4.2 Grounding Electrode Conductor Sizing (250.66)

Exam Role of 250.66

On the Arkansas Journeyman Prov exam, grounding electrode conductor (GEC) sizing is a high-yield Article 250 calculation. The stem usually gives the ungrounded service-entrance conductor size (or kcmil), names the electrode type, and asks for the minimum GEC. Your workflow is always:

1. Find the largest ungrounded service-entrance conductor (or equivalent area for parallel conductors).
2. Open Table 250.66 and read the required copper (or aluminum) GEC.
3. Check whether the GEC terminates only to a rod/pipe/plate, concrete-encased electrode, or ground ring — if so, apply 250.66(A), (B), or (C) so you do not oversize beyond the Code maximum required for that electrode type.
4. Still obey installation rules in 250.64 (continuous, protected, splicing limits).

Do not size the GEC from the overcurrent device rating using Table 250.122 — that table is for equipment grounding conductors, covered in the next chapter.

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What Table 250.66 Uses as the Input

250.66 sizes the GEC from the largest ungrounded service-entrance conductor or equivalent area for parallel conductors. For a typical 120/240 V residential or light-commercial service, that is the hot service conductor size (for example, 4/0 AWG aluminum or 3/0 AWG copper).

If service-entrance conductors are paralleled, compute the equivalent area (sum of the circular mil areas of the paralleled ungrounded conductors in each set) and enter Table 250.66 with that equivalent size — do not pick one conductor of the parallel set alone.

Where the ungrounded supply conductors are larger than the service-entrance conductors (unusual exam setups), read 250.66 and related notes carefully; most Prov items stick to a clear service-entrance size.

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Table 250.66 — Copper GEC Snapshot (verify in your NEC)

Open the live table in NEC 2023. The copper column relationships you will use constantly are:

Aluminum or copper-clad aluminum GEC sizes are larger for the same service conductor row — always read the matching column in your book. On timed exams, confirm whether the stem asks for copper or aluminum GEC.

Worked Example 1 — Table only

Service: 4/0 AWG copper ungrounded service-entrance conductors to a service disconnect that bonds to a metal water pipe and building steel (full electrode system).

• 4/0 AWG copper is over 3/0 through 350 kcmil → Table 250.66 copper GEC = 2 AWG.
• No 250.66(A)/(B)/(C) reduction applies to the water-pipe/building-steel connections that require the full table size.

Answer: 2 AWG copper GEC (minimum).

Worked Example 2 — Parallel service conductors

Service: two paralleled 3/0 AWG copper ungrounded conductors per phase.

• Equivalent area ≈ 2 × 167,800 cmil = 335,600 cmil → treat as over 3/0 through 350 kcmil for Table 250.66 entry (verify cmil values in Chapter 9 Table 8).
• Copper GEC from Table 250.66 = 2 AWG.

If the stem instead gave two paralleled 500 kcmil copper conductors, equivalent area = 1,000,000 cmil → over 600 through 1100 → copper GEC 2/0 AWG.

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The Three Electrode Caps — 250.66(A), (B), (C)

These are not free passes to undersize every GEC on the job. They apply when the GEC is the sole connection to that electrode type (read each subsection’s exact connection language).

250.66(A) — Rod, pipe, or plate electrodes

Where the GEC is connected to rod, pipe, or plate electrodes as permitted in 250.52(A)(5) or (A)(7), that portion of the conductor that is the sole connection to the rod/pipe/plate electrode shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.

Worked Example 3 — Rod only:

Service ungrounded conductors: 500 kcmil copper. Electrodes present: two ground rods only (no water pipe, no ufer, no ring).

• Table 250.66 alone would call for 1/0 AWG copper (over 350 through 600).
• Because the GEC is the sole connection to rod electrodes, 250.66(A) caps the required size at 6 AWG copper.

Exam answer: 6 AWG copper, not 1/0.

250.66(B) — Concrete-encased electrode

Where connected to a concrete-encased electrode as permitted in 250.52(A)(3), that portion of the conductor that is the sole connection to the concrete-encased electrode shall not be required to be larger than 4 AWG copper wire.

Worked Example 4 — Ufer only:

Service: 750 kcmil copper ungrounded service-entrance conductors. Only electrode: concrete-encased rebar electrode.

• Table 250.66 → 2/0 AWG copper (over 600 through 1100).
• 250.66(B) → not required larger than 4 AWG copper for the sole connection to the ufer.

250.66(C) — Ground ring

Where connected to a ground ring, that portion of the conductor that is the sole connection to the ground ring shall not be required to be larger than the conductor used for the ground ring.

Worked Example 5 — Ground ring:

Ground ring installed with 2 AWG bare copper (minimum size under 250.52(A)(4)). Service conductors would otherwise require 1/0 GEC from Table 250.66.

• Sole connection to the ring → GEC need not be larger than the 2 AWG ring conductor.

If the ring were installed with 1/0 AWG copper (larger than Code minimum), the sole-connection GEC would not need to exceed that 1/0 ring conductor — still verify Table 250.66 does not require something smaller; you never install smaller than both the table and the applicable cap require as a minimum pathway.

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Mixed Electrode Systems — Do Not Over-Apply the Cap

If the building has a metal water pipe electrode and rods, the GEC that serves the water pipe (and the bonding that creates the full system) is still sized from Table 250.66 without using the 6 AWG rod cap for the water-pipe connection. The tap or bonding jumper that is the sole connection to the rods may be limited per 250.66(A).

Worked Example 6 — Water pipe + rods:

• Service: 3/0 AWG copper → Table 250.66 = 4 AWG copper GEC to the service grounding point / water-pipe electrode system.
• A bonding jumper that is the sole connection from that system to a supplemental ground rod need not be larger than 6 AWG copper per 250.66(A).

Prov will often ask only one piece: either the main GEC size from the table or the capped size to the rod. Match the stem.

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Installation Notes That Appear Beside Sizing Questions

Sizing is 250.66; routing is 250.64:

• GEC shall be installed in one continuous length without a splice or joint except as permitted (irreversible compression-type connectors listed as grounding and bonding equipment, exothermic welding, busbars, etc.).
• Protect where exposed to physical damage; raceway protection rules differ for smaller GECs.
• Aluminum or copper-clad aluminum GEC terminations have exterior/enclosure restrictions — read 250.64(A).

On an open-book Arkansas exam, if the options mix a correctly capped rod GEC with an incorrectly applied Table 250.122 EGC size, pick the Article 250.66 path.

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Quick Drill Pattern for Prov Timing

Memorize the lookup order; verify every AWG in the 2023 tables on exam day.