TN-C-S:
Regulation 544.1.1: "...Except for highway power supplies and street furniture, where PME conditions apply the main protective bonding conductorshall be selected in accordance with the neutral conductor of the supply and Table 54.8..."
Table 54.8 indicates the minimum CSA of 10mm², where the neutral conductor is 35mm² or less.
16mm² is for where the neutral conductor is greater than 35mm² up to 50mm².
Regulation 542.3.1: "Every earthing conductor shall comply with Section 543 and, where PME conditions apply, shall meet the requirements of Regulation 544.1.1 (see above) for the cross-sectional area of a main protective bonding conductor..."
As the neutral is 25mm², which is smaller than 35mm², the minimum CSA required for both the Main earthing conductor and the Main protective bonding conductors is 10mm².
As for supplementary bonding conductors, the minimum CSA depends upon what it is being used for, and whether it is sheathed or otherwise mechanically protected.
If it is not sheathed or otherwise mechanically protected it shall be a minimum of 4mm².
However if it is sheathed or mechanically protected:
(i) Connecting two exposed-conductive-parts, then it shall be no smaller than the smallest CPC connected to the exposed-conductive-parts.
(ii) Connecting an exposed-conductive-part to an extraneous-conductive-part then it shall be no less than half the CSA of the smallest CPC connected to the exposed-conductive-part.
(iii) Connecting two extraneous-conductive-parts, the minimum CSA is 2.5mm².
TN-S & TT:
Regulation 542.3.1: "Every earthing conductor shall comply with Section 543..."
Regulation 543.1.1: The cross-sectional area of every protective conductor, other than a protective bonding conductor, shall be:
(i) calculated in acordance with Regulation 543.1.3, or
(ii) selected in acordance with Regulation 543.1.4...
...the cross-sectional area shall not be less than 2.5mm² copper equivalent if protection against mechanical damage is provided, and 4mm² copper equivalent if mechanical protection is not provided (see also Regulation 543.3.1)."
Regulation 544.1.1: "...a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation and not less than 6mm²."
The minimum CSA for a Main earthing conductor is 2.5mm² if mechanically protected, or 4mm² if not.
The minimum CSA for the Main protective bonding conductors is 6mm² or half the CSA required for the Main earthing conductor, whichever is the greater.
