Dimensioning

Cross-Sectional Area of Protective Conductors

NOTE: The following section deals with dimensioning of the Protective Conductor (the PE conductor).

Reproduced Articles from IEC 60364-5-54

543 Protective Conductors

543.1 Minimum cross-sectional areas

543.1.1 The cross-sectional area of every protective conductor shall satisfy the conditions for automatic disconnection of supply required in clause 413.1 of IEC 60364-4-41 and be capable of withstanding the prospective fault current.

The cross-sectional area of the protective conductor shall either be calculated in accordance with 543.1.2, or selected in accordance with table 54.3. In either case, the requirements of 543.1.3 shall be taken into account.

Terminals for protective conductors shall be capable of accepting conductors of dimensions required by this subclause

543.1.2. The cross-sectional areas of protective conductors shall not be less than the value determined either

in accordance with IEC 60949;
or by the following formula applicable only for disconnection times not exceeding 5 s:

where

S	is the cross-sectional area, in mm²
I	is the value (r.m.s) in A of prospective fault current for a fault negligible impedance, which can flow through the protective device (see IEC 60909-0);
t	is the operating time of the prospective device for automatic disconnection in s; Note 1: Account should be taken of the current-limiting effect of the circuit impedances and the limitation of I²t of the protective device.
k	is the factor dependent on the material of the protective conductor, the insulation and other parts and the initial and the final temperatures (for calculation of k, see annex A).

If application of the formula produces non-standard sizes, conductors of a higher standard cross-sectional area shall be used.

Note 2: For limitations of temperatures for installations in potentially explosive atmospheres, see IEC 60079-0.

Note 3: As the metallic sheaths of mineral insulated cables according to IEC 60702-1 have an earth fault capacity greater than that of the line conductors, it is not necessary to calculate the cross-sectional area of the metallic sheaths when used as protective conductors.

543.1.3 The cross-sectional area of every protective conductor which does not form part of the cable or which is not in a common enclosure with the line conductor shall be not less than

2,5 mm² Cu/16 mm² Al of protection against mechanical damage is provided
4 mm² Cu/16 mm² Al of protection against mechanical damage is not provided

Cross-sectional area of line conductor S mm²		Minimum cross-sectional area of the corresponding protective conductor mm²
Cross-sectional area of line conductor S mm²		If the protective conductor is of the same material as the line conductor	If the protective conductor is not of the same material as the line conductor



where
k₁	is the value of k for the line conductor, selected from table A.54.1 or from the tables in IEC 60364-4-43, according to the materials of the conductor and insulation;
k₂	is the value of k for the protective conductor selected from tables A.54.2 to A.54.6 as applicable
^a	For a PEN conductor, the reduction of the cross-sectional area is permitted only in accordance with the rules for sizing of the neutral conductor (see IEC 60364-5-52).

Values of k for Phase Conductor

	Conductor isolation
	PVC ≤300 mm²	PVC >300 mm²	EPR XLPE	Rubber 60 °C	Mineral
	PVC ≤300 mm²	PVC >300 mm²	EPR XLPE	Rubber 60 °C	PVC	Bare
Initial temperature °C	70	70	90	60	70	105
Final temperature °C	160	140	250	200	160	250
Material of conductor: Copper Aluminum Tin-soldered joints in copper conductors	115 76 115	103 68 –	143 94 –	141 93 –	115 – –	135/115* – –
Note 1: Other values of k are under consideration for: small conductors (particularly for cross-sectional areas less than 10 mm²); duration of short-circuit exceeding 5 s; other types of joints in conductors; bare conductors. Note 2: The nominal current of the short-circuit protective device may be greater than the current-carrying capacity of the cable. Note 3: The above factors are based on IEC 60724.