Cable fault rating calculator
Cable fault rating calculator for New Zealand and Australia. The k factor comes from AS/NZS 3008.1.2:2017, or AS/NZS 3008.1.1:2025 with 2017 selectable.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Prospective fault current
- 6 kA
- Clearing time
- 0.1 s
- Standard
- NZ · AS/NZS 3008.1.2
- Conductor size
- 25 mm²
- Material
- Copper
- Insulation
- PVC 75 °C
- Insulation class
- From the cable insulation
- Joint in the run
- No joint
Minimum conductor size
17.09mm²
- Selected size
- 25mm²
- Withstand fault current
- 8.78kA
- k factor
- 111
- Temperature limit
- 160°C
This result is a design aid. Verify it against the current standard.
Calculation steps
- Conductor start temperature75°CAS/NZS 3008.1.2:2017 Table 53
- Short-circuit temperature limit160°CAS/NZS 3008.1.2:2017 Table 53
- k factor111xAS/NZS 3008.1.2:2017 Table 52 p.121
- Minimum conductor size17.09mm²AS/NZS 3008.1.2:2017 Cl 5.3
- Withstand fault current8.78kAAS/NZS 3008.1.2:2017 Cl 5.3
About this calculator
How this is calculated
The tool checks that a conductor survives a fault current for the time the protective device takes to clear it. It uses the adiabatic equation, S = √(I²t) / k. It assumes that no heat leaves the conductor during the fault.
- Temperatures. It reads the start temperature and the short-circuit temperature limit for the insulation. The insulation class select can name a 110 °C cross-linked type. A joint in the run can set a lower limit than the insulation.
- k factor. It reads k from Table 52 for the conductor material and the two temperatures. A larger temperature rise gives a larger k.
- Minimum size. S = √(I²t) / k gives the smallest conductor, in mm², that stays within the limit. I is the fault current in amperes and t is the clearing time in seconds.
- Withstand. It turns the equation round for the size you pick: I = k × S / √t. That is the largest fault current the conductor survives for that time.
- Result. The size you pick passes when it is at or above the minimum.
The Standard select decides which part it reads. It reads AS/NZS 3008.1.2 for New Zealand or AS/NZS 3008.1.1 for Australia. Take the clearing time from the device curve at the fault current. A longer time needs a larger conductor.
| Step | Table or clause | What it decides |
|---|---|---|
| Temperatures | Table 53 | The start temperature and the limit for the insulation |
| Joint | Table 55 | A lower limit where the joint sets it |
| k factor | Table 52 | The k for the material and the two temperatures |
| Minimum size and withstand | Cl 5.3 | The adiabatic check itself |
Tables without a standard named are in the part of AS/NZS 3008.1 you pick.
Common faults, worked with the tool
Each line states its conductor. Every line uses copper and the New Zealand tables, with no joint unless the line names one.
- Submain, 6 kA for 0.1 s on 25 mm² PVC. The minimum size is 17.09 mm² with a k of 111. The 25 mm² cable withstands 8.78 kA.
- The same fault on 25 mm² XLPE. The minimum size is 13.27 mm² with a k of 143. The cable withstands 11.31 kA.
- 25 mm² XLPE with a soldered joint. The joint sets the limit at 160 °C. The minimum size is 18.99 mm² with a k of 99.9. The cable withstands 7.9 kA.
- 10 kA for 0.2 s on 16 mm² PVC. The minimum size is 40.29 mm² with a k of 111, so it fails. The 16 mm² cable withstands 3.97 kA.
Keep the result
Every input sits in the page address, so the link in your browser reproduces the calculation. The PDF button prints the result with its steps and table references. Saving a calculation to a project needs a free account.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the fault rating calculator do?
The tool applies the adiabatic check to a conductor. From the prospective fault current and the clearing time, it works out the minimum size. It also reports the fault current the entered size can withstand for that time. The check passes when the installed size is not below the minimum.
Where does the k factor come from?
The k factor comes from AS/NZS 3008.1.2:2017 Table 52. It depends on the conductor material and the insulation type. A failed check cites AS/NZS 3008.1.2:2017 Clause 5.3, which gives the adiabatic equation I²t = K²S². Change the material or the insulation and the k factor changes with it.
Which inputs matter most?
The prospective fault current and the clearing time drive the minimum size. The clearing time must match the device that clears the fault, not the downstream device. Material and insulation set the k factor. Cables in parallel are treated as one conductor of the combined cross section.
What does this check not cover?
The adiabatic method assumes that no heat leaves the conductor during the fault. It does not check the current rating, the voltage drop or the loop impedance. Use the cable size tool for those checks. The inputs sit in the page address, so a link reproduces the same check.
Does the conductor size or a joint change the k factor?
Yes. AS/NZS 3008.1.2:2017 Table 53 drops the thermoplastic limit from 160 to 140 degrees above 300 square millimetres. Table 55 limits a soldered joint to 160 degrees. The tool reads both, then takes the k factor for that start and limit pair from Table 52.
Worked examples
- AS/NZS 3008 vs BS 7671: cable sizing comparedHow BS 7671 and AS/NZS 3000 with AS/NZS 3008.1 size a cable, where they differ, and why a BS current rating never converts into an AS/NZS one.
- Prospective fault current at a main switchboardA 16 kA fault cleared in 0.1 s on 70 mm² copper XLPE, with the minimum conductor size, the withstand current and the k factor the adiabatic check returns.
- AS/NZS 3008.1.1:2025: what changed from 2017The 2025 AU part adds d.c. ratings to 1500 V, renumbers every table and moves some ratings. Four circuits, each run on both editions, show what moves.
Background
- MEN system: multiple earthed neutral explainedMEN means multiple earthed neutral. In NZ, earth and neutral join at the main switchboard. A fault on a 32 A C curve circuit then draws 343.8 A and clears.
- IB ≤ IN ≤ IZ: the overload rule on a 32 A circuitThe overload rule is IB ≤ IN ≤ IZ. A grouped 32 A circuit fails on 6 mm² and passes on 10 mm², with an IZ of 37.6 A. Why the order matters.
- Short-circuit withstand and the k factorThe adiabatic check in plain terms: prospective fault current, clearing time and k, worked on 25 mm² copper XLPE at 10 kA for 0.1 s.
- Industrial switchboards: fault rating, form, IPWhat the three ratings on an industrial switchboard or motor control centre mean, how each one is chosen, and which AmpSize tools feed the numbers behind them.
Clauses and tables this tool reads
- AS/NZS 3008.1.2:2017 Cl 5.3Short-circuit performance
- AS/NZS 3008.1.2:2017 Cl 5.5.1Short-circuit duration
- AS/NZS 3008.1.2:2017 Table 52Values of constant K for determination of permissible short-circuit currents
- AS/NZS 3008.1.2:2017 Table 52, 53, 54, 55Short-circuit K values and temperature limits
- AS/NZS 3008.1.1:2017 Table 52Values of constant K for determination of permissible short-circuit currents
- AS/NZS 3008.1.1:2017 Table 52, 53, 54, 55Short-circuit K values and temperature limits
- AS/NZS 3008.1.1:2025 Table 5.1Values of constant K for determination of permissible short-circuit currents
- AS/NZS 3008.1.1:2025 Table 5.1, 5.2, 5.3, 5.4Short-circuit K values and temperature limits
Every clause and table, with how each was verified
Tables last checked on 27 Sept 2026.
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