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Earthing & Bonding Calculator

Calculate minimum CPC, main earthing and bonding conductor sizes per BS 7671

Reference Info & Formulas
What This Calculates
  • Minimum CPC size (Table 54.7)
  • Adiabatic CPC sizing
  • Main earthing conductor
  • Main bonding conductor
  • Supplementary bonding
Key References

Table 54.7: Min CPC sizes

Reg 544.1: Main earthing

Reg 544.2: Bonding

Table 54.4: k values

Circuit Details
Line conductor size for CPC calculation

Phase conductor of the circuit

Supply Details
For main bonding calculation

Incoming supply neutral conductor

Adiabatic Calculation (Optional)
For fault current based CPC sizing
A

Prospective fault current

s

0.4s for 32A, 5s for distribution

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Data: BS 7671:2018+A2:2022 — Tables 54.7, 54.8, 54.4

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Earthing and bonding conductor sizing uses BS 7671 tables for the simplified method, with the adiabatic equation available for precise verification.

CPC sizing follows Table 54.7: for line conductors up to 16mm², the CPC is equal in size; for 16–35mm², the CPC is 16mm²; above 35mm², the CPC is half the line conductor. Main bonding conductors for PME systems are sized from Table 54.8, based on the supply neutral cross-section.

S = √(I²t) / k
S
= Minimum conductor cross-sectional area (mm²)
I²t
= Energy let-through of protective device (A²s)
k
= Factor for conductor material and insulation (e.g. 115 for Cu/PVC)

BS 7671 Regulation 543.1.3

The adiabatic equation provides an exact check that the chosen CPC can withstand the thermal energy of a fault without its insulation exceeding its limiting temperature. This is particularly useful where non-standard arrangements make the simplified table approach overly conservative.

Quick Reference — PME Main Bonding

Main bonding conductor sizes for PME (TN-C-S) supplies

BS 7671 Table 54.8
Supply Neutral (Cu)Main Bonding Conductor
≤35mm²10mm²
50mm²16mm²
70mm²16mm²
95mm²25mm²
120mm²25mm²

Copper conductors throughout. For aluminium equivalents, consult the full table.

Practical Notes

PME Bonding Is Not Optional

PME bonding requirements from Table 54.8 are significantly larger than the general "half the line conductor" rule. These increased sizes are mandatory — they limit touch voltages in the event of a PEN conductor fault, which could otherwise energise all bonded metalwork.

The 22kΩ Extraneous Part Test

Use a 22kΩ resistance test to determine whether a metallic part is extraneous. If the resistance to earth exceeds 22kΩ, the part is not considered extraneous and does not require bonding. Amendment 2 confirms that plastic incoming water pipes with an insulating section need no bonding.

Verify the Earthing System

Many supplies originally installed as TN-S (separate earth) have been upgraded to TN-C-S (PME) by the DNO without notification to the property owner. Always verify the earthing arrangement at the intake before sizing bonding conductors — the PME requirements are substantially larger.

Frequently Asked Questions

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