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 |
Frequently Asked Questions - earth conductor size calculator:
What is the formula to size an earth conductor?
Use S = (I · √t) / k, where I is fault current, t is disconnection time, and k is a material constant from the standard.
What are typical k values?
Illustrative values: copper PVC ≈115, bare copper ≈143, aluminum ≈76. Verify with your standard and insulation class.
How do I get the fault current and time?
From short-circuit studies and the protective device’s time-current curve for the fault location.
Do I round up the result?
Yes, select the next standard cross-section and check mechanical/thermal limits and installation conditions.
How does NEC size grounding conductors?
NEC uses Table 250.122 based on the overcurrent device rating, not the IEC thermal formula.
Can aluminum be used for the earth conductor?
Yes where permitted; use the correct k and adjust for terminations and code allowances.
Does ambient temperature change k?
k depends on material and permissible temperature rise; choose the appropriate k per insulation and standard.
Is this sizing valid for PEN or bonding jumpers?
Bonding jumpers and PEN conductors have additional rules; check your specific standard sections.
What about parallel conductors?
For parallel runs, size each in parallel per code rules; NEC and IEC provide specific guidance.
Is code compliance guaranteed by this calculator?
No. Use it as a design aid and confirm final sizes against the applicable code and manufacturer data.
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