Existing circuit check
Check an installed circuit: cable, device, drop and Zs.
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 the breaker on a 4 mm² circuitWhether a 32 A breaker suits 4 mm² cable, and the largest breaker the cable allows.
- Check an existing three-phase submainWhether 16 mm² on a 63 A breaker over 80 m passes the cable, drop and Zs checks.
- Check a 20 A breaker on 2.5 mm²Whether a 20 A breaker suits a 2.5 mm² circuit.
- Check a 32 A breaker on 6 mm²Whether a 32 A breaker suits a 6 mm² circuit.
- Check a 16 A breaker on 1.5 mm²Whether a 16 A breaker suits a 1.5 mm² circuit.
- Check a 63 A breaker on 16 mm²Whether a 63 A breaker suits 16 mm² mains.
- Check a 10 A breaker on 1 mm²Whether a 10 A breaker suits a 1 mm² lighting circuit.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Phases
- Single phase
- Voltage
- 230 V
- Design current
- 20 A
- Power factor
- 0.9
- Standard
- NZ · AS/NZS 3008.1.2
- Run length
- 30 m
- Installation method
- Enclosed in air
- Ambient temperature
- 30 °C
- Grouped circuits
- 1
- Installed cable size
- 2.5 mm²
- Material
- Copper
- Insulation
- PVC 75 °C
- Cores
- Multicore
- Cables in parallel
- 1
- Voltage drop limit
- 5 %
- External impedance Ze
- 0.35 Ω
- Prospective fault current
- 0 kA
- Device curve
- Curve C
- Device rating
- 20 A
Circuit on 2.5 mm²
Passes
- Corrected rating Iz
- 23A
- Voltage drop
- 4.25%
- Earth fault loop impedance
- 0.89Ω
- Maximum Zs for the device
- 1.53Ω
- Smallest size that passes
- 2.5mm²
- Largest curve C device the cable allows
- 20A
- Combined correction factor
- 1x
This result is a design aid. Verify it against the current standard.
Calculation steps
- Installed conductor size2.5mm²
AS/NZS 3008.1.2:2017 Table 13 p.62–64
- Tabulated rating of 2.5 mm²23A
AS/NZS 3008.1.2:2017 Table 13 p.62–64
- Ambient correction factor1x
AS/NZS 3008.1.2:2017 Tables 27(1), 27(2) p.88
- Grouping correction factor1x
One circuit, no grouping factor
- Combined correction factor1x
- Corrected rating Iz23A
AS/NZS 3000:2018 Cl 3.4
- Voltage drop9.78V
- Voltage drop percent4.25%
AS/NZS 3000:2018 Cl 3.6.2
- Device rating In20A
AS/NZS 3000:2018 Cl 2.5.3.1
- Earth conductor size2.5mm²
AS/NZS 3000:2018 Table 5.1
- Earth fault loop impedance at 230 V phase to earth0.89Ω
AS/NZS 3000:2018 Table 8.1
- Maximum Zs for the device1.53Ω
AS/NZS 3000:2018 Table 8.1
- Smallest size that passes2.5mm²
AS/NZS 3000:2018 Cl 3.4
About this calculator
How this is calculated
The tool takes a circuit that is already installed and runs the five checks of the cable size calculator at its size. It uses the same code, so the two tools agree on every size.
- Current capacity. It reads the tabulated rating of the installed size for your material, insulation, core count and installation method. It multiplies the rating by the correction factors for the route to get Iz. Iz must be at or above the design current Ib.
- Voltage drop. It works out the drop for the run length, the power factor and the conductor. It compares the drop with the limit you set.
- Breaker rating. The breaker rating In must sit between the two: Ib at or below In, and In at or below Iz. The tool gives both margins.
- Loop impedance. It adds the active and earth conductor resistance to the external impedance Ze. The total Zs must stay under the maximum for the breaker curve and rating.
- Fault withstand. If you enter a prospective fault current, the conductor must withstand it for the clearing time.
- Result. The circuit passes when every check it runs passes. The tool gives the smallest standard size that passes with the same breaker, and the largest breaker of the same curve that the cable allows.
| Step | Table or clause | What it decides |
|---|---|---|
| Current rating | Table 13 or 14 for multicore, Tables 4, 5, 7 and 8 for single core | The tabulated rating of the installed size |
| Correction factors | Tables 22 to 29 and Table 2 | The factors for the route |
| Voltage drop | Tables 30, 34 and 35, AS/NZS 3000 Cl 3.6.2 | The drop over the run and its limit |
| Breaker rating | AS/NZS 3000 Cl 2.5.3.1 | That In sits between Ib and Iz |
| Earth and Zs | AS/NZS 3000 Table 5.1 and Table 8.1 | The earth conductor size and the maximum loop impedance |
| Fault withstand | Table 52 and Cl 5.3 | The fault current the conductor survives |
Tables without a standard named are in the part of AS/NZS 3008.1 you pick. AS/NZS 3008.1.1:2025 numbers them by section.
Common circuits, worked with the tool
Each line states its circuit. Every line uses multicore copper cable with PVC insulation and one circuit on the route, enclosed in air at 30 °C. Every breaker is a C curve and Ze is 0.35 Ω, on the New Zealand tables.
- Socket-outlets, 2.5 mm² on a 20 A breaker over 30 m. It passes every check. The drop is 4.25 %, close to the 5 % limit.
- The same cable on a 32 A breaker with a 25 A load. It fails the current, drop and breaker checks. 6 mm² is the smallest size that passes.
- 4 mm² on a 32 A breaker over 20 m. It fails the breaker check, because Iz is 29 A. The largest breaker the cable allows is 25 A, and 6 mm² is the smallest size that passes on 32 A.
- Three-phase submain, 16 mm² on a 63 A breaker over 80 m. It fails on loop impedance: Zs is 0.76 Ω against a maximum of 0.49 Ω. With this breaker, 70 mm² is the smallest size that passes.
New Zealand and Australia
The Standard select picks the part of AS/NZS 3008.1 the tool reads. For Australia, the Edition select picks 2025 or the superseded 2017 edition. The breaker and Zs rules come from AS/NZS 3000, which is the same in both countries.
What the tool does not check
The tool works from the inputs, not from a test. Measure Ze and Zs on site and compare the reading with the maximum it gives. It does not check RCD rules or discrimination between breakers.
To size a new circuit, use the cable size calculator. To read the factors for a route on their own, use the correction factors calculator.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What size breaker can go on 4 mm² cable?
It depends on the route. Enclosed in 30 °C air on its own, 4 mm² copper rates 29 A in New Zealand, so a 25 A breaker is the largest standard size. Grouping, heat or insulation lowers it. Enter your route to read the largest breaker the cable allows.
What does Ib ≤ In ≤ Iz mean?
Ib is the design current of the load, In the breaker rating and Iz the cable rating after correction. The breaker must carry the load and the cable must carry the breaker rating. AS/NZS 3000 Clause 2.5.3.1 sets the rule.
Why does a long circuit fail on loop impedance?
A long or small cable has a high earth fault loop impedance. The fault current is then too low to trip the breaker on its magnetic setting in time. Use a larger cable, a lower breaker rating, a B curve, or check the RCD rules for the circuit.
Does the tool measure my circuit?
No. It works from the cable size, the route and the external impedance Ze you enter. Measure Ze and Zs on site and compare the reading with the maximum Zs the tool gives.
What size does the tool suggest when the circuit fails?
The smallest standard size that passes every check with the same breaker, route and load. It is the answer the cable size calculator gives for the same circuit.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 2.5.3.1Protective device coordination
- AS/NZS 3000:2018 Cl 3.4Current-carrying capacity
- AS/NZS 3000:2018 Cl 3.6.2Voltage drop limit
- 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 13Current ratings, multicore cable with three or four cores, thermoplastic 75 °C
- AS/NZS 3008.1.2:2017 Table 14Current ratings, multicore cable with three or four cores, cross-linked 90 °C
- AS/NZS 3008.1.2:2017 Table 22, 25(1), 25(2)Grouping factors for bunched circuits, buried circuits and underground wiring enclosures
Every clause and table, with how each was verified
Tables last checked on 27 Sept 2026.
Related tools
- Fault level calculatorProspective fault current at a board from its source.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- IDMT grading calculatorPlot the trip curves of inverse-time overcurrent relays.IEC 60255IEEE C37.112
- RCD selection calculatorCheck if an RCD is required and test the trip time.AS/NZS 3000Electricity (Safety) Regulations 2010
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