Correction factor calculator
Read each correction factor and the rating a cable needs.
Reads AS/NZS 3008.1.2:2017 (NZ), AS/NZS 3008.1.1:2017 and 2025 (AU) and AS/NZS 3000:2018. Tables checked 27 Sept 2026.
Try a job 7
Each job fills in example values. Replace them with your values.
- Find the correction factors in a hot ceilingThe factor for four circuits in a 50 °C ceiling space, and the rating the cable needs.
- Find the correction factors for a buried cableThe soil, depth and grouping factors for two buried three-phase circuits.
- Find the factor for nine circuitsThe grouping factor for nine circuits on a tray.
- Find the factor on a 50 °C roofThe ambient factor at 50 °C.
- Find the factors in poor soilThe soil and depth factors at 2.5 K·m/W and 1 m.
- Find the factor for a harmonic loadThe factor for 40 % third harmonic in a four-core cable.
- Compare the factors in AustraliaThe same conditions on the Australian tables.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Standard
- NZ · AS/NZS 3008.1.2
- Installation method
- Enclosed in air
- Ambient temperature
- 40 °C
- Grouped circuits
- 3
- Phases
- Single phase
- Design current
- 32 A
- Cable size
- 6 mm²
- Material
- Copper
- Insulation
- PVC 75 °C
- Cores
- Multicore
- Cables in parallel
- 1
Combined correction factor
0.62x
- Required tabulated current
- 51.95A
- Tabulated rating of 6 mm²
- 38A
- Corrected rating Iz
- 23.41A
- Ambient correction factor
- 0.88x
- Grouping correction factor
- 0.7x
This result is a design aid. Verify it against the current standard.
Calculation steps
- Ambient correction factor0.88x
AS/NZS 3008.1.2:2017 Table 27(1) p.88
- Grouping correction factor0.7x
AS/NZS 3008.1.2:2017 Tables 22, 25(1), 25(2) p.79, p.84–85
- Combined correction factorResult0.62x
- Required tabulated current51.95A
AS/NZS 3000:2018 Cl 3.4
- Tabulated rating of 6 mm²38A
AS/NZS 3008.1.2:2017 Table 13 p.62–64
- Corrected rating Iz23.41A
AS/NZS 3000:2018 Cl 3.4
About this calculator
How this is calculated
The tool reads every factor that lowers or raises the current a cable can carry on your route. It is the same code the cable size calculator runs before it tests each size.
- Ambient. It reads the factor for the air temperature, or the soil temperature on a buried 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, so a cooler route gives a factor above 1.
- Grouping. It reads the factor for the circuits that share the route, by how they sit. On a tray, ladder, rack or cleats, pick the cable support. Each parallel cable set counts as a circuit.
- Buried cable. For a buried route it reads the soil thermal resistivity factor and the burial depth factor. Cable buried direct reads its depth factor by conductor size.
- Harmonics. On three phases with third harmonic current, it reads the reduction factor. Above 33 %, it sizes on the neutral current.
- Product. It multiplies the factors into one correction factor (derating factor in 2017). It divides the design current by that factor to give the tabulated current the cable needs.
- One size. It reads the tabulated rating of the size you pick and multiplies it by the factor. The result is Iz, which it checks against the design current.
| Step | Table or clause | What it decides |
|---|---|---|
| Ambient | Tables 27(1) and 27(2) | The factor for the air or soil temperature |
| Grouping | Tables 22, 25(1) and 25(2) | The factor for circuits run together |
| Cable support | Tables 23 and 24 | The factor on a tray, ladder, rack or cleats |
| Buried cable | Table 29, Tables 28(1) and 28(2) | The factors for soil resistivity and depth |
| Harmonics | Table 2 and Cl 3.5.9 | The reduction for third harmonic current in the neutral |
| Required current | AS/NZS 3000 Cl 3.4 | The current the cable must carry in the table |
| Current rating | Table 13 or 14 for multicore, Tables 4, 5, 7 and 8 for single core | The tabulated rating of the size you pick |
Tables without a standard named are in the part of AS/NZS 3008.1 you pick. AS/NZS 3008.1.1:2025 numbers them by section, such as Table 3.44 for Table 27(1).
Common routes, worked with the tool
Each line states its route. Every line uses multicore copper cable with PVC insulation, except where a line names single-core cable.
- Three circuits enclosed in 40 °C air, 32 A on 6 mm². The factors multiply to 0.62 x. The cable needs a tabulated 51.95 A, and 6 mm² carries only 23.41 A on this route.
- The same route on 16 mm². The factors still multiply to 0.62 x, and the cable needs a tabulated 51.95 A. 16 mm² carries 40.66 A, above the 32 A design current.
- Buried direct in Australia, 100 A on 35 mm² single-core. Soil at 20 °C with a resistivity of 2 K·m/W, 1 m deep. The factors multiply to 0.8 x, and the cable needs a tabulated 125.31 A.
- Four three-phase circuits touching, 80 A with 40 % third harmonic on 50 mm². The neutral carries 96 A, so the tool sizes on it. The factors multiply to 0.56 x, and the cable needs a tabulated 171.74 A.
New Zealand and Australia
The Standard select picks the part of AS/NZS 3008.1 the tool reads. For Australia, the Edition select picks 2025 or the superseded 2017 edition. The 2025 edition changed several ambient and grouping factors, so run both when you check an older design.
To size the cable from these factors, use the cable size calculator. To check a cable that is already installed, use the circuit check.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
Is a correction factor the same as a derating factor?
Yes. AS/NZS 3008.1.1:2025 calls them correction factors. The 2017 parts call them derating factors. Both multiply the tabulated rating to give the current the cable carries on your route.
How do I combine the factors?
Multiply them. Ambient, grouping, soil resistivity, burial depth and the harmonic factor each apply to the rating. Divide the design current by the product to get the tabulated current the cable needs.
Why does an Australian cable at 30 °C get a factor above 1?
AS/NZS 3008.1.1 rates cables for 40 °C air and 25 °C soil. A cooler route lets the cable carry more. AS/NZS 3008.1.2 rates New Zealand cables for 30 °C air and 15 °C soil.
Do parallel cables count as grouped circuits?
Yes. Each cable set of a parallel circuit counts as a circuit in the group. Three circuits of two parallel sets read the factor for six circuits.
Why is the depth factor different for a bigger cable?
For a cable buried direct, the depth table bands the factor by conductor size: up to 50 mm², above 50 up to 300 mm², and above 300 mm². The tool reads the band of the size you pick.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 3.4Current-carrying capacity
- AS/NZS 3008.1.2:2017 Cl 3.5.9Third harmonic current in the neutral
- AS/NZS 3008.1.2:2017 Table 13Current ratings, multicore cable with three or four cores, thermoplastic 75 °C
- AS/NZS 3008.1.2:2017 Table 2Reduction factors for harmonic currents in 4- and 5-core cables
- AS/NZS 3008.1.2:2017 Table 22, 25(1), 25(2)Grouping factors for bunched circuits, buried circuits and underground wiring enclosures
- AS/NZS 3008.1.2:2017 Table 23Grouping factors for single-core circuits on trays, ladder supports, racks and cleats in air
- AS/NZS 3008.1.2:2017 Table 24Grouping factors for multicore cables on trays, ladder supports, racks and cleats in air
- AS/NZS 3008.1.2:2017 Table 27(1), 27(2)Rating factors for air and soil ambient temperature
Every clause and table, with how each was verified
Tables last checked on 27 Sept 2026.
Related tools
- Cable size calculatorFind the smallest cable size that passes each check.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Voltage drop calculatorCalculate voltage drop on a cable run and check the limit.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Existing circuit checkCheck an installed circuit: cable, device, drop and Zs.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
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