Grounding Electrode Resistance (Dwight / IEEE 142)
Driven rod / ring / plate / Ufer resistance to earth from soil resistivity, with the 25-ohm NEC advisory and a supplemental-electrode count.
Formula and source
Driven rod (Dwight 1936): R = (rho / (2*pi*L)) * (ln(8L/d) - 1). Ring: R = (rho / (4*pi^2*D)) * (ln(8D/d) + ln(4D/s)). Plate: R = (rho / 4) * sqrt(pi / A). Ufer: rod formula with concrete-cylinder effective diameter, times 0.5 empirical reduction.
IEEE 142-2007 (Green Book) §4. NEC 2023 (NFPA 70). §250.53 governs adoption.
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