6.4 Grounding Connections at the Service

Why Grounding-at-Service Is Chapter 6 Material

Prov weights Grounding and Bonding at 6 items and Services/OCP at 8 items. Many stems sit on the seam: the only place a premises wiring system normally bonds the grounded conductor (neutral) to the grounding electrode system and to equipment grounding is at the service (or at a separately derived system — Chapter 5 in this guide). This section teaches the service connection rules in 250.24 and the companion pieces you must open during service questions. Electrode types and deep GEC sizing drills also appear in Chapters 4–5; here the focus is what gets landed where at the service equipment.

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The Service Bonding Picture (Big Picture)

At the service disconnect enclosure for a grounded AC system you typically find:

1. Ungrounded service conductors (hots) landing on the service disconnect / main OCPD.
2. Grounded service conductor (neutral) landing on a neutral terminal bar.
3. Main bonding jumper connecting the neutral terminal bar to the metal enclosure / equipment grounding terminal bar (250.24(B), 250.28).
4. Grounding electrode conductor (GEC) connecting the grounded service conductor (or service enclosure, as permitted) to the grounding electrode system (250.24(A), 250.64, 250.66).
5. Equipment grounding conductors leaving on the load side bonded to the enclosure/EGC bar — not re-bonded to neutrals in subpanels.

If you remember only one sentence for Prov: neutral and ground meet at the service; they stay separated on the load side.

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System Grounding Connections — 250.24(A)

250.24(A) requires that a grounded AC system be grounded by connecting the grounded service conductor to a grounding electrode at the building/service as specified. Practical exam points:

• The connection is made at an accessible location from the load end of the service drop/lateral or underground service conductors to the service disconnecting means / service equipment.
• Common landing points: grounding electrode terminal bar in the service equipment, or other locations permitted by 250.24.
• 250.24(A)(5) (and related rules) generally prohibit bonding the grounded conductor to equipment grounding conductors/enclosures on the load side of the service disconnect — that is the “no neutral-ground bond in subpanels” rule candidates mix up.

Objectionable current: If you bond neutral to ground at both the service and a downstream panel, neutral return current can flow on metal pipes, EGCs, and enclosures. That is both a shock hazard and a classic Code failure mode.

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Main Bonding Jumper — 250.24(B) and 250.28

The main bonding jumper is the connection between the grounded conductor and the equipment grounding conductor / service enclosure at the service.

250.28 covers MBJ material, construction, attachment, and size:

• MBJs may be a wire, bus, screw, or similar suitable conductor as permitted.
• Screws used as MBJs must be identified (often green) — factory bonding screws matter.
• Sizing: wire-type MBJs are sized from Table 250.102(C)(1) based on the ungrounded service conductors (same family of sizing logic used for supply-side bonding jumpers). Do not use Table 250.122 (EGC table) to size an MBJ.

Exam trap: Using the EGC table (250.122) for an MBJ or supply-side bonding jumper is a frequent wrong answer. Service-side bonding jumpers follow 250.102(C)(1) / 250.28 pathways.

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Grounded Conductor Brought to Service Equipment — 250.24(C)

250.24(C) generally requires that the grounded conductor be run to each service disconnecting means and bonded to each disconnecting means enclosure. Key ideas:

• The grounded conductor must be routed with the ungrounded conductors.
• Sizing of the grounded conductor for services follows 250.24(C) references (including Table 250.102(C)(1) pathways for certain conditions).
• Where multiple disconnects are used in permitted 230.71(B) arrangements, understand how the grounded conductor and bonding are required at those enclosures — open 250.24(C) rather than inventing a single shared landing rule.

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GEC Connection, Routing, and Protection — 250.64 / 250.66

At the service, the GEC is what ties the system to earth electrodes (rods, water pipe, concrete-encased electrode, etc.).

Sizing snapshot (detail in Chapter 4)

Table 250.66 sizes GECs based on the largest ungrounded service entrance conductor. Memorize the lookup habit, not a fake universal size:

• Many 100–125 A copper service examples land on smaller GEC sizes in the table.
• 2/0 and 3/0 copper services commonly require 4 AWG copper GEC (verify in Table 250.66).
• 3/0 or 250 kcmil ranges step up — always open the table.
• Electrode-type limits (for example, GEC to a rod not required larger than 6 AWG copper under 250.66(A) conditions) can reduce the conductor where only those electrodes are present — read the subsection; do not apply the rod limit when a water-pipe/concrete electrode requires the full Table 250.66 size.

Routing and protection

250.64 highlights for service installs:

• GEC must be continuous to the electrode connection as required (splices only by irreversible means or listed connectors where permitted).
• Protect GECs against physical damage; raceways used for protection have bonding rules when ferrous.
• 250.64(E) ferrous metal raceways/enclosures for GECs must be bonded at both ends (and as required) so the raceway does not choke fault/lightning current.

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Equipment Grounding vs Grounded Conductor on the Load Side

After the service disconnect:

Subpanels get isolated neutrals and bonded EGCs to the enclosure. Reinstalling an MBJ screw in a subpanel recreates a second bonding point — illegal for typical premises wiring and a gift to Prov item writers.

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Meter Sockets, CT Cans, and Outside Disconnects

Modern dwelling services often use meter-main combination equipment or outdoor emergency disconnects (230.85). Grounding/bonding still must satisfy 250.24 at the service disconnecting means. When equipment is listed as service equipment and includes the MBJ provision, use the manufacturer’s bonding means. Do not assume the meter enclosure alone completes every bonding obligation if the service disconnect is elsewhere — follow where 250.24 says the connections must be made.

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Interaction with Article 230 GFP and Fault Clearing

Grounding at the service creates the low-impedance path that lets service OCPDs and, where required, 230.95 ground-fault protection operate. A missing MBJ can leave metal parts energized without a reliable trip path. On mixed outline questions (“why did the breaker not clear?”), check bonding continuity at the service before blaming the breaker curve.

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Timed Open-Book Tips for §6.4

• Tab 250.24, 250.28, Table 250.66, and Table 250.102(C)(1) together.
• Ask first: is this the service or a subpanel/SDS? Bonding rules change.
• MBJ/supply-side bonding jumper → 250.102(C)(1) family; EGC → 250.122.
• If the stem mentions only ground rods, check 250.66(A) electrode-type limits before forcing a huge GEC.
• Pair this section with §6.2: outdoor emergency disconnects and meter-mains still need a Code-correct 250.24 bonding scheme.