Sizes of main Bonding Conductors…
This can be confusing as the size of the main bonding conductor differs depending on the type of supply.
In a TN-S or TT system the main bonding conductor is in relation to the size of the main earthing conductor which is dependant on the size of the incoming line conductor.
e.g. A 25mm line supply conductor requires a 16mm main earth conductor (see table 54.7 BS 7671). The main bonding conductor in a TN-S or TT supply system must be at least half the cross sectional area of the main earth conductor (see 544.1.1 BS 7671).
Half the size of 16mm is 8mm but there’s no such cable size so we’ll go up to the next available size, which is 10mm for the main bonding conductor.
In a TN-C-S the main bonding conductor size is dependant on the size of the incoming neutral conductor (see 544.1.1 second paragraph and Table 54.8 BS 7671).
e.g. A TN-C-S supply has a 25mm neutral conductor, which is less than 35mm (Table 54.8) so a 10mm main bonding conductor is required.
Either way it’s 10mm.