NEC Article 250 Grounding and Bonding: The Rules Tested Most on the Journeyman Exam (2026)
Grounding vs. bonding explained for the journeyman electrician exam, plus the specific NEC Article 250 rules and table values that show up most often: GEC sizing (250.66), EGC sizing (250.122), ground-rod resistance, and the main bonding jumper.
TL;DR
Grounding and bonding (NEC Article 250) is the single most-failed topic on the journeyman electrician exam, mostly because candidates blur the two terms and because the section is full of small table values that are easy to mix up. Grounding is the connection to earth; bonding is the connection of metal parts together to create an effective ground-fault current path. On exam day, the questions that recur most are grounding-electrode-conductor sizing (Table 250.66), equipment-grounding-conductor sizing (Table 250.122), the 25-ohm ground-rod rule, the 6-foot rod spacing rule, and the difference between the main bonding jumper and the system bonding jumper. Drill those and you will clear most of Article 250.
Grounding vs. Bonding — Get This Distinction First
Almost every Article 250 question gets easier once you stop using the two words interchangeably. Per NEC Article 100, grounding is the connection of a system or equipment to earth or to a conductive body that serves in place of earth. Bonding is the connection of metallic parts to establish electrical continuity and conductivity. The reason this matters: earth itself is a poor fault-clearing path. When a hot conductor faults to a metal enclosure, what actually trips the breaker is the low-impedance bonding path back to the source — not the dirt under the building. NEC 250.4(A)(3) calls this the effective ground-fault current path. Exam writers love to offer 'connection to earth' as a tempting wrong answer for a question that is really about bonding. Read the stem carefully: if it is about clearing a fault, the answer involves bonding, not the grounding electrode.
Sizing the Grounding Electrode Conductor (Table 250.66)
The grounding electrode conductor (GEC) connects the service to the grounding electrode system, and it is sized from Table 250.66 based on the size of the largest service-entrance conductor. Memorize a few anchor points: 2 AWG copper service conductors take an 8 AWG copper GEC; 3/0 AWG copper takes a 4 AWG GEC; over 1100 kcmil copper takes a 3/0 GEC. Then learn the three caps that override the table. Per 250.66(A), where the GEC connects only to a ground rod, that part is never required to be larger than 6 AWG copper. Per 250.66(B), where it connects to a concrete-encased (Ufer) electrode, it is never required to be larger than 4 AWG copper. Per 250.66(C), to a ground ring, never larger than the ring conductor. These three caps are tested constantly, and candidates who only memorized the table miss them.
Ground Rods: The 25-Ohm Rule and 6-Foot Spacing
Two ground-rod facts appear on nearly every exam. First, per 250.53(A)(2) Exception, a single rod, pipe, or plate electrode that does not have a resistance to earth of 25 ohms or less must be supplemented by one additional electrode. Note the word 'one' — you are not required to keep adding rods until you hit 25 ohms; one supplement satisfies the code. In practice, electricians drive two rods by default to avoid having to measure resistance at all. Second, per 250.53(A)(3), the supplemental rod must be at least 6 feet from the first. The exam may try to trick you with 3-foot or 5-foot distractors. Both numbers — 25 ohms and 6 feet — are worth committing to memory exactly.
Sizing the Equipment Grounding Conductor (Table 250.122)
Do not confuse the equipment grounding conductor (EGC) with the GEC. The EGC runs with the circuit conductors and is sized from Table 250.122 by the rating of the overcurrent device protecting the circuit — not by the load and not by the conductor ampacity. A 15- or 20-amp circuit takes a 14 AWG copper EGC; a 60-amp circuit takes a 10 AWG copper EGC; a 100-amp circuit takes an 8 AWG copper EGC. One important nuance the exam tests: when ungrounded conductors are upsized for voltage drop, 250.122(B) requires the EGC to be increased proportionally. Knowing whether a question is asking about the GEC (Table 250.66, sized by service conductor) or the EGC (Table 250.122, sized by breaker) is half the battle.
Main Bonding Jumper vs. System Bonding Jumper
At the service, the grounded (neutral) conductor and the equipment grounding system are tied together at exactly one point by the main bonding jumper (250.24(B), 250.28). Downstream of that point, neutrals and grounds must be kept separate — 250.142 prohibits re-bonding the neutral in a subpanel, and a question showing a neutral bonded to the enclosure in a downstream panel is testing whether you catch that violation. For a separately derived system, such as a transformer secondary or a generator, the equivalent connection is the system bonding jumper (250.30(A)(1)), made at a single point. Same idea, different name and different code section — the exam expects you to use the correct term for the correct location.
Three Article 250 Traps That Cost Points
First, mixing up the GEC and the EGC. The GEC ties the service to the earth electrode and is sized from Table 250.66 by the service-entrance conductor; the EGC rides with the branch circuit and is sized from Table 250.122 by the breaker. Decide which one the question is about before you open the codebook. Second, forgetting the 250.66 caps. Candidates memorize the table and then oversize the conductor to a ground rod or Ufer, missing the 6 AWG and 4 AWG ceilings. Third, re-bonding the neutral downstream. The neutral-to-ground bond is made once, at the service (or at the system bonding jumper for a separately derived system); any neutral bonded to an enclosure in a subpanel is a 250.142 violation. Exam questions often present a wiring scenario and ask you to spot the error — these three are the usual suspects.
How to Lock This In Before Exam Day
Grounding and bonding rewards repetition more than reading, because most of the misses come from confidently picking a value that belongs to a different rule. Tab Article 250 in your codebook, then drill mixed questions until the table lookups are automatic. We built a free 10-question NEC Grounding and Bonding practice set covering exactly these rules — try it at /questions/electrician/grounding-bonding-essentials. When you are hitting 75% or higher on timed practice with only your codebook, you are ready. For the full state-by-state breakdown and registration details, see /electrician-exam-prep, and warm up with the broader /free-electrician-practice-test.
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