earth conductor size calculator
Size earth (ground) conductors from fault current and trip time using S = (I·√t)/k. Includes IEC/NEC references, copper/aluminum tables, and examples
NEC equipment grounding size
To size an earth conductor on IEC basis, consider S = (I) (√t)/k where I = Prospective earth-fault current (A), t = Protective device disconnection time (s) and k = The material constant in the standard. Round it up to the nearest size in the standard and check against.
Formula & Table Summary:
- IEC: S (mm²) = ( I(A) · √t(s) ) / k
- Typical k (illustrative): Cu PVC ≈115, Bare Cu ≈143, Al ≈76
- NEC: Use Table 250.122 → EGC size by OCPD rating
- Selection: Round up to next standard size; check installation deratings
grounding conductor size
The size of protective earth/ground conductor calculated by the calculator of the Earth Conductor depends on the prospective fault current ( I ), time to automatic disconnection ( t ) and material constant ( k ). According to IEC usage, apply S = (I · sqrt (t)) / k, where S= cross-section (mm 2 ), and k = a function of conductor material and the temperature rise of the insulation. Standard values: k=115 typical for copper PVC-insulated, k=143 bare copper, k=76 aluminum (example only--check by your convention). In the U.S., the NEC provides Table 250.122 to size the Equipment Grounding Conductor (EGC) with respect to the overcurrent device ratings without reference to I or t. Use this calculator to double check designs, and then round up to the next standard size and check against relevant code (IEC 60364-5-54, 250 NEC). Make sure to always take into account the method of Installation, temperature, and correction parameters as per the local regional regulation.
earth conductor size calculator
| I (kA) | t (s) | Scalc (mm²) | Std (mm²) |
|---|---|---|---|
| 5 | 0.2 | 19.44 | 25 |
| 10 | 0.2 | 38.88 | 50 |
| 10 | 0.1 | 27.50 | 35 |
| 20 | 0.2 | 77.76 | 95 |
| OCPD (A) | Min Cu EGC |
|---|---|
| 15–20 | 14 AWG |
| 60 | 10 AWG |
| 100 | 8 AWG |
| 200 | 6 AWG |
| 400 | 3 AWG |
