Table 52 of AS/NZS 3008.1.2:2017 holds the constant k used in the adiabatic check. One value from that table decides whether a conductor survives a short circuit.
What the table governs
Cl 5.3 gives the adiabatic equation, written I²t = k²S². The left side is the energy the protective device lets through. The right side is the energy the conductor can absorb.
k is the conductor constant. It depends on the conductor material and on the insulation. Those two inputs set the temperature the conductor starts at and the temperature it may reach. Table 53 holds the short-circuit temperature limits behind k.
The word adiabatic states the assumption: no heat leaves the conductor during the fault. That is close to true for the first few cycles and pessimistic for longer faults.
Cl 2.5.4.1 of AS/NZS 3000:2018 requires the check. Cl 5.3.3.1.3 applies it to an earthing conductor.
What k does not depend on
k does not change with the fault current, the clearing time or the conductor size. Those three sit elsewhere in the equation.
AmpSize holds one k value for each combination of conductor material and insulation type. Change either input and the k factor changes. Change the fault current and it does not.
The encoded set covers conductors up to and including 300 mm². Above that size the temperature limit changes, and the tool does not carry a separate value for it.
Which tools read the table
- The fault rating tool applies the check on its own. It reports the minimum size, the withstand current for the size you entered and the k factor it used.
- The cable size tool applies the same check to every candidate conductor once you enter a fault current and a clearing time.
The inputs
A submain at a commercial board.
- Prospective fault current: 10 kA
- Clearing time: 0.1 s
- Installed conductor: 25 mm² copper
- Insulation: XLPE
The steps
The engine reads a k factor of 143 for copper with XLPE insulation.
It then rearranges the equation for S. The fault current and the clearing time give the let-through energy, and k converts that energy into a cross-sectional area.
The result
The minimum conductor size is 22.11 mm². The installed 25 mm² conductor is above that, so the check passes.
The engine also reports the withstand current for the installed size: 11.31 kA at the same 0.1 s. That is the fault current this conductor could take, and it sits above the 10 kA at the board.
Why clearing time carries the weight
The energy scales with the square of the current and directly with the time. Halve the clearing time and the minimum size falls by about three tenths. Halve the current and it falls by half.
In practice the supply fixes the current at a board. The clearing time is a design choice, and the device that clears the fault sets it. Use the clearing time of the upstream device that will actually open, not the downstream one.
What the check does not cover
The adiabatic check says nothing about the current rating, the voltage drop or the loop impedance. A conductor can pass it and still fail every other check. Run the cable size tool for the full set.
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 52 is now Table 5.1. It gives k to one more significant figure.
- Tables 53 to 55 are now Tables 5.2 to 5.4. The temperature limits do not change.
- Table 5.5 is new. It gives short-circuit currents.
- Cl 5.3 keeps its number.
The same fault on the AU part moves a little. Take 10 kA for 0.1 s on 25 mm² copper with XLPE. On the 2017 edition k is 143, and the minimum size is 22.11 mm². On the 2025 edition k is 142.9, and the minimum size is 22.13 mm². The 25 mm² conductor passes on both. Open it on 2017 and open it on 2025. 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.