Tables 4 to 21 of AS/NZS 3008.1.2:2017 hold the tabulated current-carrying capacity of a conductor. Every cable size result starts in one of them.
What the tables govern
The family covers the combinations a designer has to choose between. The tables split by conductor material, by insulation type, by core count and by cable construction. Within a table, the columns split by installation method.
Each figure is a rating at a stated reference condition. For a cable in air that condition is an ambient temperature of 30 °C. Buried methods carry their own reference soil temperature, soil thermal resistivity and depth of cover.
That matters because a tabulated figure is never the final answer. Cl 3.4 of AS/NZS 3000:2018 asks for the capacity in the conditions of the actual route. Tables 22 to 29 supply the factors that take you there.
Which table AmpSize reads
AmpSize encodes six tables. Single-core cable reads Table 4 or Table 5 on a single-phase circuit. On a three-phase circuit it reads Table 7 or Table 8. Tables 4 and 7 cover thermoplastic insulation, and Tables 5 and 8 cover cross-linked insulation. Multicore cable reads Table 13 for thermoplastic and Table 14 for cross-linked insulation.
Each single-core table gives copper and aluminium columns for all ten installation methods the picker offers. The multicore tables give eight. A multicore cable spaced apart reads the spaced column. A multicore cable alone in its duct reads the duct column.
The engine reads the printed value and applies no ratio. Where a table prints a dash for a size, the tool leaves that size out. In six cells of NZ Table 4 the print is higher than the columns beside it. AmpSize uses the lower value there and says so on the result.
The engine does not yet encode the other tables in the 4 to 21 family. Those cover two-core cables, 110 °C and 150 °C insulation, flexible cords, mineral-insulated cable and aerial cable. Every result names the table it used.
Which tools read the tables
The cable size tool is the only one that reads them. It tests each standard size in turn and reports the smallest that passes every check.
The inputs
A domestic subcircuit.
- Supply: 230 V single phase
- Design current: 40 A
- Route length: 20 m
- Installation method: enclosed in air
- Ambient temperature: 30 °C
- Device: 40 A, C curve
- External impedance Ze: 0.35 Ω
The steps
The ambient is at the reference condition, so the ambient factor is 1. No grouping, no soil and no burial applies, so the total derating factor is 1. The required tabulated current is therefore 40 A.
The engine then walks the sizes. Table 13 gives the 6 mm² conductor 38 A in this method. That sits below both the 40 A design current and the 40 A device rating. It fails on capacity and on the Cl 2.5.3.1 device check. The 10 mm² conductor carries 50 A and passes.
The result
The answer is 10 mm², with a derated capacity of 50 A. The voltage drop is 1.42 % and the earth fault loop impedance is 0.51 Ω. The earth conductor is 4 mm².
The circuit passes every check. The 16 mm² conductor also passes, at 66 A. It is larger than the design needs.
Reading the result on site
Two habits keep the tabulated figure honest.
Match the installation method to the route, not to the cable. One cable in a wall cavity and the same cable clipped in open air read different columns.
Check the ambient against the reference condition. A 30 °C figure is generous for a roof space in February and pessimistic for a cool plant room. The tabulated rating only applies at the reference condition, and the derating tables handle the rest.
2025 numbering
The 2017 parts share the table numbers on this page. AmpSize reads AS/NZS 3008.1.2:2017 for New Zealand with these numbers. The Australian part, AS/NZS 3008.1.1:2025, numbers each table by its section:
- Table 4 is now Table 3.9, and Table 5 is now Table 3.10.
- Table 7 is now Table 3.12, and Table 8 is now Table 3.13.
- Table 13 is now Table 3.18, and Table 14 is now Table 3.19. The Table 13 explainer reads its columns into the calculator.
- Tables 3.21 to 3.26 are new. They give d.c. ratings up to 1500 V d.c.
In these 2025 tables, the buried-direct ratings up to 10 mm² rise. New ratings for large enclosed cables fill the 2017 dashes. The other columns do not change.
The 40 A circuit above, on the AU part at its 40 °C reference, also selects 10 mm². On the 2025 edition its steps cite Table 3.18. Open it on the 2025 edition. The table number map gives the 2025 number of every table that AmpSize reads.
This page is a design aid. Verify every value against the current edition of the standard.