arrangements, circuit counts, spacings and temperatures AS/NZS 3008.1.1 actually tabulates. Where the Standard publishes no row there is no factor, and the calculator says so rather than returning a number: a combination with no published factor is a design that has to change, not one that derates to zero.

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 untreated tabulated rating only to find it non-compliant once grouping is applied.

Worked example: how quickly mild factors compound

Take a 16 mm² copper XLPE cable with a tabulated capacity of 85 A, installed in a roof space in Western Sydney alongside five other circuits on the same ladder, touching, in one row. None of the conditions is extreme on its own. A 45 °C ambient gives k1 = 0.95 for X-90 insulation from Table 3.44. Six multicore circuits touching in a single row on a ladder gives k2 = 0.73 from Table 3.35. Neither soil resistivity nor burial depth applies, so k3 and k4 are 1.0.

The combined factor is 0.95 × 0.73 = 0.69, so Iz falls from 85 A to about 59 A. A 63 A protective device that looked comfortably inside the cable rating no longer coordinates: In = 63 A now exceeds Iz = 59 A and the Ib ≤ In ≤ Iz chain fails. The fix is either a larger conductor, spacing the circuits on the ladder to lift k2 to 0.87, or a lower device rating if the load allows it.

This is the ordinary case rather than the pathological one, and it is why derating is not a refinement applied at the end of a cable calculation. Grouping alone took more than a quarter of the cable's capacity here, which is frequently the difference between one standard size and the next.

Where the correction factors do not help

Derating addresses steady-state heating, and it is silent on the other three checks a compliant cable has to pass. Voltage drop is governed by conductor impedance and route length, so a cable can be thermally sound after derating and still breach the AS/NZS 3000 Clause 3.6 5% budget on a long run.

The short-circuit withstand check (I²t ≤ k²S²) is a fault-energy calculation over milliseconds, during which no meaningful heat leaves the conductor, so the ambient and grouping factors have no bearing on it. Earth fault loop impedance is likewise unaffected. On a long or heavily protected circuit any of these can be the binding constraint, and the correction factors will not reveal it.

The practical consequence is that a derating result is a necessary input to cable selection rather than an answer in itself. The ElecAS cable size calculator runs all six checks together for that reason.

Correction factor terms

Correction factor (derating factor): A multiplier applied to a cable's tabulated current-carrying capacity to account for installation conditions that differ from the basis of the table. Factors multiply together, so several mild ones can bite harder than one severe one.
k1 (ambient temperature): The factor for an ambient air or soil temperature other than the table basis. Above the basis it is less than 1, because a hotter environment removes less heat from the conductor.
k2 (grouping): The factor for other current-carrying circuits installed alongside. Each neighbouring circuit adds heat, so the more cables in a group and the closer they touch, the lower k2 becomes.
k3 (soil thermal resistivity): The factor for buried cables where the soil conducts heat differently from the 1.2 °C·m/W the tables assume. Dry sand is far worse than moist clay, and the difference is significant.
k4 (depth of burial): The factor for a burial depth other than the table basis. Deeper cable is surrounded by more thermal resistance, so its rating falls.
Current-carrying capacity (Iz): The current a cable can carry continuously without exceeding its insulation temperature limit. Iz is the tabulated value multiplied by every applicable correction factor.
Ib ≤ In ≤ Iz: The protection coordination rule: the design current must not exceed the device rating, and the device rating must not exceed the derated cable capacity. It is where the correction factors actually decide the outcome.

Reviewed by

Wisam Tozah: Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). See how these calculations are verified. LinkedIn. Updated 7 September 2026.

Cable derating 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 3008.1.1:2025 Tables 3.33 to 3.43 applies when multiple loaded cables are installed touching or in close proximity (in trays, conduits, or enclosed runs). Spacing the cables apart lifts the factor but does not remove it: a spaced single layer on a wall or floor still sits at 0.90 in Table 3.33, and six circuits spaced on a ladder are 0.87 rather than the 0.73 they take touching.
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, burial deeper than 0.5 m, or direct buried with closely spaced circuits. The applicable factors from Tables 3.33 to 3.48 are multiplied together.
What if AS/NZS 3008 has no factor for my arrangement?: Then there is no correction factor to apply, and that is a design answer rather than a calculation failure. The tables stop where the Standard stops: Table 3.33 tabulates bunched-in-air groups up to six circuits, the single-core tray table (3.34) up to three circuits per row, and the multi-row buried and enclosure tables up to six or twelve depending on which one applies. Beyond those the arrangement has to change, or the factor has to come from a manufacturer or a thermal calculation. ElecAS only offers the combinations the Standard publishes, so it will not hand you a number the tables do not support.
What is the AS/NZS 3008 derating factor for 4 cables in a conduit?: For 4 circuits bunched on a surface or enclosed in a conduit, AS/NZS 3008.1.1:2025 Table 3.33 gives a grouping factor of 0.65. Single-core circuits in trefoil use a different set of tables (3.34 to 3.43): pick the one matching your arrangement.
How does ambient temperature affect cable rating?: AS/NZS 3008.1.1 rates cables in air at a 40 °C ambient, so no correction applies at 40 °C. Above that the factor drops: for a 75 °C PVC cable it is 0.93 at 45 °C and 0.85 at 50 °C. XLPE factors are higher (0.95 and 0.89), because the insulation tolerates a higher conductor temperature. Below 40 °C the factor goes above 1.0, so a cool location can actually gain you capacity.
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.