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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.

1. 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.
2. 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.
3. 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.
4. Harmonics. On three phases with third harmonic current, it reads the reduction factor. Above 33 %, it sizes on the neutral current.
5. 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.
6. 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.

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.