Earthing & Bonding Calculator - BS 7671 Compliant

Calculate main earthing conductors, protective bonding conductors, and circuit protective conductors (CPC) according to British Standards BS 7671:2018

Main Earthing Conductor Size (Table 54.7)

Calculate the minimum size of the main earthing conductor based on the line conductor size

Size of the incoming supply line conductor

About Earthing & Bonding Calculations

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.

Main Earthing Conductor

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

Main Protective Bonding

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

CPC (Adiabatic Equation)

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 Tables Used

Earthing & Bonding

  • • Table 54.7 - Minimum earthing conductor sizes
  • • Table 54.8 - Main protective bonding for PME
  • • Table 54.2 - k values for separate bare conductors
  • • Table 54.3 - k values for conductors in cables

k Values

  • • Copper 70°C PVC: k = 115 (30°C) or 103 (70°C)
  • • Copper 90°C XLPE: k = 143 (30°C) or 134 (90°C)
  • • Aluminium 70°C PVC: k = 76 (30°C) or 68 (70°C)
  • • Aluminium 90°C XLPE: k = 94 (30°C) or 88 (90°C)

When to Use This Calculator

Main Earthing:

  • ✓ Designing new electrical installations
  • ✓ Upgrading consumer units
  • ✓ Checking existing earthing adequacy
  • ✓ Commercial and industrial installations

Main Bonding:

  • ✓ PME (TN-C-S) supply connections
  • ✓ Bonding water and gas services
  • ✓ Oil tank bonding
  • ✓ Structural steelwork bonding

Important Notes

  • ⚠️ PME Systems: Contact your Distribution Network Operator (DNO) to confirm the PEN conductor size for Table 54.8 calculations.
  • ⚠️ Minimum Sizes: Main protective bonding conductors must be at least 6mm² (non-PME) or 10mm² (PME), regardless of calculations.
  • ⚠️ Adiabatic Equation: Only applies where disconnection time does not exceed 5 seconds. For longer times, use alternative calculation methods.
  • ⚠️ Professional Advice: These calculations are for guidance. Always consult BS 7671:2018+A2:2022 and seek professional electrical engineer advice for complex installations.

Frequently Asked Questions

What is the difference between earthing and bonding?

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.

What is PME and why does it affect bonding size?

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.

What is the adiabatic equation used for?

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).

Can I use aluminium for earthing conductors?

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.

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