4.3 Main Bonding Jumper & Service Bonding

Service Bonding on the Arkansas JW Outline

Grounding and Bonding is 6 items on the Prov Arkansas Journeyman outline. After electrode types and GEC sizing, the next cluster is service bonding: the main bonding jumper, bonding of service raceways and enclosures, and the connection of the grounded conductor to the grounding electrode system at the service disconnecting means. These rules live primarily in 250.24, 250.28, and 250.102, with sizing help from Table 250.102(C)(1).

If you confuse three conductors, you will miss easy points:

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System Grounding at the Service — 250.24 Snapshot

For a grounded AC system supplying a building, 250.24 requires the premises wiring system to have a grounding electrode conductor connected to the grounded conductor at each service. Key exam ideas (verify exact text):

• The grounded conductor (neutral) is connected to the grounding electrode system via the GEC.
• The connection is made at an accessible location from the load end of the service drop/lateral to the service disconnecting means (including the meter enclosure where allowed).
• Metal service raceways, cable trays, cablebus framework, auxiliary gutters, or service cable armor/sheath that serve as grounding electrode conductors or that enclose service conductors must be bonded as required.
• Load-side neutral-to-case bonds are generally prohibited except where specifically allowed (separately derived systems, certain existing installations, etc.) — Journeyman items often test that the MBJ is at the service, not at every subpanel.

On a typical dwelling service, you should be able to point to: (1) neutral landed on the neutral bus, (2) MBJ bonding neutral bus to the enclosure/EGC bus, (3) GEC leaving to the electrode system, (4) EGCs leaving with feeders/branch circuits.

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Main Bonding Jumper Defined — 250.28

250.28 states that the main bonding jumper shall be a wire, bus, screw, or similar suitable conductor for connecting the equipment grounding conductors and service-equipment enclosure to the grounded conductor of a solidly grounded system at each service disconnect. That single connection creates the low-impedance fault-return path onto the grounded conductor back toward the utility source.

Material and construction — 250.28(A) and (B)

• Material: copper or other corrosion-resistant material. Aluminum or copper-clad aluminum is permitted where suitable for the environment and termination rules.
• Construction: MBJ may be a wire, bus, screw, or similar suitable conductor. Screws must be identified (often green) where used as the MBJ.

Attachment — 250.28(C)

Main bonding jumpers shall be connected in a manner that creates a permanent, reliable path using listed methods appropriate to the connector type (exothermic weld, listed pressure connectors, listed clamps, and other methods recognized in Article 250 bonding sections).

Size — 250.28(D)

This is the calculation core for Prov:

• Where the MBJ is a wire or busbar, size it per 250.28(D)(1) by referencing Table 250.102(C)(1) based on the ungrounded supply conductors / equivalent area.
• Where the service-entrance conductors are larger than the sizes in the table, size the MBJ not less than 12½ percent of the area of the largest ungrounded supply conductor (with parallel-conductor rules in the section).
• A screw used as the MBJ must comply with 250.28(D) requirements for screw-type jumpers (manufacturer design within listed equipment typically handles this — exam stems may ask whether a green bonding screw in listed service equipment can serve as the MBJ).

Worked Example 1 — Wire-type MBJ:

Ungrounded service-entrance conductors: 3/0 AWG copper. Service equipment uses a wire-type main bonding jumper.

1. Open Table 250.102(C)(1).
2. Find the row for 2/0 or 3/0 AWG copper ungrounded conductors.
3. Read the copper bonding jumper / MBJ size column (commonly 4 AWG copper for that row — verify in your 2023 table).

Worked Example 2 — Large parallel service:

Two paralleled 600 kcmil copper ungrounded conductors per phase.

1. Equivalent area = 1,200 kcmil.
2. If that exceeds Table 250.102(C)(1) ranges, apply the 12½ percent area rule in 250.28(D) / 250.102(C) as applicable: 0.125 × 1,200,000 cmil = 150,000 cmil → select the next standard size conductor not smaller than that area (often 3/0 or 4/0 copper depending on Chapter 9 Table 8 cmil values — verify).

Always finish by checking the live table rather than memorizing edge-case AWG picks.

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Table 250.102(C)(1) vs Table 250.66 vs Table 250.122

Prov distractors deliberately swap tables. Use this filter:

Notice that for many common service sizes, Table 250.66 copper GEC sizes and Table 250.102(C)(1) copper bonding jumper sizes look similar — but they are different tables for different functions. On exam day, open the table named by the section that matches the conductor’s job.

Supply-side bonding jumpers (bonding service raceways, meter enclosures, CT cans, etc. on the supply side of the service disconnect) also size from 250.102(C) rules. Load-side bonding jumpers for equipment follow 250.102(D) and often track EGC sizing — read which side of the service disconnect the stem describes.

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Bonding Service Enclosures and Raceways

Metal parts of service equipment that are likely to become energized must be bonded together and to the grounded conductor. Typical bonding candidates on a service:

• Service raceways and elbows
• Meter socket enclosure
• Service disconnect enclosure
• Metal cable armor/sheath used as an enclosure
• Bonding bushings/jumpers where concentric/eccentric knockouts or reducing washers impair bonding continuity

Where a bonding jumper is required around impaired connections, size it as a supply-side bonding jumper per 250.102(C). Do not leave a meter can or service nipple relying on a precarious locknut-only path when the Code requires a bonding bushing or jumper for that installation condition.

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Neutral-to-Ground Bond Location — Subpanel Trap

A frequent Journeyman conceptual item: after the service disconnect, do not install another main bonding jumper in a downstream panel. In a subpanel:

• Keep neutrals isolated on an insulated neutral bar.
• Keep EGCs on the equipment grounding bar bonded to the enclosure.
• Run a four-wire feeder (two hots, neutral, EGC) on a 120/240 V single-phase feeder as required.

The service already established the system bonding point. Extra neutral-to-case bonds on the load side create parallel neutral current on raceways and EGCs — unsafe and a Code violation except where a specific section permits a separately derived system bond.

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Putting §§4.1–4.3 Together on One Service

Walk a complete dwelling service in Code order:

1. Identify present electrodes (250.50 / 250.52) and install/supplement as required (250.53).
2. Size and install the GEC (250.64 / 250.66), applying rod/ufer/ring caps only where they truly apply.
3. At the service disconnect, land the grounded conductor, install the MBJ (250.28), and bond metal service enclosures/raceways (250.24 / 250.92 / 250.102 as applicable).
4. Leave the load side with proper EGCs (250.122) and no extra neutral-case bonds.

That sequence matches how Prov stems are written: one question may isolate the electrode, the next the GEC size, the next the MBJ or bonding jumper.

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Timed Open-Book Tips

• Permanent-tab 250.24, 250.28, Table 250.102(C)(1), 250.66, and 250.122 as a five-stop grounding kit.
• On every bonding/grounding size question, first name the conductor’s function in one word: electrode, main bond, or equipment ground.
• For MBJ screws in listed service equipment, look for identification requirements in 250.28 rather than inventing a wire size.
• If the stem says “supply side of the service disconnect,” think 250.102(C); if it says “feeder equipment grounding conductor,” think 250.122.

Arkansas Journeyman rewards clean Article 250 navigation under the 3-hour / 60-question clock — function first, table second, exception third.