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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.

1. 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 For Australia, the Edition select picks 2025 or the superseded 2017 edition.
2. 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.
3. 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 %.
4. 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.
5. 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.
6. Result. The tool lists each size that fails and the check it failed. Each step names its table or clause where one applies.

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.