Calculate main earthing conductors, protective bonding conductors, and circuit protective conductors (CPC) according to British Standards BS 7671:2018
Calculate the minimum size of the main earthing conductor based on the line conductor size
Proper earthing and bonding are critical safety requirements in all UK electrical installations. This calculator helps electricians and electrical engineers determine the correct conductor sizes according to BS 7671:2018 regulations.
Sized using Table 54.7 based on the line conductor size. For line conductors ≤16mm², the earth conductor must be the same size. For conductors >35mm², it can be half the size.
Regulation 543.1.4
For PME (TN-C-S) systems, use Table 54.8 based on PEN conductor size. For TN-S and TT systems, bonding is half the main earthing conductor size (minimum 6mm², maximum 25mm² if mechanically protected).
Regulation 544.1.1 & Table 54.8
Circuit protective conductors are sized using the adiabatic equation: S = √(I²t) / k, where I is fault current, t is disconnection time, and k is a factor based on conductor material and insulation.
Regulation 543.1.3
BS 7671:2018+A2:2022 requirements for earthing and bonding are critical for installation safety. This guide covers the regulatory framework, calculation methods, and practical applications.
The DNO provides different earthing arrangements, each with unique impedance characteristics and safety implications for bonding conductor sizing.
| System | Earth Path | Max Ze | Min Bonding |
|---|---|---|---|
| TN-S | Separate conductor (cable sheath) | 0.8Ω | 6mm² min |
| TN-C-S (PME) | Combined PEN conductor | 0.35Ω | 10mm² min (Table 54.8) |
| TT | Local earth electrode | 21Ω (variable) | 6mm² min |
PME "Open PEN" Risk: In TN-C-S systems, if the PEN conductor is severed in the street, neutral current seeks alternative paths through your bonding conductors. This is why PME bonding sizes (Table 54.8) are larger - they must carry diverted network current without overheating.
A critical update in Amendment 2 changed the approach to bonding. Regulation 411.3.1.2 now states that metallic pipes entering the building with an insulating section at entryneed not be bonded.
The 22kΩ Test: To determine if a metallic part requires bonding, measure resistance between the part and the MET:
Technical Rationale: Modern utilities use polyethylene (MDPE) service pipes. Internal copper plumbing isolated from earth is not "extraneous" - it doesn't introduce earth potential. Bonding such isolated systems is unnecessary and potentially hazardous: in a broken PEN scenario, bonding would export dangerous voltage to taps and radiators.
Practical Check: If the water/gas service pipe is plastic at the property boundary and the internal copper has no contact with earth (verified by 22kΩ test), bonding is not required. Document your test result on the EIC/EICR.
For PME (TN-C-S) installations, bonding conductor sizes are determined by the supply neutral (PEN) conductor size, not the earthing conductor size. This ensures the bonding can carry diverted network current safely.
| Supply Neutral Size | Min Bonding (Copper) | Typical Application |
|---|---|---|
| Up to 35mm² | 10mm² | Most domestic (25mm² tails) |
| 35mm² to 50mm² | 16mm² | Large domestic / small commercial |
| 50mm² to 95mm² | 25mm² | Commercial installations |
| 95mm² to 150mm² | 35mm² | Industrial |
| Over 150mm² | 50mm² | Large industrial |
Common Domestic Scenario: Most UK PME supplies have 25mm² meter tails. The supply neutral is also 25mm² (within the "up to 35mm²" range), so main bonding must be minimum 10mm² to gas and water.
In commercial installations using Steel Wire Armoured (SWA) cable, the armour often serves as the CPC. However, steel has a much lower k value than copper, meaning it handles fault energy less efficiently.
k Values Comparison:
Practical Impact:
The lower k value (46-51 vs 115-143) means steel armour must be physically larger to provide the same protection as copper. Always verify armour adequacy using the adiabatic equation with manufacturer CSA data.
Table 54.7 (selection method) often suggests armour is insufficient, but adiabatic calculation usually validates it because modern breakers clear faults so quickly that the I²t energy is low.
Verification of Zs proves your earthing system works. The loop impedance must be low enough to guarantee protective device operation within required times (0.4s or 5s).
The 80% Design Rule: BS 7671 tables assume cables at maximum operating temperature (70°C). Testing at ambient temperature gives lower readings. Apply the 80% rule during verification:
| MCB Rating | Type B Max Zs | Type C Max Zs | Type D Max Zs |
|---|---|---|---|
| 16A | 2.73Ω | 1.37Ω | 0.68Ω |
| 32A | 1.37Ω | 0.68Ω | 0.34Ω |
| 63A | 0.70Ω | 0.35Ω | 0.17Ω |
Values from BS 7671 Table 41.3 (incorporating Cmin = 0.95)
Regulation 701.415.2 allows supplementary bonding in bathrooms to be omitted if all three conditions are met:
Condition 1
All final circuits have additional protection by 30mA RCDs
Condition 2
All extraneous-conductive-parts are connected to main protective bonding
Condition 3
Disconnection times (ADS) are met for all circuits
In modern domestic installations with full RCD protection and proper main bonding, supplementary bonding is rarely required. However, it remains common in older properties without RCDs.
PME earthing presents specific hazards for outdoor EV charging points. Regulation 722.411.4.1 restricts PME use for outdoor EV installations.
The Risk: A broken PEN in the street can raise the car chassis to 230V relative to the ground the user is standing on. Since users touch the car while standing on earth, this creates an electrocution hazard.
Solutions:
C2: Undersized Bonding on PME
6mm² bonding on a PME supply where Table 54.8 requires 10mm². Potentially dangerous - bonding cannot safely carry diverted neutral current.
C2: No Supplementary Bonding (No RCDs)
Older property with rewirable fuses, no RCDs, and no supplementary bonding in bathroom. The omission conditions are not met.
C3: Inaccessible Earth Connection
Connection to earth electrode is buried or corroded. BS 7671 requires accessible connections for inspection (except exothermic welds).
C3: Bonding to Plastic Pipes
Bonding conductor connected to internal copper with plastic supply pipe. Unnecessary per Amendment 2 - verify with 22kΩ test and remove if appropriate.
Earthing connects exposed conductive parts of electrical equipment to the main earthing terminal, providing a path for fault current. Bondingconnects extraneous conductive parts (pipes, steelwork) to ensure they remain at the same electrical potential, preventing dangerous voltage differences.
PME (Protective Multiple Earthing) or TN-C-S is a supply system where the neutral and earth are combined in the supply network. PME requires larger bonding conductors (Table 54.8) because if the neutral is lost, dangerous voltages can appear on bonded metalwork.
The adiabatic equation (S = √(I²t) / k) calculates the minimum size of circuit protective conductors (CPC) to withstand fault current for the disconnection time without thermal damage. It assumes all heat remains in the conductor (adiabatic heating).
Aluminium can be used but has lower conductivity than copper, requiring larger conductor sizes. Copper is more common due to better conductivity, corrosion resistance, and ease of termination. Always use appropriate connectors for aluminium.
Looking for earthing and bonding products? Browse our directory of electrical distributors across London carrying leading cable brands.
View All DistributorsWas this calculation helpful?