Fault level calculator
Prospective fault current at a board from its source.
Reads AS/NZS 3008.1.2:2017 (NZ), AS/NZS 3008.1.1:2017 and 2025 (AU) and AS/NZS 3000:2018. Tables checked 27 Sept 2026.
Try a job 8
Each job fills in example values. Replace them with your values.
- Find the fault level at a sub-boardThe fault current at a board 45 m from a 300 kVA transformer, against 6 kA breakers.
- Check breakers beside a 1000 kVA transformerWhether 25 kA breakers suit a main board 20 m from a 1000 kVA transformer.
- Find the fault level at a house boardThe fault current at the end of 20 m of 16 mm² mains.
- Find the fault level at a shedThe fault current 60 m down a 16 mm² submain.
- Find the fault level at an MCCThe fault current 30 m from a 500 kVA transformer.
- Check 6 kA breakers at a tenancy boardWhether 6 kA devices suit the fault level at the board.
- Find the fault level on parallel feedersThe fault current with two cables per phase.
- Find fault level from a 10 kA serviceThe fault current at a board in Australia.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Source
- Transformer
- Transformer rating
- 500 kVA
- Transformer impedance
- 4.5 %
- Line voltage
- 400 V
- Standard
- NZ · AS/NZS 3008.1.2
- Submain length
- 30 m
- Conductor size
- 35 mm²
- Material
- Copper
- Insulation
- PVC 75 °C
- Cores
- Multicore
- Cables in parallel
- 1
- Circuit-breaker breaking capacity
- 15 kA
Three-phase fault current at the board
10.01kA
- Phase-earth fault current, maximum
- 3.18kA
- Phase-earth fault current, cable hot
- 2.63kA
- Source impedance per phase
- 14.4mΩ
- Breaking capacity
- 15kA
This result is a design aid. Verify it against the current standard.
Calculation steps
- Transformer impedance per phase, U² ÷ S × Z%0.01Ω
- Fault current at the source16.04kA
- Submain phase resistance at operating temperature0.02Ω
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- Submain phase resistance at 20 °C0.02Ω
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- Submain phase reactance0Ω
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- Protective conductor size10mm²
AS/NZS 3000:2018 Table 5.1
- Protective conductor resistance at 20 °C0.06Ω
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- Three-phase fault current at the board, U0 ÷ |Z|Result10.01kA
- Phase-earth fault current at the board, maximum3.18kA
- Phase-earth fault current, cable at operating temperature2.63kA
- Breaking capacity of the circuit-breaker15kA
About this calculator
How this is calculated
The tool finds the fault current at a board. The source is a transformer or a stated supply fault level. A submain then adds impedance. The tool compares the result with the breaking capacity of your device.
- Source impedance. For a transformer, the tool works Z = U² ÷ S × Z% per phase. It takes the source as all reactance. That gives the highest current for the size.
- Source current. It gives the fault current at the transformer terminals.
- Submain impedance. It reads the resistance and reactance of the submain for your size, length and material. The tables are Table 30 for reactance and Tables 34 and 35 for resistance. It scales the resistance to 20 °C for the maximum figure.
- Protective conductor. It reads the earth conductor size from Table 5.1 of AS/NZS 3000:2018. It adds that resistance to the earth loop.
- Fault currents. It gives the three-phase current as U0 ÷ |Z|. It also gives the phase-earth current, maximum and with the cable hot.
- Breaking capacity. It compares the three-phase current with the rating you enter.
| Step | Table or clause | What it decides |
|---|---|---|
| Submain reactance | AS/NZS 3008.1.2 Table 30 | The reactance of the submain |
| Submain resistance | AS/NZS 3008.1.2 Tables 34 and 35 | The resistance of the submain |
| Earth conductor | AS/NZS 3000 Table 5.1 | The size of the protective conductor |
Worked example
- 500 kVA transformer with a 30 m 35 mm² submain. The transformer has 4.5 % impedance. The three-phase fault current at the board is 10.01 kA. The maximum phase-earth current is 3.18 kA. A 15 kA device passes.
Limits
- The tool works at nominal voltage. It does not apply the IEC 60909 voltage factors.
- It takes the network above the transformer as infinite. A real network lowers the current a little.
- It takes the phase-earth loop at the transformer as its phase impedance, as for a Dyn transformer.
- It does not add motor contribution. It does not grade devices. Use the IDMT tool for grading.
- The result is an estimate for sizing. Ask the network company for the supply fault level at the point of supply.
See also: Prospective fault current at a board, Fault level at a sub-board on a 300 kVA transformer and Fault rating.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How does the fault level tool work out the current?
It adds impedances per phase. A transformer gives U² ÷ S × Z%, and a supply gives the phase voltage over its fault level. The submain adds its resistance and reactance from the AS/NZS 3008.1 impedance tables. The phase voltage over the total is the three-phase fault current at the board.
Why are there two phase-earth figures?
The tables hold the resistance at the conductor operating temperature. A cold cable gives the highest current, so the maximum scales the resistance to 20 °C. The hot figure is the lower one, for disconnection time checks in the loop impedance tool.
What does the tool assume about the source?
It takes the source as all reactance and the network above a transformer as infinite. Both give the highest current for the figures you enter. It uses the nominal voltage with no IEC 60909 voltage factor. A supply source needs Ze at the origin for the phase-earth figures.
What do I do with the result?
Compare it with the breaking capacity of the circuit-breakers at the board. Then carry it to the fault rating tool, which checks that the cables withstand the fault for the clearing time.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Table 5.1Minimum earthing conductor size by active conductor size
- AS/NZS 3008.1.2:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2025 Table 4.1(A), 4.1(B), 4.5(A), 4.5(B)Reactance at 50 Hz and a.c. resistance at 50 Hz
Every clause and table, with how each was verified
Tables last checked on 27 Sept 2026.
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