Grounding Electrode Conductor Sizing (250.66)

GEC vs EGC — the distinction that trips up most test-takers

Article 250 uses two conductors whose names and abbreviations look almost identical, and confusing them costs points. The grounding electrode conductor (GEC) is the conductor that runs from the service equipment (or separately derived system) down to the grounding electrode — the rod, concrete-encased rebar, metal water pipe, or other electrode buried in or bonded to the earth. It connects the electrical system to the ground reference.

The equipment grounding conductor (EGC) is an entirely different conductor. It runs alongside the circuit conductors in every branch circuit and feeder and bonds the metal frames, enclosures, and raceways back to the source so that a line-to-ground fault has a low-impedance return path to clear the overcurrent device. The GEC is sized by Section 250.66; the EGC is sized by Section 250.122. Knowing which rule applies to which conductor is the first thing the exam tests in this area.

How Table 250.66 works

Table 250.66 takes one input: the size of the largest ungrounded service-entrance conductor. If the service uses multiple conductors in parallel per phase, you use the equivalent combined circular-mil area as your starting point. The table then gives you the minimum GEC size in copper or aluminum, stepping the GEC upward as the service conductors grow larger — but the relationship is not one-to-one. A GEC does not need to be as large as the service conductor it serves; it scales more slowly because its job is low-impedance fault clearance and earth reference, not carrying the full load current.

The table does cap out: above a certain service conductor size, the required GEC size does not continue to grow. Very large services are not penalized with proportionally enormous GECs. This cap is one of the few Table 250.66 nuances that shows up in journeyman exam scenarios involving large commercial services.

Conductors are copper or aluminum. The table provides separate columns for each metal, and aluminum GECs must be larger in circular-mil area than their copper equivalents to carry the same fault energy — the same relationship that applies throughout Article 310 ampacity tables.

Sole-connection limits for specific electrode types

Sections 250.66(A), 250.66(B), and 250.66(C) create reduced-size allowances when a GEC connects to only one type of electrode. These are among the most-tested details in this section, so they are worth memorizing by type:

The word sole is the key qualifier. These reduced sizes apply only when that particular electrode is the only one being connected. When a GEC ties into a grounding electrode system that combines a rod with a water pipe or a Ufer ground, the sole-connection exception no longer applies and the full Table 250.66 size is required. Because the NEC requires all electrodes that are present at a structure to be bonded into one system under Section 250.50, the sole-connection scenario is more common on smaller structures — a detached garage with only a rod electrode, for example — and becomes the trap on larger installations where multiple electrode types are present.

Why this section is on the journeyman exam

The exam tests GEC sizing because it sits at the intersection of two skills: knowing which NEC table to use and understanding what drives the table's input. Many candidates incorrectly size the GEC from the overcurrent device rating — that is the EGC rule from Table 250.122, not the GEC rule. Getting those two inputs swapped is the most common mistake.

A second common trap is applying the rod-electrode cap universally. If an exam question mentions that the structure has both a ground rod and a metal water pipe, the 6 AWG cap for rods no longer controls — the full table value based on the service conductor size applies. Read the scenario carefully for which electrodes are present before picking a GEC size.

Pair this section with the grounding electrode system article to understand which electrodes must be present and bonded, and use the grounding conductor calculator to practice reading Table 250.66 before test day.