NEC grounding electrode conductor sizing: Table 250.66 vs Table 250.122

Where to turn

• 250.66 is the section for grounding electrode conductor size, and Table 250.66 is its table. You enter it with the size of the largest ungrounded service-entrance conductor, or the equivalent area for parallel sets.
• 250.66(A), (B) and (C) set limits that can cap the size when the conductor goes only to a rod, a concrete-encased electrode or a ground ring. Confirm the paragraph letters in your 2026 book.
• 250.122 and Table 250.122 size the equipment grounding conductor, entered with the rating of the overcurrent device ahead of the circuit.
• 250.102 and Table 250.102(C)(1) size bonding jumpers on the supply side. Separately derived systems send you back to 250.30.

The method

1. Decide which conductor the question is about.
2. For a grounding electrode conductor, find the largest ungrounded conductor. If sets run in parallel, add the circular mil areas of one phase.
3. Enter Table 250.66 with that size and read the copper or aluminum value.
4. Check the electrode. If the conductor goes only to a rod, a concrete-encased electrode or a ring, the matching paragraph of 250.66 may set a smaller maximum.
5. For an equipment grounding conductor, enter Table 250.122 with the overcurrent device rating, not the conductor size.

Worked example

A service has two parallel sets of 3/0 AWG copper per phase. Circular mil area of one 3/0 AWG is 167,800, so one phase totals 335,600.

• Table 250.66 for that total, which falls in the over 3/0 AWG through 350 kcmil row, gives 2 AWG copper.
• If the grounding electrode conductor runs to a metal water pipe or building steel, 2 AWG is the answer.
• If it runs only to a driven rod, the rod paragraph caps it at 6 AWG copper, so 6 AWG.

A feeder protected by a 200-ampere breaker is a different problem. Table 250.122 gives 6 AWG copper for the equipment grounding conductor, whatever the size of the feeder conductors.

Common traps

• Using the wrong table. Table 250.66 is entered with conductor size. Table 250.122 is entered with the device rating.
• Forgetting parallel sets. Add the areas before entering the table.
• Missing the electrode cap. A rod, a concrete-encased electrode or a ring can each limit the size. Check which one the problem describes.
• Mixing copper and aluminum. Each has its own column.
• Upsized conductors. If feeder conductors are increased for voltage drop or other reasons, the equipment grounding conductor increases in proportion under 250.122(B). That does not apply to the grounding electrode conductor.
• Bonding jumpers. A main bonding jumper or supply-side jumper uses 250.102, not 250.66.
• Edition drift. Confirm every paragraph letter you rely on against the 2026 NEC.

Working it with the book open

Tab Table 250.66 and Table 250.122 in different colors if your tabs allow it, and write the entering quantity on each tab: conductor size for one, device rating for the other. Before you flip to either table, say aloud which conductor the question is about and which electrode or circuit it serves. That ten-second habit prevents the most common error. Then check whether a paragraph of 250.66 caps the size.

How to practice

Write ten service and feeder descriptions with different sizes and electrodes. For each, name the conductor, name the table and then read the value. Sort by which table you used. The sorting trains the first step, which is where most misses start.

Related guides: conductor ampacity and motor calculations.

Start with the free 10-question check

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Sources

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• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• Electrician exam information, Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.