3.2 Service & Feeder Grounding & Bonding (0202)

Grounding vs. Bonding (Article 100)

• Grounding — connecting to earth or to a conductive body that extends the earth connection. Grounding establishes a zero-voltage reference, not a low-impedance fault path.
• Bonding — the permanent mechanical and electrical connection of conductive parts to establish electrical continuity and fault-current-carrying capability.

A grounded system is not necessarily a safe one. The main bonding jumper (250.28) ties the grounded conductor (neutral) to the equipment grounding conductor at the service, which is what creates the low-impedance fault path that clears a line-to-case fault. Without bonding, grounding alone will not trip an OCPD.

Grounding Electrode System (250.50, 250.52)

250.50 requires that all grounding electrodes described in 250.52(A)(1) through (A)(7) that are present at each building or structure served be bonded together to form the grounding electrode system. "Present" means it exists — you do not get to skip the concrete-encased electrode because nobody stubbed out a rebar tail. Where none of those electrodes is present, one or more of 250.52(A)(4) through (A)(8) must be installed. The permitted electrodes, with the correct 250.52(A) numbering:

Where an interior metal water pipe is used as a conductor to interconnect electrodes, 250.68(C)(1) limits that use to the first 5 ft of the water pipe from the point of entrance into the building (with a relaxation for industrial, commercial, and institutional buildings under qualified maintenance where the entire length is exposed).

GEC Sizing — Table 250.66

The grounding electrode conductor (GEC) connects the grounding electrode system to the grounded conductor at the service. Size per 250.66 and Table 250.66, based on the size of the largest ungrounded service-entrance conductor (or equivalent area for parallel conductors). Excerpt of common values:

Three caps modify the table, and the exam loves them:

• 250.66(A) — where the GEC connects to a rod, pipe, or plate electrode (or the bonding jumper to that electrode), that portion need not be larger than 6 AWG copper or 4 AWG aluminum.
• 250.66(B) — where it connects to a concrete-encased electrode, that portion need not be larger than 4 AWG copper.
• 250.66(C) — where it connects to a ground ring, that portion need not be larger than the conductor used for the ground ring (2 AWG copper minimum for the ring itself).

Note that these caps apply only to the portion of the GEC running to that electrode. A system with both a driven rod and a metal water pipe can legitimately have a 6 AWG run to the rod and a full table-sized run to the water pipe.

The 25-Ohm Rule — 250.53(A)(2)

250.53(A)(2) requires a single rod, pipe, or plate electrode to be supplemented by an additional electrode. The Exception relieves that requirement only where the single rod, pipe, or plate electrode has a resistance to earth of 25 ohms or less — which must be demonstrated by measurement, not assumed. Because almost nobody measures, the practical field rule is: drive two rods. Where two rods are used, 250.53(A)(3) requires them to be not less than 6 ft (1.8 m) apart. Note the direction of the logic on the exam: the supplemental electrode is the default and the 25-ohm test is the escape hatch, not the other way around. A concrete-encased electrode or ground ring is not subject to the 25-ohm test at all.

Bonding the Service (250.92, 250.28)

250.92(A) requires the metal raceways, cable armor, enclosures, meter fittings, and boxes containing service conductors to be bonded together, and 250.92(B) lists the acceptable methods: bonding to the grounded service conductor, connections using threaded couplings or hubs made up wrenchtight, threadless couplings and connectors made up tight, or other listed bonding devices such as bonding jumpers, bonding-type locknuts, bushings, or bushings with bonding jumpers. Standard locknuts and bushings alone are not a bonding means on the service. The grounded conductor is bonded to the equipment grounding conductor by the main bonding jumper (250.28) at the service disconnect — this is the connection that makes the fault-current path work. Bonding methods permitted by 250.8(A) include listed pressure connectors, terminal bars, exothermic welding, machine screws engaging at least two threads, and listed clamps; sheet-metal screws are not permitted (250.8(B)).

250.24(C) requires the grounded conductor (neutral) to be bonded to the service disconnect enclosure at every service disconnect — not just at a single point. At the service, the grounded conductor is also used as the equipment grounding conductor; beyond the service (subpanels), the neutral must be isolated from the equipment grounding conductor to prevent parallel neutral current paths.

Bonding to Other Systems — 250.94

Where communications (CATV, telephone, network-powered broadband) and other systems are present, 250.94 requires an intersystem bonding terminal (IBT) at the service equipment or at the grounding electrode conductor. The IBT provides a single accessible point to bond all external system grounding conductors, reducing differences in potential between systems.

EGC Sizing for Feeders — Table 250.122

The equipment grounding conductor (EGC) for a feeder is sized per 250.122 and Table 250.122, based on the rating of the feeder OCPD (not the feeder conductor ampacity). Common values:

Read the left column as "not exceeding" — there is no 30 A or 40 A row in the current table, so a 30 A or 40 A device drops to the 60 A row and takes a 10 AWG copper EGC. Where conductors are increased in size (for example for voltage drop), 250.122(B) requires the EGC to be increased proportionally in circular-mil area.

Worked GEC Sizing Example

A commercial service is supplied by 4/0 AWG Cu ungrounded service-entrance conductors. Determine the minimum GEC size per Table 250.66:

• From Table 250.66: a 4/0 AWG Cu service conductor corresponds to a #2 AWG Cu GEC (or 1/0 AWG Al).
• If the electrodes are only rod/pipe/plate, 250.66(A) caps the GEC at #6 Cu — so use #6 Cu (the table value of #2 is required only when a water pipe, building steel, concrete-encased, or ground ring electrode is part of the system).

Larger example: a service uses 500 kcmil Cu ungrounded conductors. Table 250.66 returns 1/0 AWG Cu GEC, used because a metal water pipe and concrete-encased electrode are present in the grounding electrode system.