Chapter G

Sizing and protection of conductors


Sizing of protective earthing conductor

From Electrical Installation Guide

Figure G59 below is based on IEC 60364-5-54. This table provides two methods of determining the appropriate c.s.a. for both PE or PEN conductors.

Fig. G59 – Minimum cross section area of protective conductors
Method c.s.a. of phase
conductors Sph (mm2)
Minimum c.s.a. of
PE conductor (mm2)
Minimum c.s.a. of
PEN conductor (mm2)
Cu Al
Simplified method[a] Sph ≤ 16 Sph[b] Sph[c] Sph[c]
16 < Sph ≤ 25 16 16
25 < Sph ≤ 35 25
35 < Sph ≤ 50 Sph/2 Sph/2
Sph > 50 Sph/2
Adiabatic method Any size   [c][d]
  •      - 2.5 mm2 if the PE is mechanically protected
  •      - 4 mm2 if the PE is not mechanically protected
  1. ^ Data valid if the prospective conductor is of the same material as the line conductor. Otherwise, a correction factor must be applied.
  2. ^ When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected:
  3. ^ 1 2 3 For mechanical reasons, a PEN conductor, shall have a cross-sectional area not less than 10 mm2 in copper or 16 mm2 in aluminium.
  4. ^ Refer to table A.54 of IEC60364-4-54 or Figure G60 to get values of k factor.

The two methods are:

  • Adiabatic (which corresponds with that described in IEC 60724)
This method, while being economical and assuring protection of the conductor against overheating, leads to small c.s.a.’s compared to those of the corresponding circuit phase conductors. The result is sometimes incompatible with the necessity in IT and TN schemes to minimize the impedance of the circuit earth-fault loop, to ensure positive operation by instantaneous overcurrent tripping devices. This method is used in practice, therefore, for TT installations, and for dimensioning an earthing conductor[1]
  • Simplified
This method is based on PE conductor sizes being related to those of the corresponding circuit phase conductors, assuming that the same conductor material is used in each case.
Thus, in Fig. G58 for:
Sph ≤ 16 mm2 : SPE = Sph
16 < Sph ≤ 35 mm2 : SPE = 16 mm2
Sph > 35 mm2 : SPE = Sph / 2

Note: when, in a TT scheme, the installation earth electrode is beyond the zone of influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 25 mm2 (for copper) or 35 mm2 (for aluminium).

The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger than 10 mm2 (copper) or 16 mm2 (aluminium).

Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that necessary for the neutral, as discussed in Sizing the neutral conductor.

This c.s.a. cannot be less than that of the phase conductors unless:

  • The kVA rating of single-phase loads is less than 10% of the total kVA load, and
  • Imax likely to pass through the neutral in normal circumstances, is less than the current permitted for the selected cable size.

Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-conductor protection (described in Protection of the neutral conductor)

Values of factor k to be used in the formulae

These values are identical in several national standards, and the temperature rise ranges, together with factor k values and the upper temperature limits for the different classes of insulation, correspond with those published in IEC60364-5-54, Annex A.

The data presented in Figure G60 are those most commonly needed for LV installation design.

Fig. G60 – k factor values for LV PE conductors, commonly used in national standards and complying with IEC60364-5-54 Annex A
k values Nature of insulation
Polyvinylchloride (PVC) Cross-linked-polyethylene (XLPE)

Ethylene-propylene-rubber (EPR)

Final temperature (°C) 160 250
Initial temperature (°C) 30 30
Insulated conductors not incoporated in cables or bare conductors in contact with cable jackets Copper 143 176
Aluminium 95 116
Steel 52 64
Conductors of a multi-core-cable Copper 115 143
Aluminium 76 94

Notes

  1. ^ Grounding electrode conductor
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