Earthing & bonding
Earthing and bonding conductor size
Find the minimum main earthing conductor and main protective bonding conductor for a supply, from the supply conductor size and the earthing arrangement.
Quick answer: for the typical UK domestic supply (25 mm² tails on a PME / TN-C-S supply) the main earthing conductor is 16 mm² and the main protective bonding to gas and water is 10 mm², from BS 7671 Tables 54.7 and 54.8. Other supplies: use the calculator below.
Main earthing from BS 7671 Table 54.7, main bonding from Table 54.8 (PME) or Reg 544.1 (TN-S / TT). Copper. Guide only; the adiabatic method may permit a different size.
Two different conductors, two different rules
The main earthing conductor connects the installation's main earthing terminal to the supply earth. The main protective bonding connects extraneous parts (gas, water, structural steel) to that terminal. They are sized differently, and mixing the rules is a common slip.
Main earthing conductor
BS 7671 Table 54.7 gives the simple sizing: equal to the line conductor up to 16 mm², 16 mm² for line conductors between 16 and 35 mm², and half the line conductor above 35 mm². The adiabatic equation can sometimes justify a smaller conductor, but the table is the quick, safe answer.
Main protective bonding
On a PME (TN-C-S) supply, Table 54.8 sets the minimum from the supply neutral: 10 mm² up to 35 mm² tails, then 16, 25, 35 and 50 mm² as the supply grows. The typical domestic 25 mm² tails give 10 mm² bonding and a 16 mm² earthing conductor. On TN-S or TT the bonding is at least half the earthing conductor, minimum 6 mm². Confirm the result on the EICR where undersized bonding is a common C2.
BS 7671 Table 54.8: main bonding on a PME supply
| Copper supply neutral (line) conductor | Minimum main bonding conductor |
|---|---|
| Up to 35 mm² | 10 mm² |
| Over 35 up to 50 mm² | 16 mm² |
| Over 50 up to 95 mm² | 25 mm² |
| Over 95 up to 150 mm² | 35 mm² |
| Over 150 mm² | 50 mm² |
These are the copper values. The usual domestic 25 mm² tails sit in the first row, so the main bonding is 10 mm². For supplementary bonding, the bathroom omission rule and how these sizes are coded on a report, see the full guide to earthing and bonding conductor sizes.
BS 7671 Table 54.7: main earthing conductor
| Line conductor | Minimum earthing conductor |
|---|---|
| Up to 16 mm² | Same as the line conductor |
| Over 16 up to 35 mm² | 16 mm² |
| Over 35 mm² | Half the line conductor |
FAQs
What size is the main protective bonding for a 25mm2 supply?
On a PME (TN-C-S) supply with tails up to 35 mm2, the main protective bonding is 10 mm2 copper, from BS 7671 Table 54.8. Larger supplies need 16, 25, 35 or 50 mm2.
How big should the main earthing conductor be?
By BS 7671 Table 54.7 it equals the line conductor up to 16 mm2, is 16 mm2 for line conductors of 16 to 35 mm2, and half the line conductor above 35 mm2. A 25 mm2 supply gives a 16 mm2 earthing conductor.
Is bonding different on TN-S or TT?
Yes. Without PME, the main bonding is at least half the cross-section of the earthing conductor, with a minimum of 6 mm2, rather than following the PME Table 54.8 values.
What size earth bonding cable do gas and water pipes need?
On the common domestic PME supply, 10 mm2 copper to both the gas and water incomers, connected within 600 mm of the meter or point of entry. The size comes from BS 7671 Table 54.8 and grows with the supply neutral, to 16, 25, 35 or 50 mm2 on larger supplies.
What is BS 7671 Table 54.8?
The table that sets the minimum main protective bonding conductor on a PME (TN-C-S) supply, from the copper supply neutral: up to 35 mm2 gives 10 mm2 bonding, over 35 to 50 gives 16 mm2, over 50 to 95 gives 25 mm2, over 95 to 150 gives 35 mm2, and above that 50 mm2.
What happens if main bonding is undersized on an EICR?
Undersized or missing main protective bonding is normally coded C2 (potentially dangerous) on an EICR, because fault current on an extraneous part could not be carried safely. It is one of the most common C2s on domestic reports.