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Practice question · NEC Table 250.66
What Size Grounding Electrode Conductor for 500 kcmil Copper?
Grounding electrode conductor sizing is a straight table read — if you know which table to reach for. Here is the exact lookup path, so it takes you ten seconds on test day instead of five minutes of page-flipping.
Exam-style question (Sparky QOTD)
Service-entrance ungrounded conductors are 500 kcmil copper. What is the minimum copper grounding electrode conductor per Table 250.66?
A. 1/0 AWG
B. 3/0 AWG
C. 4 AWG
D. 2 AWG
Answer: A) 1/0 AWG. Over 350 through 600 kcmil copper reads across to 1/0 copper on Table 250.66.
Where to Find It
Open your Code book to Table 250.66. Run down the left column of service-entrance ungrounded conductor sizes until you hit 500 kcmil — it falls in the "over 350 through 600 kcmil" row. Read across that row and the minimum copper grounding electrode conductor is 1/0 AWG. The lookup path is: find the conductor size down the left, read across.
Why This Works
The whole idea of Table 250.66 is that the GEC follows the service conductors. Bigger service conductors can feed more fault current, so the grounding electrode conductor grows with them. You don't calculate anything here — you look it up.
That is also why this question is fast money on the exam. There is no math to botch and no exceptions to weigh. The row says what it says: over 350 through 600 kcmil copper, 1/0 copper GEC. Ten seconds, done.
If you haven't tabbed your Code book yet, start with our code book tabs guide — Table 250.66 is one of the tables you want indexed before test day.
The Trap
Option B (3/0 AWG) is the trap. "Bigger feels safer" is not how a minimum works — the question asks for the minimum, and the minimum is whatever the row says. Picking 3/0 on the job would be spending your customer's money on copper the Code never asked for. 4 AWG and 2 AWG are undersized guesses — close enough to look plausible, both wrong per the table.
Exam Tip
When the stem says "grounding electrode conductor," reach for Table 250.66. If the tab is in your book, this is a ten-second question. If it isn't, you're flipping through a thousand pages for one number — and that is the real test this question is giving you.
Keep Drilling
This is one of 33 NEC topics. Sparky's 1,087 codebook-first questions drill all of them — every question a lookup rep with your book open, 15 minutes a day.