Earth Cable Size Calculator
Accurate earth cable sizing for electrical earthing systems
Earth Cable Size Calculator
Supports IEC 60364, IEEE 80 & NEC Standards
Calculation Results
How to Use the Calculator
1. Select Standard
Choose the applicable standard for your project:
- IEC 60364: For international projects using adiabatic equation.
- IEEE 80: For substation grounding applications.
- NEC 250: For US projects based on OCPD rating.
2. Enter System Data
Depending on the standard:
- IEC/IEEE: Input Fault Current (A) and Clearing Time (s).
- NEC: Input the rating of the Overcurrent Protective Device (OCPD).
3. Select Material
Select Copper or Aluminum. The calculator automatically adjusts the material constants (k-value) or looks up the correct NEC table column.
How to Calculate Earth Cable Size As Per IEC, IEEE and NEC Standards
IEC 60364 Standard
The IEC standard uses the Adiabatic Equation to determine the minimum cross-sectional area of the protective earth conductor.
Where:
S = Cross-sectional area in mm²
I = Fault current in Amps
t = Fault clearing time in seconds
k = Material constant (e.g., 143 for Copper/PVC)
This method ensures the conductor can withstand the thermal stress of the fault current without degrading.
IEEE 80 Standard
IEEE 80 focuses on safety in AC substation grounding. While it also considers thermal limits, it places heavy emphasis on Touch and Step Potentials.
The design must ensure that the voltage difference between points on the ground (step potential) or between a structure and the ground (touch potential) does not exceed safe limits for the human body.
NEC Article 250
The National Electrical Code (NEC) uses a prescriptive method for sizing Equipment Grounding Conductors (EGC).
Instead of calculating based on fault current, the size is determined by the rating of the Overcurrent Protective Device (OCPD) upstream of the circuit.
For Service Entrance Grounding Conductors, Table 250.66 is used based on the service phase conductor size.
Real Life Earth Cable Sizing
Step-by-step sizing for a typical industrial installation (20kA Fault Level).
Identify Parameters (I & t)
From study: Fault Current I = 20 kA, Clearing Time t = 0.5s.
Select Constant (k)
For generic PVC insulated Copper cable, IEC 60364 gives k = 143.
Execute Adiabatic Calculation
Select Standard Size
Calculated 98.89 mm² Next Size: 120 mm²
Essential Engineering Notes
Earth Cable Size Chart (IEC 60364)
Minimum required size for PVC insulated conductors (Initial 30°C / Final 160°C)
| Fault Current (kA) | Duration (s) | Min Copper Size (mm²) | Standard Copper Size | Min Aluminum Size (mm²) |
|---|---|---|---|---|
| 5 kA | 1.0 s | 35.0 mm² | 35 mm² | 52.6 mm² (70 mm²) |
| 10 kA | 0.5 s | 49.4 mm² | 50 mm² | 74.1 mm² (95 mm²) |
| 10 kA | 1.0 s | 69.9 mm² | 70 mm² | 105.3 mm² (120 mm²) |
| 20 kA | 0.5 s | 98.9 mm² | 120 mm² | 148.9 mm² (150 mm²) |
| 25 kA | 0.5 s | 123.6 mm² | 150 mm² | 186.1 mm² (240 mm²) |
| 31.5 kA | 1.0 s | 220.3 mm² | 240 mm² | 331.6 mm² (400 mm²) |
| 40 kA | 1.0 s | 279.7 mm² | 300 mm² | 421.1 mm² (500 mm²) |
| 50 kA | 1.0 s | 349.7 mm² | 400 mm² | 526.3 mm² (630 mm²) |
Earth Cable Size FAQs
How do you calculate earthing cable size?
Earthing cable size is calculated using the adiabatic equation S = √(I² × t) / k as per IEC 60364, where 'I' is the fault current, 't' is the clearing time, and 'k' is the material constant. Alternatively, for NEC compliance, it is selected from Table 250.122 based on the overcurrent device rating.
What cable size is used for earthing?
The size depends on the potential fault current and system voltage. Common residential sizes are 6mm², 10mm², or 16mm². Industrial systems often use 70mm², 120mm², or larger depending on the main transformer rating and fault levels.
What is 10mm earth cable used for?
A 10mm² earth cable is commonly used for main equipotential bonding in residential properties (connecting gas/water pipes to earth) and for earthing in domestic distribution boards where the supply tails are up to 25mm².
Can I use 4mm cable for earthing?
Yes, 4mm² is often used for supplementary bonding or individual circuit protective conductors (CPCs) for specific circuits like cookers or showers (depending on the circuit rating). However, it is generally too small for the main earthing conductor of a building.
What size is earth cable in 2.5 mm?
In standard "Twin and Earth" flat cables used for power circuits, the earth conductor in a 2.5mm² cable is typically 1.5mm². This is sufficient for the fault currents expected in standard ring final circuits protected by 32A breakers.
Is 6mm earth bonding ok?
For standard residential supply (TN-S or TN-C-S), 10mm² is nowadays the standard for main protective bonding. However, 6mm² was common in older installations and may still be acceptable if it shows no signs of damage and meets the adiabatic check, but upgrading to 10mm² is often recommended during renovations.
What size is earth cable in 1.5 mm?
In a standard 1.5mm² Twin and Earth cable (typically used for lighting circuits), the integral earth conductor (CPC) is usually 1.0mm². This is adequate for the low fault energy associated with 6A or 10A lighting circuits.
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