Earth fault loop impedance calculator
Check Zs against the AS/NZS 3000 Table 8.1 maximum for your device, in New Zealand and Australia.
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 7
Each job fills in example values. Replace them with your values.
- Check disconnection timeThe loop impedance of a 32 A circuit on a 45 m run, against the maximum Zs.
- Check Zs on a socket circuitThe loop impedance of a 20 A circuit on 2.5 mm² over 30 m.
- Check Zs on a lighting circuitThe loop impedance of a 10 A circuit on 1.5 mm² over 40 m.
- Check Zs on a motor circuitThe loop impedance of a 25 A D‑curve circuit over 35 m.
- Check Zs on a shed submainThe loop impedance of a 63 A submain on 16 mm² over 60 m.
- Check Zs on a weak rural supplyThe loop impedance of a 20 A circuit with 0.8 Ω at the supply.
- Check Zs in AustraliaThe loop impedance of a 20 A circuit against the Australian tables.
Inputs
A worked example with the default inputs.
- External impedance Ze
- 0.35 Ω
- Voltage
- 230 V
- Device curve
- Curve C
- Device rating
- 20 A
- Measured Zs
- 0 Ω
- Standard
- NZ · AS/NZS 3008.1.2
- Run length
- 30 m
- Active conductor size
- 2.5 mm²
- Earth conductor size
- From Table 5.1
- Material
- Copper
- Insulation
- PVC 75 °C
Earth fault loop impedance
0.89Ω
- Maximum permitted Zs
- 1.53Ω
- Prospective fault current
- 258.25A
- Earth conductor
- 2.5mm²
- R1 + R2
- 0.54Ω
- Maximum length at Zs
- 65.67m
This result is a design aid. Verify it against the current standard.
Calculation steps
- Earth conductor size2.5mm²
AS/NZS 3000:2018 Table 5.1
- R1 + R2 per km at the conductor operating temperature18.02Ω/km
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- R1 + R20.54Ω
- Earth fault loop impedanceResult0.89Ω
- Maximum permitted Zs1.53Ω
AS/NZS 3000:2018 Table 8.1
- Prospective fault current258.25A
About this calculator
How this is calculated
The tool works out the earth fault loop impedance of one circuit. It compares that impedance with the maximum for the protective device.
- Earth conductor. If you leave the earth size on From Table 5.1, the tool picks it from the active conductor size.
- R1 + R2. It reads the resistance of the active and the earth conductors at their operating temperature. 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. It adds the two and multiplies by the run length.
- Zs. It adds the external impedance: Zs = Ze + (R1 + R2).
- Maximum Zs. It reads the maximum for the device curve and rating from AS/NZS 3000 Table 8.1. The circuit passes when Zs is at or below that maximum.
- Fault current. It divides the voltage by Zs to get the prospective fault current.
- Maximum length. It works out the run length at which Zs reaches the maximum.
Ze is a site value. Measure it, or ask the network operator for it. The tool warns when Ze alone takes 90 % or more of the limit. After you install the circuit, test it and compare the measured Zs with the calculated one.
| Step | Table or clause | What it decides |
|---|---|---|
| Earth conductor | AS/NZS 3000 Table 5.1 | The earth size when you do not name one |
| Conductor resistance | Tables 34 and 35 | R1 and R2 at the operating temperature |
| Maximum Zs | AS/NZS 3000 Table 8.1 | The limit for the device curve and rating |
| Disconnection time | AS/NZS 3000 Cl 5.7.2 | The times behind the Table 8.1 limits |
| Failed check | AS/NZS 3000 Cl 5.7.4 | The clause a circuit over the limit cites |
Tables without a standard named are in the part of AS/NZS 3008.1 you pick. The Table 8.1 guide explains why the B, C and D curve limits differ.
Common circuits, worked with the tool
Each line states its route. Every line uses copper cable with PVC insulation, a Ze of 0.35 Ω at 230 V and the New Zealand tables. The earth size comes from Table 5.1.
- Socket‑outlets, 16 A B curve over 30 m of 2.5 mm². Zs is 0.89 Ω against a maximum of 3.59 Ω. The prospective fault current is 258.25 A.
- Cooker, 32 A C curve over 25 m of 6 mm². Zs is 0.67 Ω against a maximum of 0.96 Ω. The earth conductor is 2.5 mm².
- The same cooker circuit over 40 m. Zs is 0.86 Ω against a maximum of 0.96 Ω. The run can reach 47.68 m.
- Motor, 20 A D curve over 50 m of 2.5 mm². Zs is 1.25 Ω against a maximum of 0.92 Ω, so it fails. The run can reach 31.63 m.
New Zealand and Australia
The Standard select picks the part of AS/NZS 3008.1 for the conductor resistance. Table 8.1 and Table 5.1 are in AS/NZS 3000, which is the same in both countries.
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
The result is a design aid. Verify against the current standard.
What does the loop impedance calculator do?
The tool takes the active and earth resistance over the run length, and adds the external impedance Ze. The sum is Zs, the loop impedance for an earth fault. It compares Zs with the maximum for the protective device. It also reports the prospective fault current and the longest run that still passes.
Which tables set the maximum Zs?
The maximum Zs comes from AS/NZS 3000:2018 Table 8.1 for the device curve and rating. The earth conductor size comes from Table 5.1. Conductor resistance comes from AS/NZS 3008.1.2:2017 Tables 34 to 35. A failed check cites AS/NZS 3000:2018 Clause 5.7.4.
What affects earth fault loop impedance the most?
Ze, run length and active conductor size change Zs the most. The device curve and rating set the limit that Zs is compared with. The earth conductor size is taken from Table 5.1 unless you enter one. Cables in parallel divide the active resistance only, not the earth resistance.
Does the loop impedance calculator measure Ze?
No. Ze is a site value and the tool does not measure it. A warning appears when Ze alone uses 90 % or more of the maximum Zs. The prospective fault current is worked out at the phase to earth voltage, which defaults to 230 V. Inputs sit in the page address for sharing.
Worked examples
- Lighting circuit loop impedance, B curve 10 AA 60 m shop lighting circuit on 1.5 mm² cable with a B curve 10 A device, showing Zs, the maximum Zs, the fault current and the longest run that passes.
- A test sheet for a house boardA test sheet for a six-circuit New Zealand house board, with the tests in the AS/NZS 3017 order and the limit for each reading, then the Australian sheet.
- Max cable length for a 20 A C curve subcircuitA 20 A curve C socket circuit on 2.5 mm² runs 40 m. See the Zs the engine returns, the Table 8.1 limit and the maximum length before the circuit fails.
- Fault level at a sub‑board, 300 kVA transformerWork the prospective fault current at a sub‑board 45 m from a 300 kVA transformer, three‑phase and phase-earth, and check 6 kA and 10 kA breakers against it.
- AS/NZS 3000 Table 8.1: max Zs by curve and ratingTable 8.1 max Zs for a 16 A device, by curve. B allows 3.59 Ω, C allows 1.92 Ω and D allows 1.15 Ω. See why, with a 30 m circuit at 0.89 Ω.
- Earth fault loop impedance and maximum ZsWhat earth fault loop impedance measures, how the maximum Zs check works, and a 32 A circuit on 6 mm² over 25 m that lands on 0.67 ohms against a 0.96 limit.
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.
- Checking an existing circuitThe five checks AmpSize runs on an installed circuit, from Iz and voltage drop to Zs and fault withstand, and what a site test adds that a calculation cannot.
- 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.
- IEC 60364 and the Wiring RulesIEC 60364 is the international series that AS/NZS 3000 stays aligned with. See how its parts are numbered, what TN-C-S, TT and IT mean, and where MEN fits.
- Loop impedance and disconnection timeZe, R1+R2, Zs and Ia, and the two disconnection times, worked on a 32 A curve C circuit over 30 m that returns 0.79 Ω against a 0.96 Ω limit.
- RCD types and where they are requiredType AC, A, F and B, what each one detects, and the clauses that place them. The fault path behind them, worked on a 20 A circuit over 25 m.
Clauses and tables this tool reads
- AS/NZS 3000:2018 App B Para B5.2.2 Note 2Conductor temperature in the loop
- AS/NZS 3000:2018 Cl 5.7.4Earth fault loop impedance
- AS/NZS 3000:2018 Table 5.1Minimum earthing conductor size by active conductor size
- AS/NZS 3000:2018 Table 8.1Maximum earth fault loop impedance multipliers by MCB curve
- 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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