Cable size calculator

How the calculator picks a size

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 for New Zealand or AS/NZS 3008.1.1 for Australia. Both are 2017 editions.
2. Derating. It multiplies the ambient, grouping, soil, burial depth and harmonic reduction factors into one factor. 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.

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. Both are the 2017 editions. The checks from AS/NZS 3000 are the same in both countries.

A single-phase circuit at 32 A runs 25 m on the Australian tables. 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.

AmpSize does not hold AS/NZS 3008.1.1:2025. No figure from that edition appears on this site.

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.