Grounding Electrode Conductor Sizing Under NEC 250.66

Two conductors in a service look similar, get called the same thing in conversation, and are sized from two completely different tables. Mixing them up is the most common grounding error in the trade, and it shows up on exams as reliably as it shows up in panels.

GEC and EGC are different animals

Table 250.66: sizing the GEC

Size from the largest ungrounded service-entrance conductor, or the equivalent area for parallel conductors.

The caps that override the table

This is the part that makes the whole thing manageable, and it is the part people forget. The required size to a specific electrode type is capped regardless of what the table says.

That example is why "just run 1/0 to be safe" is the wrong instinct here. It is expensive, hard to terminate on a rod clamp, and unnecessary.

Table 250.122: sizing the EGC

Size from the rating of the overcurrent device ahead of the circuit, not from the conductor size.

Installation rules that fail inspections

Continuity

The GEC must be installed without splice or joint except by specific permitted means: irreversible compression connectors listed for the purpose, exothermic welding, or connection to busbars and certain listed fittings. A wire nut is not one of them.

Protection

A 6 AWG or larger GEC may generally be run along the surface without additional protection where it is not subject to physical damage. Smaller than 6 AWG requires a raceway or armor. Where a ferrous raceway is used, it must be bonded at both ends to the conductor inside it, otherwise the raceway acts as a choke and dramatically increases the impedance of the path.

Aluminum restrictions

Aluminum and copper-clad aluminum grounding conductors are generally not permitted where in direct contact with masonry or earth, or where subject to corrosive conditions. Terminations outdoors have a minimum height requirement above earth. This effectively rules aluminum out for most electrode connections.

Electrode spacing

Where rods are used and a single rod does not meet the resistance requirement, a second is required, and rods in a group must be separated by at least 6 feet. Driving two rods 3 feet apart because that is where the conduit came out is a citation.

The electrode system, briefly

All electrodes present at a building must be bonded together into a single grounding electrode system. That includes:

• A metal underground water pipe in direct contact with earth for the required length
• The metal frame of the building where it qualifies
• A concrete-encased electrode, where present in the foundation
• A ground ring, where installed
• Rod, pipe and plate electrodes

You do not get to pick your favorite. If a qualifying electrode exists at the building, it goes in the system. A metal water pipe electrode additionally requires a supplemental electrode.

Quick check before you close the panel

• GEC sized from the service conductors, then capped by electrode type where applicable.
• EGC sized from the overcurrent device, and increased if the phase conductors were upsized.
• No splices in the GEC except by permitted means.
• Ferrous raceway around the GEC bonded at both ends.
• All present electrodes bonded into one system.
• Main bonding jumper installed at the service, and only at the service.
• Downstream panels have grounds and neutrals separated.

That last one and the main bonding jumper are the two that turn up most often on the common violations list, and both belong on your rough-in walkthrough.