What grounding resistance does NEC require?

NEC 250.56: if single 8-foot electrode has R > 25Ω, must add a second electrode. Two rods spaced 6+ feet apart typically achieve < 25Ω in most soils. Rod driven in moist clay: R = 5-10Ω.

How does soil resistivity affect grounding?

Soil resistivity ρ: wet clay 10 Ωm, moist soil 100 Ωm, sandy soil 1000 Ωm, dry rock 10,000 Ωm. Ground electrode resistance scales linearly with ρ. Salt and carbon soil treatments reduce ρ.

What are grounding requirements for lightning protection?

UL 96A and NFPA 780 recommend < 10 Ω for lightning protection system. Ground ring conductor, multiple rods, counterpoises, and ground enhancement materials used to achieve low resistance in high-resistivity soils.

Grounding Resistance Calculator

Calculate ground electrode resistance: single rod R = (ρ/2πL)·[ln(4L/d)-1]. Wenner 4-probe test method. NEC requires ≤ 25Ω for single electrode. Typical lightning protection < 10Ω, telecom < 5Ω.

2D stress state at a point. All stresses in MPa.

σx — Normal Stress X (MPa)

σy — Normal Stress Y (MPa)

τxy — Shear Stress (MPa)

⚙ Mechanical Note: Results are based on standard textbook formulas and ISO/AASHTO references. Always verify critical designs with applicable codes (ISO 898, AASHTO 1993, AISC, Eurocode 3) and a qualified engineer.

How to Use

Enter the values for the patient or scenario you are assessing. Calculate ground electrode resistance: single rod R = (ρ/2πL)·[ln(4L/d)-1]. Wenner 4-probe test method. NEC requires ≤ 25Ω for single electrode. Typical lightning protection < 10Ω, telecom < 5Ω. Use the grounding resistance result to inform your clinical assessment.

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Frequently Asked Questions

What grounding resistance does NEC require?

How does soil resistivity affect grounding?

What are grounding requirements for lightning protection?