Earth Size Calculator
Enter the line conductor size, and if you know them, the fault current, the disconnection time and how the earth is installed. The calculator gives the earth conductor size by the simple table and the adiabatic check.
- Earth size
- Table method
- Adiabatic check
Fault current comes from the earth loop impedance at the far end of the circuit.
Up to 16 mm², the earth matches the line conductor. A 6 mm² circuit needs a 6 mm² earth by the simple table. With a 1,000 A fault cleared in 0.4 s, the adiabatic check needs 5.5 mm² at k = 115, so 6 mm² passes.
Earth conductor size chart
Copper, simple method. Search for a size.
| Line (mm²) | Earth (mm²) |
|---|---|
| 1.5 | 1.5 |
| 2.5 | 2.5 |
| 4 | 4 |
| 6 | 6 |
| 10 | 10 |
| 16 | 16 |
| 25 | 16 |
| 35 | 16 |
| 50 | 25 |
| 70 | 35 |
| 95 | 47.5 |
| 120 | 60 |
| 150 | 75 |
| 185 | 92.5 |
| 240 | 120 |
mm².
Line size and earth
Simple table. Slide the line size.
Both conductors are drawn to scale.
How to size an earth conductor
Line to earth. Step through it.
Line
Size.
Test continuity of every earth before energising.
Simple table or adiabatic check
Methods. Compare them.
Quick.
Precise.
Twin and earth.
The adiabatic check often allows a smaller earth than the table.
Before you install
Earthing checks. Tick each item.
Tick each item as you go.
Record test results on the installation certificate.
What you get
Earth sizing. Each tile explains one result.
mm².
Method.
Check.
Disconnection.
Factor.
Twin and earth cable has a smaller earth than its line cores.
Frequently Asked Questions
No question matches that search.
What size earth cable do I need?
Up to sixteen square millimetres, the same as the line conductor.
What size earth for a twenty five mm² cable?
Sixteen square millimetres by the simple table.
What is the adiabatic equation?
Earth size equals the square root of fault current squared times time, divided by k.
What is k in the adiabatic equation?
A factor for the conductor and insulation, one hundred and fifteen for copper in PVC cable.
Is a smaller earth allowed?
Yes, if it passes the adiabatic check.
What size is the earth in two point five twin and earth?
One point five square millimetres.
Who should size earthing?
A qualified electrician, to your local wiring rules.
A safe path to earth
Size the earth conductor right and faults clear quickly and safely.
More size calculators
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