Cable Sizing

Circuit Protective Conductor (CPC) in Cable Sizing

What a circuit protective conductor is, common CPC forms, and how CPC selection affects fault current, earth-loop impedance and thermal withstand.

Updated June 5, 2026

The circuit protective conductor, or CPC, provides a low-impedance path for earth-fault current. Its purpose is to help protect against electric shock and allow sufficient fault current to flow so the protective device disconnects the supply within the required time.

CPC requirements are defined by national and international wiring standards. In cable sizing, the CPC is important for both fault current calculations and earth-fault loop impedance checks.

Common forms of CPC

Depending on the installation standard and arrangement, a CPC may be provided by:

  • A single-core protective conductor.
  • A conductor within a multicore cable.
  • An insulated or bare conductor in a common enclosure with insulated live conductors.
  • A fixed bare or insulated conductor.
  • The metallic sheath or armour of a cable.
  • A metal conduit, cable-management system or electrically continuous support system.
  • An extraneous-conductive-part, where permitted by the applicable regulations.

CPC options in myCableEngineering

For cable calculations, the CPC may be represented in several practical ways:

  1. Cable armour by itself.
  2. A separate protective conductor by itself.
  3. Cable armour combined with a separate protective conductor.

Although combining armour with a separate conductor is common in practice, it can be difficult to evaluate accurately because the fault current is divided between the parallel metallic paths. That division depends on impedance, bonding, installation geometry and the return path.

Checks involving the CPC

  • Earth-fault loop impedance: the CPC impedance contributes to the fault-loop impedance, thereby affecting disconnection time.
  • Thermal withstand: the CPC must withstand the fault current until the protective device clears the fault.
  • Continuity and bonding: metallic paths used as a CPC must be electrically continuous and suitably bonded.
  • Mechanical and installation suitability: the selected CPC arrangement must remain reliable throughout the cable life.

For related calculations, see Fault Current Calculations for Cable Sizing and Cable Thermal Withstand Under Fault Conditions.

For the loop impedance calculation that uses the CPC path, see Earth Fault Loop Impedance.

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