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Sizing earthing and bonding conductorsHow big does a main bonding conductor need to be?

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The question at the end: How big does a main bonding conductor need to be? The answer is in the notes below.

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CSA (cross-sectional area) is the size of the copper only, not including the insulation. A 10 mm² bonding conductor has 7 strands.

Use the table, not a rule of thumb. There is no "half the tails, rounded up" rule in the book: read both sizes off Table 3.3.2a (TN-S) or Table 3.3.2b (PME). On PME (TN-C-S) main bonding is never below 10 mm² (up to 35 mm² PEN); on TN-S it can be 6 mm² for 6 and 10 mm² tails. The distributor may require a 16 mm² earthing conductor (see the notes under the table).
Example (PME): 25 mm² tails → table → 16 mm² earthing conductor and 10 mm² main bonding
TN-C-S (PME) · Table 3.3.2b
Supply line/neutral (mm²)Earthing conductor (mm²)Main bonding (mm²)
61010
101010
161610
251610
351610
502516
703525
TN-S · Table 3.3.2a
Line conductor (mm²)Earthing conductor, not buried (mm²)Main bonding (mm²)
666
10106
161610
251610
351610
502516
703525

Highlighted rows = typical domestic. Always check the tables in your own book: Tables 3.3.2a and 3.3.2b.

Electrician's Guide to the Building Regs · Tables 3.3.2a (TN-S) and 3.3.2b (PME): earthing and main protective bonding conductor sizes · p.59

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