Grounding Electrode Resistance (Dwight / IEEE 142)
Driven rod / ring / plate / Ufer resistance to earth from soil resistivity.
Example
You enter
- Electrode type driven_rod
- Soil resistivity (ohm-cm) 10000
- Rod diameter (in; rod / Ufer) 0.625
- Rod length (ft; rod / Ufer) 8
You get
- Resistance to remote earth (ohms) 39.9
- Electrodes in parallel to reach 25 ohms 2
- Meets NEC 25-ohm advisory No
Checked against published worked examples from: IEEE / Dwight.
Details, formula, and sources
ScopeWith the 25-ohm NEC advisory and a supplemental-electrode count.
Formula or tableDriven rod (Dwight 1936): R = (rho / (2*pi*L)) * (ln(8L/d) - 1). Ring: R = (rho / (2*pi^2*D)) * (ln(8D/d) + ln(4D/s)), with s = 2 x burial depth (the distance to the image ring). Plate: R = (rho / 4) * sqrt(pi / A). Ufer: rod formula with the concrete-cylinder effective diameter (no further reduction; the concrete's own resistivity is not modeled).
Source and editionIEEE 142-2007 (Green Book) §4. NEC 2023 (NFPA 70). §250.53 governs adoption.
Free accessstandards.ieee.org for IEEE 142 bibliographic data; Free read-only access at nfpa.org/freeaccess. for NEC 250.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field nameselectrode_type, soil_resistivity_ohm_cm, rod_diameter_in, rod_length_ft, resistance_ohms, supplemental_count_to_25_ohm, meets_25_ohm
- Soil resistivity user-supplied (ohm-cm); varies seasonallyfield megger reading is the authoritative value at the time of inspection
- 25-ohm advisory NEC 250.53(A)(2) two-electrode rule when a single electrode exceeds 25 ohmsNEC 2023 §250.53(A)(2)
- Mutual impedance supplemental-rod count ignores mutual impedance at typical 6 ft spacingengineering practice; field check required
- Ufer model effective diameter = concrete cylinder; no extra reduction factorIEEE 142 §4.2.4 (concrete-encased electrodes)