Earth Conductor Size Calculator (Adiabatic Equation)
By Santaji GadepatilinUpdated October 27, 2026
A protective earth conductor must carry the full fault current for the time it takes a breaker or fuse to trip — without overheating. The adiabatic equation gives the minimum safe size, as explained in this earth conductor size calculation; the fault current itself comes from a short circuit current calculation.
How to use the earth conductor size calculator
Enter the prospective earth fault current, the protective device’s disconnection time at that current, and choose the conductor and insulation type. The result is the minimum cross-section; select the next standard size up.
For more background, read Earth Fault Loop Impedance (Zs) Testing.
The adiabatic equation
S = √(I² × t) ÷ k
- S = cross-section in mm²
- I = fault current in A (rms)
- t = disconnection time in seconds
- k = material factor
Minimum sizes (copper, PVC, k = 143)
| Fault current | 0.1 s | 0.4 s | 1 s | 5 s |
|---|---|---|---|---|
| 1 kA | 2.2 mm² | 4.4 mm² | 7.0 mm² | 16 mm² |
| 2 kA | 4.4 mm² | 8.8 mm² | 14.0 mm² | 31 mm² |
| 5 kA | 11.1 mm² | 22.1 mm² | 35.0 mm² | 78 mm² |
| 10 kA | 22.1 mm² | 44.2 mm² | 69.9 mm² | 156 mm² |
| 20 kA | 44.2 mm² | 88.5 mm² | 139.9 mm² | 313 mm² |
| 31.5 kA | 69.7 mm² | 139.3 mm² | 220.3 mm² | 493 mm² |
Standard sizes
1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 and 300 mm².
Simplified method (IEC 60364-5-54 Table 54.2)
| Line conductor S | Protective conductor (same material) |
|---|---|
| S ≤ 16 mm² | S |
| 16 < S ≤ 35 mm² | 16 mm² |
| S > 35 mm² | S ÷ 2 |
Check the earth fault loop too
A conductor large enough thermally must also have low enough impedance for the protective device to trip within the required time. Verify the earth fault loop impedance Zs on site.
Frequently asked questions
What is the adiabatic equation?
S = √(I²t) ÷ k, the minimum conductor cross-section that can carry the fault current for the disconnection time without exceeding its final temperature. It is given in IEC 60364-5-54 and BS 7671.
Where does the k factor come from?
It depends on the conductor material and the insulation’s initial and final temperatures. IEC 60364-5-54 Annex A tabulates the values.
What disconnection time should I use?
The actual operating time of the protective device at the fault current, from its time–current curve. Final circuits often require 0.4 s; distribution circuits 5 s.
Can I use the simple table instead?
Yes, IEC 60364-5-54 also allows sizing by the line conductor size — equal up to 16 mm², 16 mm² up to 35 mm², and half above that.