Short answer: Ask what the conductor connects to. A grounding electrode conductor is sized under 250.66, and 250.66(A) and 250.66(B) limit the size needed when it connects to a rod or a concrete-encased electrode. An equipment grounding conductor is sized under 250.122 from Table 250.122, by the rating of the overcurrent device ahead of it (250.122(A)). For a transformer or other separately derived system, start at 250.30.
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Two conductors, two sections
Both tables live in Article 250, and both give a conductor size, so they are easy to mix up under time. The difference is what the conductor connects.
| The conductor connects | Section to open | What you size from |
|---|---|---|
| The grounding electrode | 250.66, Table 250.66 | Read the table headings in your code book for the value you enter with |
| Only a rod or a concrete-encased electrode | 250.66(A), 250.66(B) | These limit the size needed for that connection |
| Equipment (enclosures, raceways, frames) | 250.122, Table 250.122 | The rating of the overcurrent device ahead of it, 250.122(A) |
| Equipment, on parallel runs | 250.122(H), formerly 250.122(F) | Same table, plus the parallel rule |
| Equipment, on feeder taps | 250.122(I), formerly 250.122(G) | Same table, plus the tap rule |
| A transformer or other separately derived system | 250.30 | Follow 250.30 to the sizing section it sends you to |
Two more locations come up on the same questions: 250.118 lists the types of equipment grounding conductors, and 250.52 lists the electrodes, with installation in 250.53.
This page does not give any conductor size from Table 250.66 or Table 250.122. Read the size from the table in your own code book; that is exactly the skill the exam tests.
Read the question in four steps
- Find the noun. “Electrode” points to 250.66. “Equipment”, “enclosure” or “raceway” points to 250.122.
- Find the number you were given. If the question gives a breaker or fuse rating, you are almost always in Table 250.122, because that table is entered by the rating of the overcurrent device ahead of the conductor.
- Look for a modifier. A rod or concrete-encased electrode only, a parallel run, a feeder tap or a transformer each sends you to a different section in the table above.
- Read, do not compute. Go across the correct row and read the size. All conductors are copper unless the question says otherwise.
The PSI bulletin also says not to consider code exceptions or optional calculation methods unless the question specifically directs you to. On a grounding question, answer from the main rule unless the question points you to an exception.
The renumbering trap
The 2026 NEC moved two equipment grounding conductor rules that show up on calculation questions: parallel runs went from 250.122(F) to 250.122(H), and feeder taps went from 250.122(G) to 250.122(I). If your notes or an older study guide cite the old letters, update them before you tab and highlight your book.
Article 250 is the second most cited article in our workbook’s 486 questions, with 38 citations, behind only Article 210. A fast route into Article 250 saves time on every question that lands there. The bulletin allows only permanent tabs made or provided by the NEC publisher, plus your own highlighting and notes made before the exam.
Pick the section to open
Which grounding section to open
Answer from the wording of the question. The result names the section to open first and what you size from. Read the size itself from your code book.
- What does the conductor connect to?
- What number does the question give you?
- Does the question name a parallel run or a feeder tap?
[ETOS_ACCOUNT_CARD]
Is the equipment grounding conductor sized by the wire or by the breaker?
By the breaker or fuse. Table 250.122 is entered by the rating of the overcurrent device ahead of the conductor (250.122(A)). Read the size from that table in your code book.
When is the grounding electrode conductor allowed to be smaller?
250.66(A) and 250.66(B) limit the size needed when the grounding electrode conductor connects to a rod or a concrete-encased electrode. Read the exact limit in your own code book.
Where are the grounding rules for a transformer on the journeyman exam?
Start at 250.30, which covers separately derived systems. Electrodes are listed in 250.52 and their installation is in 250.53.
Sources checked October 2026
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