Earth Cable Size Calculator UK
Choosing the correct earth cable size is critical for electrical safety. In the UK, this choice depends on factors like fault current, installation type, conductor material, and system voltage. An earth cable size calculator UK helps electricians, engineers, and installers select the right size with confidence. Using the right size not only ensures safety but also compliance with BS 7671 regulations. This guide explains how these calculators work, the formulas involved, and what standards apply in the UK.
Earthing Cable Size Calculation Formula
The Earthing Cable Size Calculation Formula used is based on the adiabatic equation:
Where:
S = cross-sectional area (mm²)
I = fault current (A)
t = fault duration (s)
k = constant based on conductor material and insulation
This formula is critical in both manual and automated earth cable size calculator UK tools.
Typical K Values for Different Materials (as per IEC 60364-5-54)
MaterialInitial Temp (°C)Final Temp (°C)K Value (A·s½/mm²)Copper (bare)30250226Copper (insulated)90250143Aluminium (bare)30250148Aluminium (insulated)9025094Steel3025050
Earthing Cable Size Standard in the UK
In the UK, the Earthing Cable Size Standard is governed by BS 7671. The minimum size is usually:
- 2.5 mm² (if protected against mechanical damage)
- 4 mm² (if not protected)
This varies depending on installation type and earthing system.
Below is a reference table for minimum earthing conductor sizes based on the live conductor:
Live Conductor Size (mm²)Minimum Earth Conductor (mm²)Up to 16Same as live conductor16 – 3516 mm²Above 35Half of live conductor size
These are general guidelines, and final sizing should still be confirmed using an earth cable size calculator UK.
Earthing Cable Size as Per IEC Standards
IEC 60364 also provides guidance. According to Earthing Cable Size as Per IEC, the same adiabatic equation is applied with material-specific constants. For aluminium, k = 76, and for copper, k = 115 (PVC insulation).
IEC standards often supplement BS standards, especially for international projects or installations in commercial buildings.
Size of Earth Wire for Domestic Wiring in SWG
In older installations, SWG (Standard Wire Gauge) was used. In modern UK practice, we use mm². However, for Size of Earth Wire for Domestic Wiring in SWG, here’s a conversion example:
SWGmm² (approx)Use Case142.0 mm²Lighting circuits123.3 mm²Small power outlets105.3 mm²Cooker, heavy loads
You can use a Wire Gauge Conversion Calculator to switch between SWG and mm².
Factors That Affect Earth Cable Sizing
The size of an earth cable is influenced by many practical and regulatory factors:
- Type of earthing system: TN-C-S systems may require larger earth conductors.
- Length of cable run: Longer runs may require larger sizes due to resistance.
- Material: Copper conducts better than aluminium, so copper cables can be smaller.
- Installation method: Enclosed installations may need derating of cable size.
- Temperature: Higher ambient temperature reduces current-carrying capacity.
All these variables are built into most earth cable size calculator UK tools.
Using Cable Size for Motor Calculator to Support Earth Sizing
Sometimes, earth cable sizing is done in tandem with phase conductor sizing. The Cable Size for Motor Calculator helps determine phase conductors, and the earth is sized in proportion.
For example, for a 3-phase motor, if the phase conductor is 25 mm², the earth might be 16 mm² or 10 mm² depending on protection methods.
This approach aligns the results of the Cable Size for Motor Calculator with earth cable sizing requirements. Read More.
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