NEC 250.66 Grounding Electrode Conductor Sizing

NEC 250.66 GEC Sizing Lookup

NEC 250.66 is a service-grounding sizing checkpoint. Use this page to identify the conductor type, service conductor basis, electrode connection, and GEC vs EGC boundary before relying on a grounding conductor size in the field.

Grounding Rule Separation

GEC vs EGC

Sizing Basis

Sizing Basis

NEC 250.66 is generally reviewed from the largest ungrounded service-entrance conductor or equivalent area. Service size, conductor material, parallel service conductors, and service conductor equivalent area can all affect the GEC review. For dwelling service conductor context, keep this review separate from NEC 310.12 Dwelling Service and Feeder Conductors.

• Service conductor basis: The service conductor set must be identified before grounding electrode conductor sizing is trusted.
• Copper vs aluminum: Conductor material and equivalent area matter when the service conductor basis is reviewed.
• Parallel conductors: Parallel service conductors may require an equivalent conductor area review instead of a simple single-conductor assumption.

Electrode Review

Electrode Type Review

A grounding electrode conductor review is not complete until the grounding electrode system is identified. Ground rods, metal water pipe, concrete-encased electrodes, structural metal, and supplemental electrodes can change the field review and inspection path.

• Ground rods: Rod, pipe, and plate electrodes often raise maximum-size and supplemental-electrode questions.
• Water pipe: Metal water pipe grounding requires continuity, bonding, and supplemental-electrode review.
• UFER: Concrete-encased electrode connections need electrode identification and inspection-specific review.
• Local requirements: Utility service standards and AHJ practices can affect the final grounding electrode conductor review.

Field Conditions

Where This Rule Shows Up

NEC 250.66 usually appears when the work touches the service grounding system, service equipment, grounding electrode system, or a detached building grounding electrode review.

• Service upgrades: Panel, meter-main, or service conductor changes can require renewed GEC sizing review.
• Detached buildings: Detached structures with feeders may require grounding electrode system and bonding review.
• Meter-main replacement: Changing service equipment can trigger grounding electrode conductor and bonding verification.

Field Example

GEC vs EGC Sizing

A grounding electrode conductor and an equipment grounding conductor are not sized from the same rule. The GEC is tied to the service grounding electrode system and is sized from the service conductor basis under NEC 250.66, while branch-circuit and feeder EGCs are sized from the OCPD basis under NEC 250.122.

• GEC: service grounding electrode conductor sized from the service conductor basis.
• EGC: equipment grounding conductor sized from the circuit overcurrent device basis.
• Field check: do not use the EGC table to size a service grounding electrode conductor.

Calculator Use

TradeHub Calculator Application

Use the service conductor basis to separate NEC 250.66 GEC sizing from NEC 250.122 EGC sizing, then coordinate the result with service, load, and conductor planning.

Field Checks

Checks Before Trusting GEC Sizing

• The conductor being sized is actually a grounding electrode conductor, not an equipment grounding conductor.
• The grounding electrode system has been identified before applying any sizing limit.
• The largest ungrounded service conductor or equivalent conductor area has been reviewed.
• Copper vs aluminum service conductor basis is known.
• Parallel service conductors are reviewed using equivalent area logic where applicable.
• The service bonding point and GEC connection point are confirmed.
• Utility service standards, local amendments, and AHJ requirements have been checked.

Common Misses

Common Field Misses

• The GEC is sized from NEC 250.122 by mistake.
• The EGC is sized from NEC 250.66 by mistake.
• Ground rods, water pipe, UFER, and supplemental electrodes are treated as the same field condition.
• Parallel service conductor equivalent area is skipped.
• Bonding jumpers, grounded conductors, EGCs, and GECs are mixed together in the field plan.
• A calculator wire-size result is treated as service grounding electrode approval.

Source Scope

Source Alignment and Use Scope

This page is a field reference based on NEC 250.66 grounding electrode conductor sizing and related TradeHub source alignment records. It summarizes field decision points for screening and planning only. It does not reproduce NEC sizing tables, approve service grounding layouts, verify electrode installation, determine bonding jumper sizing, approve utility service requirements, replace the adopted NEC, local amendments, equipment markings, engineered design documents, or AHJ inspection. Review the TradeHub Code Citation & Source Log for source alignment records and the TradeHub Methodology page for how field references are scoped.

FAQ

Grounding Electrode Conductor FAQ