Earthing Rod Size Calculator
Calculate required earthing rod length to achieve target resistance (IEC 60364 / IEEE 80)
Earthing Rod Size Calculator
Based on Soil Resistivity (IEC 60364 / IEEE 80)
Results:
Option 1: Single Rod Solution
How to Use the Calculator
Step 1: Input Data
- Enter Soil Resistivity (Ω·m) from site survey
- Set Target Resistance (usually < 5Ω or 1-2Ω)
- Select standard Rod Diameter
Step 2: Calculate
- Click Calculate Earthing Rod Size
- Review the required Rod Length
- Check the calculated resistance value
Step 3: Verification
- If length > 3m, consider deep driving or multiple rods
- Ensure spacing between rods is ≥ rod length
- Minimum recommended length is 2.4 m
How to Calculate Earthing Rod Size
1. Methodology Formula
We use the standard IEEE 80 / IEC 60364 formula for a single vertical rod:
R = (ρ / 2πL) × (ln(4L/d) - 1)
Where:
- ρ (Rho): Soil Resistivity (Ω·m)
- L: Length of the rod (m)
- d: Diameter of the rod (m)
2. Soil Resistivity (ρ)
This is the most critical factor. It determines how conductive the ground is.
Common Values:
- Wet Soil: ~10-50 Ω·m
- Dry Soil: ~100-500 Ω·m
- Rocky Soil: > 1000 Ω·m
Measure this on-site for accuracy.
3. Rod Dimensions
Standard rods come in specific sizes. Increasing length reduces resistance much more effectively than increasing diameter.
Standard Diameters:
12.7mm (1/2"), 14.2mm (5/8"), 16mm (3/4")
Standard Lengths:
2.4m (8ft), 3.0m (10ft)
Earthing Rod Size Chart
Reference table for standard rod performance in different soils
| Soil Type | Resistivity (Ω·m) | Rod Length (m) | Rod Dia (mm) | Approx. Resistance (Ω) |
|---|---|---|---|---|
| Marshy Ground | 30 | 2.4 | 14.2 | 11.5 |
| Clay / Loam | 100 | 2.4 | 14.2 | 38.3 |
| Clay / Loam | 100 | 3.0 | 14.2 | 31.5 |
| Sandy Soil | 500 | 3.0 | 16.0 | 157.0 |
| Rocky Ground | 1000 | 3.0 | 16.0 | 314.0 (Needs Treatment) |
Earthing Rod Size FAQs
How to calculate earth rod size?
To calculate the earth rod size, you use the formula R = (ρ / 2πL) × (ln(4L/d) - 1). You start with a known soil resistivity (ρ) and a target resistance (R). Since solving for Length (L) directly is complex due to the natural logarithm, it is standard practice to iterate: start with a standard length like 2.4m, calculate the resistance, and increase length until the target is met.
What size earthing rod do I need?
The standard minimum size for most residential and commercial applications is a 2.4 meter (8 ft) long rod with a diameter of 14.2mm (5/8 inch). However, if your soil has high resistivity (sandy/rocky), you may need a longer rod (3m) or multiple rods connected in parallel to achieve a safe earth resistance value (typically below 5Ω or 1Ω for sensitive equipment).
How to size a grounding rod?
Sizing involves two factors: Length and Diameter. Length is the primary factor—deeper rods reach moist, conductive soil layers. Diameter adds mechanical strength for driving into hard ground but has little effect on resistance (doubling diameter only reduces resistance by ~10%). Always prioritize length first.
What is the minimum distance between earthing rods?
When using multiple rods, the minimum distance between them should be equal to the length of the rod driven into the ground (e.g., if using 3m rods, space them at least 3m apart). This prevents their "resistance spheres" from overlapping, which would reduce their efficiency. Ideally, spacing them 2x the length is even better.
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