4.3 Grounding Electrode Systems for Services

Why Grounding Electrodes Appear on the TX Exam

The Texas Journeyman exam's Knowledge part consistently tests electrode types, the 250.50 "all available electrodes" rule, GEC sizing from Table 250.66, and the supplemental electrode requirement for water pipes. Misreading "supplemental" (additional electrode required) versus "supplementary" (a different concept in 250.54) is a classic exam trap.

NEC 250.50 — The Bonded Electrode System Rule

NEC 250.50 states that all grounding electrodes that are present at a building or structure must be bonded together into one grounding electrode system. You do not choose between a water pipe and a ground rod — if both exist, both must be bonded. The electrodes that make up the system are listed in 250.52(A).

Exception: A concrete-encased electrode is not required to be added to an existing building unless the foundation is being replaced or newly poured, but if one is present it must be bonded.

250.52(A) — Permitted Electrode Types

250.52(B) — NOT Permitted as Electrodes

• Aluminum electrodes
• Metal underground gas piping systems (never bond gas pipe as an electrode)
• Building steel that is not in earth contact and not bonded to a concrete-encased electrode

250.53 — Installation Requirements

250.53(A) — Rod, Pipe, and Plate Electrodes

• Each rod/pipe must be supplemented by an additional electrode unless a single rod, pipe, or plate has a measured resistance to earth of 25 ohms or less (250.53(A)(2)(Exception)).
• The supplemental electrode must be installed at least 6 ft away from the original (250.53(A)(3)).
• Rods must be driven flush or below grade; if rock is encountered, the rod may be buried horizontally in a trench at least 30 in. deep (250.53(G)).

250.53(D) — Water Pipe Electrode Supplement

A metal underground water pipe must be supplemented by an additional electrode — rod, pipe, plate, ring, concrete-encased, or building steel. The supplemental electrode is required because water pipes can be replaced with nonmetallic pipe, leaving no electrode. The bond jumper between the water pipe and the supplemental electrode must be sized from Table 250.66.

250.53(E) — Bonding Jumper Between Electrodes

When multiple electrodes form the system, the bonding jumper connecting them must be sized from Table 250.66 and is not required to be larger than the largest ungrounded service-entrance conductor.

250.66 — Sizing the Grounding Electrode Conductor

Table 250.66 sizes the GEC based on the size of the largest ungrounded service-entrance conductor (copper or aluminum). The GEC connects the grounded (neutral) bus at the service to the grounding electrode system.

Special rules:

• For rod, pipe, or plate electrodes only: GEC need not be larger than 6 AWG copper (250.66(A)).
• For a concrete-encased electrode or ground ring only: GEC need not be larger than 4 AWG copper (250.66(B) and (C)).

250.24(C) — Main Bonding Jumper at the Service

250.24(C) requires a main bonding jumper at the service equipment to connect the grounded (neutral) conductor to the equipment grounding conductor (EGC) bus. The main bonding jumper is:

• Sized from Table 250.102(C)(1) (in NEC 2023; was Table 250.66 in earlier editions — the values are essentially the same).
• Required only at the service — never on the load side of the main disconnect (that would create parallel neutral return paths).
• A screw, bus, wire, or listed strap supplied with listed panelboards.

Worked Example — GEC and Bonding Jumper Sizing

A service has 2/0 AWG copper service-entrance conductors. Size the GEC to a driven ground rod and to a concrete-encased electrode.

1. Table 250.66 lookup for 2/0 AWG Cu largest ungrounded conductor → 4 AWG Cu GEC.
2. 250.66(A) cap for a rod-only electrode → GEC need not be larger than 6 AWG Cu, so the GEC to the rod is 6 AWG Cu (the smaller of 4 AWG and 6 AWG).
3. 250.66(B) cap for a concrete-encased electrode → GEC need not be larger than 4 AWG Cu, so the GEC to the Ufer is 4 AWG Cu.
4. The main bonding jumper at the service is sized from Table 250.102(C)(1): for 2/0 AWG Cu largest ungrounded conductor → 4 AWG Cu (same value as the old Table 250.66).

Exam Pitfalls

• "Supplemental" vs. "Supplementary": 250.53(D) requires a supplemental (additional, required) electrode for water pipes. 250.54 supplementary bonding to grounding is optional and not an electrode requirement.
• Gas pipe: Never used as an electrode (250.52(B)(1)).
• Main bonding jumper location: Only at the service. A bonding jumper in a subpanel is a violation (creates objectionable neutral current on the EGC).