Grounding & Bonding: The Rules Everyone Mixes Up

Article 250 is the most-tested article on most journeyman exams, and the wrong answers are engineered from the exact confusions people carry out of the field. Here are the ones that do the most damage.

First, the distinction the exam assumes you know

Grounding connects the electrical system to the earth. Bonding connects conductive parts to each other so a fault has a low-impedance path back to the source and the overcurrent device opens. Earth is not that fault path — dirt makes a poor conductor — and several exam questions exist purely to see whether you'll reach for "the earth clears the fault" as an answer. It doesn't; the bonding path does.

Ground rods: 8 feet, in the dirt

A rod electrode must be at least 8 ft long, and it must be installed so at least 8 ft is in contact with the soil. Both halves matter: a 10-ft rod driven 6 ft deep doesn't comply. Watch for the near-miss numbers — 6 ft and 10 ft show up as distractors constantly.

Rod diameter: the "listed" qualifier is the whole question

An iron or steel rod that is not listed must be at least 5/8 in. in diameter. The 1/2 in. figure applies to listed rods and to stainless steel or nonferrous rods. When an exam question about rod diameter offers both 1/2 and 5/8, the word "listed" (or its absence) in the question stem is the entire answer.

The 25-ohm rule runs backwards from how people remember it

A single rod, pipe, or plate electrode that does not achieve 25 ohms or less to earth must be supplemented with one additional electrode. Two things trip people up. First, the direction: 25 ohms is the threshold that lets a single rod stand alone, not a value you must ever prove if you simply drive a second rod — which is why the field habit is two rods, no measurement. Second, once you add that supplemental electrode, the code asks for nothing further; there's no requirement to keep measuring until some resistance target is hit.

The GEC allowance that catches oversizers

Here's the one that snags people who know Table 250.66 too well: where the grounding electrode conductor runs to a rod, pipe, or plate electrode, that portion is never required to be larger than 6 AWG copper — even on a 400 A service where the base table would say 1/0. The exam loves this because the "safe-looking" bigger answer is wrong. Sizing from the table without checking the electrode-type allowances is the classic Article 250 mistake, in both directions.

How to study this article

Don't memorize values in isolation — memorize the qualifiers that switch them: listed vs. unlisted, sole electrode vs. supplemented, connection to a rod vs. connection to a water pipe. Then drill questions with the book open and make yourself find 250.52, 250.53, and 250.66 fast; on exam day, those three sections do a disproportionate share of the work.

Question 4 in the free set hits the GEC allowance — try it cold, then read the explanation.