Tables 22 to 29 of AS/NZS 3008.1.2:2017 hold the derating factors. They take a tabulated rating and reduce it to what the cable can carry on the actual route.
What the tables govern
Four route conditions reduce a rating. Each has its own table group and its own clause.
- Ambient temperature. A route hotter than the reference condition carries less current.
- Grouping. Cables that share a route heat each other, so each carries less.
- Soil thermal resistivity. Soil that sheds heat poorly reduces a buried rating.
- Depth of burial. A deeper cable sheds heat more slowly.
Cl 3.5.2 to Cl 3.5.5 set out when each factor applies. Soil and depth apply to buried methods only. Grouping applies when more than one circuit shares the route.
The factors multiply. One factor of 0.88 and one of 0.7 do not give 0.79; they give 0.616. Designers who apply them one at a time undersize the cable.
Which tools read the tables
The cable size tool reads them. It multiplies the factors that apply into one total factor. It then divides the design current by that factor to get the tabulated current a conductor must carry. Cl 3.4 of AS/NZS 3000:2018 is the selection clause.
The inputs
A tenancy submain on a tray.
- Supply: 400 V three phase
- Design current: 63 A
- Route length: 30 m
- Installation method: unenclosed and touching
- Ambient temperature: 40 °C
- Grouped circuits: 3
- Device: 63 A, C curve
- External impedance Ze: 0.35 Ω
The steps
The engine reads an ambient factor of 0.88 for 40 °C. It reads a grouping factor of 0.7 for three circuits in this arrangement. Soil and depth do not apply, so both sit at 1.
The product is a total derating factor of 0.616.
Divide 63 A by 0.616 and the required tabulated current is 102.27 A. The engine walks the sizes and finds the smallest that Table 13 rates at or above that figure.
The result
The answer is 35 mm² copper. Table 13 gives it 128 A, and the factors bring that down to 78.85 A. That covers the 63 A device.
The other checks pass with room to spare. Voltage drop is 0.5 % and the earth fault loop impedance is 0.44 Ω on a 10 mm² earth.
What the derating cost
Without the two factors, 63 A would need a conductor rated at 63 A. That is a much smaller cable than 35 mm².
The 25 mm² conductor shows how close the margin runs. Table 13 gives it 104 A, and the factors bring that down to 64.06 A. That still covers the 63 A device. It fails the earth fault loop impedance check instead, at 0.49 Ω on a Ze of 0.35 Ω.
So the derating does not always decide the size. The engine names the check that did.
Two habits
Count the circuits, not the cables. A three-phase circuit of four singles counts as one circuit for grouping. A bundle of four separate final subcircuits counts as four.
Use the hot part of the route. A cable that runs cool, then crosses a hot roof space, takes the roof space temperature.
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. It also names each factor a correction factor (derating factor in 2017).
- Grouping: Table 22 is now Table 3.33 (see the Table 22 explainer). Tables 23 and 24 are now Tables 3.34 and 3.35.
- Buried grouping: Tables 25(1) and 25(2) are now Tables 3.36 and 3.38.
- Underground enclosures: Tables 26(1) and 26(2) are now Tables 3.40 and 3.42.
- Ambient: Tables 27(1) and 27(2) are now Tables 3.44 and 3.45 (see the Table 27 explainer).
- Depth: Tables 28(1) and 28(2) are now Tables 3.46 and 3.47. Table 29 is now Table 3.48.
- Tables 3.37, 3.39, 3.41 and 3.43 are new. They cover more than one row of buried circuits.
What moved in 2025, by table group:
- Table 3.33 changes at 7 and 8 circuits in one touching layer.
- Tables 3.34, 3.35, 3.36, 3.38, 3.40 and 3.42 do not change.
- A new method derives the ambient factors in Tables 3.44 and 3.45.
- Table 3.46 changes for cables over 50 mm². Some factors in Table 3.47 change.
- Table 3.48 does not change.
The 63 A feeder above, on the AU part at its 40 °C reference, takes no ambient factor. Table 3.33 gives the same 0.7 for three circuits. It selects 35 mm² at 78.4 A. 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.