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Cable Size by Earth Fault Loop Impedance

Estimate the maximum route length and fault current from a target fault current and entered line/earth conductor impedances.

Inputs

Project inputs and assumptions

Range: 1–100,000 V

Enter the line-to-earth voltage U0 used by this simplified fault-current estimate. Australian low-voltage examples are commonly around 230/240 V; use the project basis.

Range: 0.01–100,000,000 A

Enter the minimum fault current required by the protective-device disconnection basis. The tool does not derive this value from a device curve or time limit.

Range: 0–100,000 Ω

Enter measured or verified external loop impedance at the supply point, in ohms, on a basis consistent with the conductors and fault path being modelled.

Range: 0–100,000 Ω/km

Enter line-conductor resistance in ohms per kilometre at the selected conductor temperature basis. Use data for the actual conductor and installation.

Range: 0–100,000 Ω/km

Enter protective-earth conductor resistance in ohms per kilometre at the same temperature and construction basis as the line value.

Range: 0–100,000 m

Enter the proposed one-way route length in metres. The line and protective-earth paths are both represented through the entered per-kilometre values.

Result

Selected-route fault current (A)
703.4
Selected-route loop impedance (Ω)
0.3270
Route-length margin (m)
16.1
Maximum one-way route length (m)
41.1
Maximum loop impedance (Ω)
0.4600
EFLI route check
The selected route is within the calculated maximum length for the entered impedance model. Confirm protective-device disconnection time and verify the installed loop by testing.

Formula

Calculation logic

Formula definitions and substituted values.

Before relying on the result

  • Solve maximum route length for a target fault current
  • Compare an entered route with the EFLI limit
  • Use the actual protective-device disconnection basis, source impedance, conductor temperature and current Wiring Rules requirements for final design.

Related calculators

Use the actual protective-device disconnection basis, source impedance, conductor temperature and current Wiring Rules requirements for final design.