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How this is calculated#

The tool finds the longest final subcircuit route that two limits each allow. It reports which limit governs. It reads AS/NZS 3000:2018 and AS/NZS 3008.1.2:2017.

1. Fault loop length. The tool reads the route length for your breaker, size and earth size from Table B1.
2. Resistance and reactance. It reads these for the active conductor. The tables are Table 30 and Tables 34 and 35, for copper at operating temperature.
3. Millivolts per ampere metre. It combines both values with your power factor.
4. Voltage drop limit. Cl 3.6.2 sets the limit. You enter it as a percentage of the supply voltage.
5. Voltage drop length. It finds the route length at which the drop equals the limit for your design current.
6. Governing length. The shorter of the two lengths is the maximum route length.

Worked example#

• 2.5 mm² on a 20 A type B breaker. The earth is 2.5 mm², with PVC insulation, on a single‑phase 230 V supply. The design current is 20 A at a power factor of 0.9. The fault loop route length is 128 m. The voltage drop route length is 35.26 m. The voltage drop governs, so the maximum route length is 35.26 m.

Limits#

The tool assumes copper conductors. The reactance is the Table 30 value for a multicore cable. A single‑core run reads a higher reactance for its formation.

The tool does not check the current rating of the cable. It does not add correction factors. Use the cable size calculator for the full check.

See also: Maximum length of a 20 A final subcircuit, The voltage drop limit in Cl 3.6.2 and the voltage drop calculator.