Ground Grid Conductor Sizing for Fault Current

by the IEEE 80 sizing relation. The conductor has to carry the fault without reaching a temperature that damages it or, worse, its joints -- and the constant encodes the material's thermal capacity and its temperature limit, where the limit is set by the JOINT rather than by the conductor. A bolted or pressure connection has to be held far below the conductor's fusing point while an exothermic or brazed connection can go much higher, which is why the same copper gets a different constant depending on how it is joined. Two properties of the relation matter in practice. It scales with the SQUARE ROOT of time, so a fault that clears in a quarter of the time needs only half the conductor, which makes protection speed a real substitute for copper. And it scales LINEARLY with current, so a system with high available fault current needs proportionally more conductor everywhere in the grid. A conductor carrying 12 kA for 0.5 s needs 59.4 kcmil of brazed copper or 135.3 kcmil of steel -- copper needs less than half the area for the same duty, which is most of the reason grids are copper. But the thermal number is a FLOOR and not a specification: a buried grid conductor is handled, tamped over, and expected to last forty years in soil, so 4/0 copper is the common practical minimum regardless of what the thermal calculation allows. This is the bare-grid counterpart to the insulated-conductor thermal withstand calculation, which works from the ICEA adiabatic relation and an insulation temperature limit instead. A screen, never a stamp; IEEE 80 in full, the soil and corrosion conditions, and the engineer of record govern.

area_kcmil = fault_current_ka x Kf x sqrt(clearing_time_s); area_cmil = area_kcmil x 1000. Kf encodes the material's thermal capacity and the temperature limit its joint type allows.

IEEE Std 80 ground-grid conductor sizing, A in kcmil = I in kA x Kf x sqrt(tc), cited by name and not reproduced; the Kf constants for copper, copper-clad steel, and steel encode the material's thermal capacity and the temperature limit its JOINT type allows. The thermal answer is a floor -- mechanical and corrosion requirements usually set a larger practical minimum. A screen, never a stamp; IEEE 80 in full and the engineer of record govern.

The sizing relation is stated and cited to its standard rather than reproduced from any table; the fault current and clearing time are the study's own values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: fault_current_ka, clearing_time_s, material, installed_kcmil, area_kcmil, area_cmil, kf

• Joint sets the limit the temperature limit is the JOINT's, not the conductor's -- bolted far below fusing, brazed or exothermic much higherIEEE Std 80
• Square root of time quartering the clearing time halves the conductor; protection speed substitutes for copperIEEE Std 80
• A floor, not a spec mechanical handling and soil corrosion usually set a much larger practical minimumsubstation grounding practice