Two cable sizing tools can return two sizes for the same circuit. The gap comes from the checks each tool runs, not from the arithmetic. One circuit makes the difference visible.
The circuit
A three-phase submain carries 63 A at 400 V. The power factor is 0.9. The route is 35 m. The cable is multicore copper with XLPE insulation, enclosed in air. The ambient temperature is 35 °C. One circuit shares the route. The protective device is a 63 A C curve. Ze is 0.3 Ω. The voltage drop limit is 5 %.
The clauses and the tables
Current ratings come from AS/NZS 3008.1.2:2017 Tables 4-21. Ambient derating comes from Tables 27(1) and 27(2). Selection follows AS/NZS 3000:2018 Cl 3.4. The drop limit is Cl 3.6.2. The earth conductor size comes from Table 5.1. The maximum Zs comes from Table 8.1.
The steps
- Ambient derating factor: 0.95.
- Grouping derating factor: 1.
- Total derating factor: 0.95.
- Required tabulated current: 66.32 A.
- 16 mm² gives a derated capacity of 71.25 A and a drop of 1.3 %.
- 16 mm² gives a Zs of 0.49 Ω, and that exceeds the Table 8.1 limit.
- 25 mm² is the smallest size that passes every check.
The result
The answer is 25 mm². The derated capacity is 95 A. The voltage drop is 0.83 %. Zs is 0.47 Ω with a 6 mm² earth conductor. The device rating is 63 A.
A tool that checks the current rating alone returns 16 mm² for this circuit. That size carries the current and meets the drop limit. It fails the loop impedance check. The check that sets the size here is Table 8.1, not the rating table.
What AmpSize cites and what it costs
AmpSize runs every calculator free of charge, and the tools index lists them all. Cable ratings, impedance, derating and the k factor come from AS/NZS 3008.1.2:2017 for New Zealand. Pick AS/NZS 3008.1.1:2017 for Australia in the Standard select instead. Maximum demand, earth size, maximum Zs and conduit fill come from AS/NZS 3000:2018. The arc flash model is IEEE 1584-2002. The relay curves follow IEC 60255.
Every calculator is open without an account. Pro costs NZ$5 a month or NZ$45 a year. Pro adds unlimited saved calculations, projects and PDF reports with a branding block. The free plan keeps 10 saved calculations and PDF reports with an AmpSize footer line.
Comparing another tool
jCalc is one of several online calculator sites used in New Zealand and Australia. Prices and feature lists change, so check the vendor's site for the current position. Three questions separate the tools.
Which standard and which edition? A tool built on a different standard gives a different rating for the same conductor. The edition matters as much as the document.
Which checks run? Ask whether the tool applies capacity, voltage drop, device rating, loop impedance and fault withstand. A single-check tool will undersize a long circuit.
Does the result show its working? A size alone cannot be reviewed. Look for the derating factors, the required tabulated current and a table reference on every line.
What AmpSize does not do
- It does not move an old Australian link to AS/NZS 3008.1.1:2025. A link with no edition reads the 2017 edition.
- It does not choose an RCD product. The RCD selection tool reports the requirement, the type and the trip limits.
- Its arc flash model is IEEE 1584-2002 only.
- It does not export a drawing or a bill of materials. It exports PDF reports, shareable links and, on Pro, a CSV schedule.
- It does not approve a design and it does not replace a designer.
This page is a design aid. Verify every value against the current edition of the standard.