NEC 250.66: Grounding Electrode Conductor Sizing (Complete Guide)

What 250.66 actually does

NEC 250.66 sizes the grounding electrode conductor (GEC). The GEC is the conductor that connects your service grounded conductor — the neutral — to the grounding electrode. Grounding electrodes are the rod, the metal underground water pipe, the building steel, the concrete-encased electrode (Ufer), or the ground ring. Article 250.52 lists the six permitted electrode types.

The GEC only carries current during a fault. Most of its life it sits there as a low-impedance reference path between the system neutral and earth. It does not run inside circuits — that’s the equipment grounding conductor (EGC), sized per Table 250.122.

Table 250.66 values

Table 250.66 sizes the GEC based on the largest ungrounded service conductor as installed:

Read the table by finding your service conductor row, then reading across to the GEC column for your conductor metal. Memorize the residential row (2 AWG service through 1/0): the answer for almost every dwelling-unit service is 8 AWG GEC.

The three electrode-type caps (where candidates lose points)

Cap 1: Rod, pipe, or plate electrode → 6 AWG copper max

Per 250.66(A), if the GEC connects only to a rod, pipe, or plate electrode (the 250.52(A)(5) and (A)(7) electrodes), the GEC need not be larger than 6 AWG copper (or 4 AWG aluminum). This applies even if your service is 600 amps. The reasoning is that a rod electrode’s resistance to earth limits the fault current that can flow through the GEC; a larger conductor doesn’t help.

Practical takeaway: most residential and small commercial services with rod-only grounding use 6 AWG copper GEC, regardless of what Table 250.66 would otherwise require.

Cap 2: Concrete-encased electrode (Ufer) → 4 AWG copper max

Per 250.66(B), if the GEC connects only to a concrete-encased electrode (the 250.52(A)(3) electrode), the GEC need not be larger than 4 AWG copper. Concrete-encased electrodes have lower resistance to earth than rod electrodes, so a slightly larger cap applies.

Cap 3: Ground ring → not larger than the ring itself

Per 250.66(C), if the GEC connects only to a ground ring electrode (the 250.52(A)(4) electrode), the GEC need not be larger than the conductor used to form the ring itself. Ground rings must be at least 2 AWG bare copper per 250.52(A)(4), so the GEC is also at least 2 AWG.

Worked examples

Example 1: 200-amp residential service to a rod electrode

• Service conductors: 2/0 AWG copper
• Table 250.66 reading: 4 AWG copper GEC required
• Electrode: rod only
• Cap applies: 6 AWG copper sufficient (250.66(A))
• Answer: 6 AWG copper GEC

Example 2: 200-amp residential service to metal water pipe

• Service conductors: 2/0 AWG copper
• Table 250.66 reading: 4 AWG copper GEC required
• Electrode: metal underground water pipe (250.52(A)(1))
• No cap applies (water pipe is not in the cap list)
• Answer: 4 AWG copper GEC

Example 3: 400-amp commercial service with multiple electrodes

• Service conductors: 600 kcmil copper
• Table 250.66 reading: 1/0 copper GEC required for the GEC system
• Electrodes: water pipe + concrete-encased + rod
• Bonding between electrodes per 250.50: required
• GEC to water pipe (primary): 1/0 copper (table value)
• GEC to rod (supplemental): 6 AWG copper (cap permits no larger)
• GEC to concrete-encased: 4 AWG copper (cap permits no larger)
• Multiple GECs are permitted; sizing each to its cap separately is correct

Common 250.66 traps on the exam

• Applying the table value to a rod electrode when the 6 AWG cap should govern
• Confusing 250.66 (GEC, Table 250.66) with 250.122 (EGC, Table 250.122) — different tables, different inputs
• Forgetting that 250.66(A) caps to rod, pipe, AND plate electrodes (all in 250.52(A)(5) and (A)(7))
• Not recognizing that 250.50 requires bonding ALL electrodes present, not just one
• Treating the supply-side bonding jumper as a GEC — it’s sized per Table 250.102(C)(1), not Table 250.66

How 250.66 questions appear on the exam

On the TDLR Journeyman, California General Electrician, and most state journeyman exams, you can expect 1-3 questions per exam that key off Table 250.66 directly. Common formats:

1. Direct table lookup: “What size copper GEC is required for a service with 350 kcmil ungrounded conductors?”
2. Cap recognition: “A 400-amp service is grounded only to a rod electrode. What is the minimum copper GEC size?”
3. Multi-electrode bonding: “A building has metal water pipe, building steel, and a rod electrode. Which must be bonded together?”
4. Discrimination from related conductors: “What table sizes the conductor between the service neutral and a ground rod?”

The discrimination questions are the trap. The exam offers Table 250.122 values as wrong answers when the question is really about Table 250.66, knowing that candidates who don’t recognize which conductor is which will pick the wrong table.