The PE wire in a distribution box should generally match or exceed the size of the phase conductors, with a minimum of 10 mm² for copper.

Sizing Guidelines

1. Based on Phase Conductor Size
   • For phase conductors ≤ 16 mm², the PE conductor should be the same size as the phase conductor.
   • For phase conductors 16–35 mm², the PE conductor should be 16 mm².
   • For phase conductors > 35 mm², the PE conductor should be half the size of the phase conductor (S_PE = S_ph / 2) if using the same material (copper or aluminum) .
2. Minimum Cross-Section
   • The PE conductor must have a minimum cross-section of 10 mm² for copper or 16 mm² for aluminum, regardless of phase conductor size .
   • In TT systems, if the earth electrode is far from the source, the PE can be limited to 25 mm² (copper) or 35 mm² (aluminum) .
3. Connection in Distribution Boxes
   • The PE conductor should be connected to a dedicated terminal on the main earthing bar.
   • It is recommended to run PE conductors close to the phase conductors to minimize inductive reactance in fault conditions .
   • Color coding should be green-and-yellow for easy identification, or green in some countries like Canada .
4. Special Considerations
   • For PEN conductors (combined protective and neutral), the cross-section must comply with neutral conductor requirements and be ≥ 10 mm² for copper or 16 mm² for aluminum .
   • If multiple PE connections are used, at least one should match the size of the mains feeder .

Practical Recommendation

For a typical low-voltage distribution box in a TN-S or TN-C-S system:

• Phase conductors ≤ 16 mm² → PE = same as phase (e.g., 2.5 mm² or 4 mm² depending on phase)
• Phase conductors 16–35 mm² → PE = 16 mm²
• Phase conductors > 35 mm² → PE = S_ph / 2, but not less than 10 mm² (copper) This ensures compliance with IEC standards, proper fault current handling, and safety for all exposed conductive parts.