Cable size calculator
Free cable size calculator for New Zealand and Australia. It reads AS/NZS 3008.1.2:2017, or AS/NZS 3008.1.1:2025 with 2017 selectable, and checks AS/NZS 3000.
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 24
Each job fills in example values. Replace them with your values.
- Size a cable for a shedThe smallest cable that carries the load over the run to a shed.
- Size a cable for an EV chargerThe cable size for a 32 A charger on a 25 m run.
- Size a submain for a tenancyThe smallest three phase submain that passes each check, on AU 3008.1.1:2025.
- Price a cable runA size and a length you can take to a price.
- Size the cable for an ovenThe cable for a 32 A cooking circuit on a 15 m run.
- Size a hot water cylinder circuitThe cable for a 3 kW cylinder on a 20 m run.
- Size a heat pump circuitThe cable for a 20 A heat pump on an 18 m run.
- Size the mains for a houseThe consumer mains for 63 A over 25 m underground.
- Size a three-phase EV charger circuitThe cable for a 22 kW charger on a 30 m run.
- Size an aluminium submainThe smallest aluminium submain for 250 A over 80 m.
- Size a cable in ceiling insulationThe cable for 16 A where insulation surrounds it.
- Size a cable on a hot roofThe cable for 25 A at 50 °C ambient.
- Size a buried cable in dry soilThe cable for 100 A at 2.5 K·m/W and 0.8 m deep.
- Size six circuits on one trayThe cable for 40 A grouped with five other circuits.
- Size parallel cables for 800 ATwo cables per phase for an 800 A feeder.
- Size a cable for a harmonic loadThe cable for 63 A with 35 % third harmonic.
- Size a 22 kW motor cableThe cable for a 41 A motor over 60 m on a 50 A D-curve breaker.
- Size a submain to a sleep-outThe submain for 40 A over 35 m.
- Size a cable with a fault checkThe cable for 32 A that also withstands 6 kA for 0.1 s.
- Size a pool pump circuitThe cable for a 1.5 kW pump on a 30 m run.
- Size a welder outlet circuitThe cable for a 32 A three-phase outlet in a workshop.
- Size a workshop submainThe three-phase submain for 63 A over 45 m.
- Size a supply to a tiny homeThe cable for a 16 A supply over 30 m.
- Size a submain to the 2025 editionA 100 A run of 50 m, read on AS/NZS 3008.1.1:2025 for Australia.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Phases
- Three phase
- Voltage
- 400 V
- Design current
- 50 A
- Power factor
- 0.9
- Standard
- NZ · AS/NZS 3008.1.2
- Run length
- 60 m
- Installation method
- Enclosed in air
- Ambient temperature
- 30 °C
- Grouped circuits
- 1
- Third harmonic content
- 0 %
- Material
- Copper
- Insulation
- PVC 75 °C
- Cores
- Multicore
- Cables in parallel
- 1
- Voltage drop limit
- 5 %
- Check protective device
- No
- Prospective fault current
- 0 kA
Conductor size
10mm²
- Derated capacity
- 50A
- Voltage drop
- 2.66%
- Earth conductor
- 4mm²
- Total derating factor
- 1x
This result is a design aid. Verify it against the current standard.
Calculation steps
- Ambient derating factor1x
AS/NZS 3008.1.2:2017 Tables 27(1), 27(2) p.88
- Grouping derating factor1x
One circuit, no grouping factor
- Soil derating factor1x
AS/NZS 3008.1.2:2017 Table 29 p.90
- Burial depth derating factor1x
AS/NZS 3008.1.2:2017 Tables 28(1), 28(2) p.89
- Total derating factor1x
- Required tabulated current50A
AS/NZS 3000:2018 Cl 3.4
- Selected conductor sizeResult10mm²
AS/NZS 3008.1.2:2017 Table 13 p.62–64
- Derated current capacity50A
AS/NZS 3000:2018 Cl 3.4
- Voltage drop2.66%
About this calculator
How this is calculated
The tool tests every standard conductor size in turn. It reports the smallest size that passes each check you ask for.
- Tabulated current. It reads the current each size can carry for your material, insulation, core count and installation method. The Standard select decides which part it reads. It reads AS/NZS 3008.1.2:2017 for New Zealand or AS/NZS 3008.1.1 for Australia. For Australia, the Edition select picks 2025 or the superseded 2017 edition.
- Correction factors. It multiplies the ambient, grouping, soil, burial depth and harmonic factors into one correction factor (derating factor in 2017). It divides the design current by that factor. Above 33 % third harmonic, it uses the neutral current instead. The result is the current the cable must carry in the table.
- Voltage drop. It works out the drop for the run length, the power factor and the conductor. It compares the drop with the limit you set. The default limit is 5 %.
- Protective device. If you name a device, its rating must sit at or below the derated capacity. The circuit's earth fault loop impedance must stay under the maximum for that device.
- Fault withstand. If you enter a prospective fault current, the conductor must withstand it for the clearing time. The k factor sets that limit. It depends on the material, the insulation and the conductor size.
- Result. The tool lists each size that fails and the check it failed. Each step names its table or clause where one applies.
| Step | Table or clause | What it decides |
|---|---|---|
| Current rating | Table 13 or 14 for multicore, Tables 4, 5, 7 and 8 for single core | The current each size carries in your installation method |
| Ambient | Tables 27(1) and 27(2) | The correction factor for the air or soil temperature |
| Grouping | Tables 22, 25(1) and 25(2) | The correction factor for circuits run together |
| Buried cable | Table 29, Tables 28(1) and 28(2) | The correction 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 after correction |
| Voltage drop | Tables 30, 34 and 35, Cl 4.5, AS/NZS 3000 Cl 3.6.2 | The drop over the run and the limit it must meet |
| Device | AS/NZS 3000 Cl 2.5.3.1 | That the rating sits between the design current and the capacity |
| Earth and Zs | AS/NZS 3000 Table 5.1 and Table 8.1 | The earth conductor size and the maximum loop impedance |
| Fault withstand | Table 52 and Cl 5.3 | The k factor and the smallest size that survives the fault |
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 3.18 for Table 13.
Common circuits, worked with the tool
Each line states its route. Change one input and the size can change, so run your own circuit before you buy cable. Every line uses multicore copper with PVC insulation and one circuit on the route. The power factor is 0.9, except where a line names a different value. Ze is 0.35 Ω and every size uses the New Zealand tables.
- Lighting, 10 A single-phase over 40 m. Enclosed in air at 30 °C with a 10 A C curve device. It needs 2.5 mm² with a 2.84 % drop.
- Socket-outlets, 20 A single-phase over 30 m. Enclosed in air at 30 °C with a 20 A C curve device. It needs 2.5 mm² with a 4.25 % drop, close to the 5 % limit.
- Cooker, 32 A single-phase over 15 m. Enclosed in air at 30 °C with a 32 A C curve device. It needs 6 mm² with a 1.43 % drop.
- Shed supply, 32 A single-phase over 40 m. Enclosed in air at 30 °C with a 32 A C curve device. It needs 6 mm² with a 3.8 % drop.
- EV charger, 32 A single-phase over 25 m. Partly in insulation at 35 °C, with no wiring enclosure. It has a power factor of 1 and a 32 A C curve device. It needs 10 mm² with a 1.55 % drop.
- Single-phase mains, 63 A over 20 m. Enclosed in air at 30 °C with a 63 A C curve device. It needs 16 mm² with a 1.42 % drop.
- Three-phase submain, 100 A at 400 V over 60 m. Enclosed in air at 30 °C. The tool checks no device on this route. It needs 35 mm² with a 1.59 % drop.
New Zealand and Australia
The Standard select picks the part of AS/NZS 3008.1 the tool reads. Part 1.2 holds the New Zealand tables. They assume 30 °C air and 15 °C soil. Part 1.1 holds the Australian tables. They assume 40 °C air and 25 °C soil. The checks from AS/NZS 3000 are the same in both countries.
For Australia, the Edition select picks AS/NZS 3008.1.1:2025 or the superseded 2017 edition. A new calculation starts on 2025. A link with no edition reads 2017, so an old link keeps its numbers.
A single-phase circuit at 32 A runs 25 m on the 2025 Australian edition. It sits enclosed in air at 40 °C, grouped with one other circuit. It needs 10 mm² with a derated capacity of 35.2 A. Open the Australian example.
A three-phase circuit at 30 A and 400 V runs 30 m, buried direct in soil at 25 °C. On the 2025 edition it needs 2.5 mm² with a derated capacity of 34 A. The 2017 edition gives 4 mm² with a derated capacity of 33 A. Open it on 2025 or on 2017.
Keep the result
Every input sits in the page address, so the link in your browser reproduces the calculation. The PDF button prints the result with its steps and table references. Saving a calculation to a project needs a free account.
See also: Cable size for a 32 A single-phase 25 m run, EV charger cable size for a 7 kW 32 A circuit and Current-carrying capacity and derating.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What size cable do I need for a 32 A circuit in NZ?
10 mm² copper on a typical route. The circuit is single-phase, 32 A, over 25 m. It runs enclosed in air at 35 °C, grouped with one other circuit. Its derated capacity is 37.6 A and the voltage drop is 1.42 %. A different route gives a different answer, so enter your own inputs.
Can a 6 mm² cable take 40 A?
Not on a typical route. The route runs 20 m at 40 A single-phase, enclosed in air at 30 °C. On that route, 6 mm² copper fails the current capacity check. The tool selects 10 mm², with 50 A of derated capacity. A shorter run does not change that, because capacity depends on the route, not the length.
What size cable for a 63 A three-phase submain?
25 mm² copper on this route, with 95 A derated capacity. It runs 35 m at 63 A, 400 V, XLPE, enclosed in air at 35 °C. The device is a 63 A C curve, and Ze is 0.3 Ω. 16 mm² meets the current and drop checks but its loop impedance of 0.49 Ω fails Table 8.1.
Does the cable size calculator work for Australia?
Yes. Set Standard to AU · AS/NZS 3008.1.1. The Edition select then picks AS/NZS 3008.1.1:2025, the default, or the superseded 2017 edition. Pick 2017 to reproduce an older design. Both editions use 40 °C air and 25 °C soil. NZ · AS/NZS 3008.1.2 is the default Standard. The AS/NZS 3000 checks, often written AS3000, are the same in both countries.
Which AS/NZS 3008 tables does the cable size calculator use?
On AS/NZS 3008.1.2:2017 or 3008.1.1:2017, single-core ratings come from Tables 4, 5, 7 and 8, multicore from 13 and 14. Derating comes from Tables 22 and 25 to 29, impedance from Tables 30 to 35, k from Table 52. AS/NZS 3008.1.1:2025 renumbers each, so Table 13 is 3.18. AS/NZS 3000:2018 Tables 5.1 and 8.1 give earth size and maximum Zs.
Worked examples
- AmpSize vs jCalc vs free cable sizing toolsCompare AmpSize with jCalc and other free cable sizing tools. One 63 A three-phase submain shows which checks decide a size, and what AmpSize leaves out.
- EV charger cable size for a 7 kW 32 A circuitA 7 kW EV charger on a 32 A dedicated circuit over 30 m needs 6 mm², with 38 A derated capacity, 2.85 percent voltage drop and 0.73 Ω loop impedance.
- AS/NZS 3008.1.2 Tables 22 to 29: deratingWhat the derating tables govern, how ambient and grouping combine into one factor, and a 63 A feeder whose capacity falls to 78.85 A on 35 mm² copper.
- AS/NZS 3008.1.1 table numbers: 2017 to 2025Each AS/NZS 3008.1.1 table AmpSize reads, with its 2017 number, its 2025 number and whether its values moved, and one submain that cites both sets.
- AS/NZS 3008.1.1 vs 3008.1.2: AU and NZ conditionsThe AU part of AS/NZS 3008.1 rates cable at 40 °C air and the NZ part at 30 °C. One 32 A circuit needs 6 mm² in Australia and 4 mm² in New Zealand.
- AS/NZS 3008.1.2 Tables 4 to 21: current ratingsWhat the current rating tables govern, which table AmpSize reads today, and a 40 A circuit that lands on 10 mm² with a derated capacity of 50 A.
Background
- Conduit fill and installation methodsHow the conduit space factor works, what the fill percent means against the 40 percent limit, and how the install method names on site map to the 3008 columns.
- Current-carrying capacity and deratingWhy one cable size has many ratings. Install method, ambient, grouping, soil and depth, with a 25 A circuit that needs 10 mm² once the factors apply.
- MEN system: multiple earthed neutral explainedMEN means multiple earthed neutral. In NZ, earth and neutral join at the main switchboard. A fault on a 32 A C curve circuit then draws 343.8 A and clears.
- Voltage drop basicsOhm's law on a cable, the mV/A·m shortcut, the percent limit and the maximum length, worked on a 25 A run over 35 m of 4 mm² copper.
- AS/NZS 3008.1.1:2025: what the AU part coversWhat AS/NZS 3008.1.1:2025 covers, a.c. to 0.6/1 kV and d.c. to 1500 V at 40 °C air and 25 °C soil, how its sections run, and where the d.c. tables are.
- IB ≤ IN ≤ IZ: the overload rule on a 32 A circuitThe overload rule is IB ≤ IN ≤ IZ. A grouped 32 A circuit fails on 6 mm² and passes on 10 mm², with an IZ of 37.6 A. Why the order matters.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 2.5.3.1Protective device coordination
- AS/NZS 3000:2018 Cl 3.4Current-carrying capacity
- AS/NZS 3000:2018 Cl 3.6.2Voltage drop limit
- AS/NZS 3000:2018 Cl 5.7.4Earth fault loop impedance
- AS/NZS 3008.1.2:2017 Cl 3.5.2.6(a)Groups of circuits in underground wiring enclosures
- AS/NZS 3008.1.2:2017 Cl 3.5.9Third harmonic current in the neutral
- AS/NZS 3008.1.2:2017 Cl 5.3Short-circuit performance
- AS/NZS 3000:2018 Table 5.1Minimum earthing conductor size by active conductor size
Every clause and table, with how each was verified
Tables last checked on 27 Sept 2026.
Related tools
- Voltage drop calculatorCalculate voltage drop on a cable run and check the limit.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Earth fault loop impedance calculatorCheck earth fault loop impedance against the maximum Zs.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Cable fault rating calculatorCheck that a conductor can withstand the fault current.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
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