NEC 250.66

Grounding Electrode Conductor Size - Table 250.66

Complete code requirements, compliance guidance, and professional analysis

Code Requirements Summary

NEC 250.66 and Table 250.66 specify the minimum size requirements for grounding electrode conductors that connect the electrical system grounded conductor to grounding electrodes such as ground rods, water pipes, and concrete-encased electrodes.

Professional Requirements

  • Size grounding electrode conductor per Table 250.66 based on largest service entrance conductor
  • Install grounding electrode conductor as a continuous run without splices where possible
  • Use copper, aluminum, or copper-clad aluminum grounding electrode conductor as specified
  • Protect grounding electrode conductor from physical damage per NEC requirements
  • Make proper connections to grounding electrodes using approved methods and materials
  • Ensure grounding electrode conductor is accessible for inspection and testing

Technical Explanation

Table 250.66 ensures the grounding electrode conductor has adequate current-carrying capacity to handle fault currents and provide a low-impedance path to earth. The sizing is based on the largest ungrounded service-entrance conductor because this determines the maximum available fault current. The grounding electrode conductor connects the electrical system to the grounding electrode system, which includes water pipes, ground rods, concrete-encased electrodes, and other qualifying electrodes. This connection is essential for system grounding, lightning protection, and fault current dissipation.

Industry Applications

Service entrance grounding electrode conductor installation for residential and commercial buildings
Separately derived system grounding electrode conductor sizing for transformers and generators
Industrial facility grounding system design and installation
Electrical service upgrades requiring grounding electrode conductor replacement
New construction electrical service grounding system installation
Grounding electrode system connections for solar and renewable energy installations

Compliance Analysis Tool

Professional Resources

NEC 2023 State Adoption Status

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Related Code Requirements

NEC 250.62->
NEC 250.64->
NEC 250.122->

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Copyright Notice:The National Electrical Code (NEC) is copyrighted by the National Fire Protection Association (NFPA). This page provides professional analysis and interpretation for educational purposes only. For official code text, consult NFPA 70.

Professional Use Notice:This analysis tool provides technical guidance based on standard NEC requirements. Local codes, amendments, and AHJ interpretations may supersede national standards.

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