6.1 Grounding Electrode Systems (Article 250 Part II)

Why This Matters on the Exam

The Virginia Journeyman exam blueprint allots 6 questions to Grounding and Bonding — one of the heaviest single-topic blocks on the test. Article 250 is the largest article in the NEC, and the electrode system rules in Part II are where many candidates lose points because they conflate grounding with bonding or misremember which electrode types require supplementation. Expect direct code-recall questions ("Which electrode must be supplemented?", "What is the minimum depth for a ground ring?") plus scenario questions that require you to size a GEC or identify a prohibited electrode.

Grounding vs. Bonding — Get the Definitions Straight

Grounding is the intentional connection to earth through a grounding electrode system. Its purpose (250.4(A)(1)) is to limit voltage imposed by lightning, line surges, and unintentional contact with higher-voltage lines, and to stabilize the system-to-earth voltage during normal operation.

Bonding is the permanent joining of electrically conductive parts to ensure electrical continuity and the capacity to safely conduct any fault current likely to be imposed (Article 100). Bonding creates the low-impedance fault-return path that trips the breaker; grounding connects that path to earth. A common exam trap is to assume the earth clears a fault — it does not. The overcurrent device clears the fault through the bonded metal path, not through the soil.

Distinguish two further terms:

• System grounding — connecting a current-carrying conductor (typically the neutral) of a system to earth (Part III).
• Equipment grounding — connecting non-current-carrying metal parts to the system grounded conductor through the EGC (Part VI).

250.50 — Bond All Electrodes Together

If more than one grounding electrode exists at a building or structure, 250.50 requires that all of them be bonded together to form a single grounding electrode system. You are not allowed to pick and choose; the metal underground water pipe, concrete-encased electrode, building steel, ground ring, and rods must all be tied together. The bonding jumpers between electrodes are sized per 250.53(C), which references Table 250.66.

250.52(A) — Permitted Electrode Types (memorize this list)

250.52(B) Prohibited electrodes: underground metal gas piping (must still be bonded per 250.104(B), but it cannot serve as a grounding electrode), aluminum, and swimming pool structural reinforcing steel.

250.53 — Installation Requirements

• 250.53(A)(2): A single rod, pipe, or plate electrode must be supplemented by an additional electrode.
• 250.53(A)(2) Exception: A single rod/pipe/plate with a resistance to earth of 25 ohms or less does not require a supplemental electrode. In practice, a single rod rarely meets 25 ohms, so most installations use two rods. Note the NEC never requires you to add a third rod if two rods still exceed 25 ohms — two is the maximum required.
• 250.53(A)(3): The supplemental electrode must be at least 6 ft from the rod it supplements.
• 250.53(A)(4): Rod/pipe electrodes must be driven to 8 ft depth; if rock bottom is hit, drive at ≤45° from vertical or bury in a trench ≥30 in. deep.
• 250.53(A)(5): The upper end of the rod must be flush with or below grade unless the GEC attachment is protected per 250.10.
• 250.53(F): Ground ring buried at least 30 in. deep.
• The metal water pipe electrode must be supplemented as if it were the only electrode (anticipating future plastic-pipe replacement).

250.66 — Sizing the Grounding Electrode Conductor (GEC)

The GEC is sized per Table 250.66 based on the largest ungrounded service-entrance conductor:

Three reductions in 250.66 cap the GEC for specific electrodes:

• 250.66(A): To rod, pipe, or plate electrodes only — GEC not required larger than 6 AWG copper.
• 250.66(B): To concrete-encased electrode only — not larger than 4 AWG copper.
• 250.66(C): To a ground ring — not larger than the ring conductor (2 AWG).

A water-pipe electrode gets no reduction; it uses the full Table 250.66. If a single GEC runs to a rod and then continues to a water pipe, the entire GEC must be sized per Table 250.66 for the water pipe.

Exam Traps

• "Underground gas piping can be a grounding electrode" — false (250.52(B)(1)).
• "A single 5/8-in. rod always satisfies the code" — false; it must be supplemented unless ≤25 ohms.
• "Two rods that measure 40 ohms require a third rod" — false; two is the max required regardless of resistance.
• The 25-ohm exception applies only to rod/pipe/plate — not to water pipe, building steel, or Ufer.

Worked Example

A 200 A residential service uses 2/0 AWG copper service-entrance conductors and a metal underground water pipe as the primary electrode, supplemented by two driven rods. The GEC to the water pipe must be sized from Table 250.66 at the 2/0 row → 4 AWG copper. The bonding jumper to the rod array may be 6 AWG copper per 250.66(A). Because the GEC runs to the rod and continues to the water pipe, the full GEC must be 4 AWG — the rod-only cap does not apply when the conductor continues to an electrode that requires the larger size.