Practice question · NEC Table 250.66

What Size Grounding Electrode Conductor for 4/0 Copper?

By Mason Garry, Master Electrician · October 22, 2026 · 2 min read

What size grounding electrode conductor for 4/0 copper service conductors is a one-table read, but you have to read a row range instead of hunting for a 4/0 row. Table 250.66 groups service conductors into ranges, and 4/0 sits inside one of them. Land in the right range and the answer reads itself.

Answer: C) 2 AWG. Table 250.66 puts service conductors over 3/0 through 350 kcmil copper in the row that requires a 2 AWG copper grounding electrode conductor — and 4/0 falls in that range.

Where to Find It

Open your Code book to Table 250.66 in Article 250. The left side of the table is a list of ranges for the service-entrance ungrounded conductors. Find the range that contains 4/0 AWG copper: over 3/0 through 350 kcmil copper. Read across that row and the copper grounding electrode conductor is 2 AWG. Tab it with the note from the table itself: Article 250, "Table 250.66" — "the GEC follows the service conductors."

Why 2 AWG

The grounding electrode conductor is sized off the service-entrance ungrounded conductors. That is the only input the table wants, and the stem hands it to you directly: 4/0 AWG copper. Your whole job is placing 4/0 inside the correct range. It is larger than 3/0, so you are past the smaller-service rows, and it is well under the 350 kcmil top of its range. That placement — over 3/0 through 350 kcmil — is what makes the answer 2 AWG.

Run the options against the ranges. Options A and B, 6 AWG and 4 AWG, belong to rows for smaller service conductors; they are what you get when you stop one or two rows too early. Option D, 1/0 AWG, overshoots into a larger-service row. You can see exactly where 1/0 does belong in the 500 kcmil grounding electrode conductor question — same table, bigger service, bigger conductor. Side by side, the two questions teach the table's whole logic: as the service conductors grow, the GEC grows with them.

One more discipline point: the stem asks for copper on both sides of the read — copper service conductors, copper grounding electrode conductor. Stay in the copper rows and columns the stem names. Table 250.66 punishes anyone who drifts.

The Trap

The trap is hunting for an exact-match row labeled 4/0, not finding one, and guessing. Table 250.66 is built on ranges, so the skill is comparison, not lookup: is 4/0 over 3/0? Yes. Is it through 350 kcmil? Yes. You are in the row. Read across and stop second-guessing it.

Exam Tip

Tab Table 250.66 and underline the row boundaries, not the answers. On test day, place the stem's service conductor size between two boundaries first, then read the GEC. "The GEC follows the service conductors" — if you can say which range you are in, the conductor size takes care of itself.

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.