6.2 Grounded Conductors & System Bonding (Article 250 Parts III-IV)

Why This Matters

The grounded (neutral) conductor and the main bonding jumper are the heart of the service grounding scheme. The exam tests where the neutral-to-case bond is permitted, how to size the main bonding jumper, and what happens if you bond on the load side of the service disconnect. Get this wrong in the field and you create objectionable neutral current on metal raceways and pipes — a shock and fire hazard.

200.6 — Identification of the Grounded Conductor

The grounded conductor of a system must be identified by white or gray insulation, or by three continuous white or gray stripes on other than green insulation (200.6). On conductors 4 AWG and larger, re-identification at termination points is permitted. Do not reach for 250.26 here — that section tells you which conductor of an AC system must be grounded, not how to identify it. The grounded conductor is the conductor intentionally connected to earth — for a 120/240 V single-phase service, that is the neutral. Do not confuse it with the equipment grounding conductor, which is identified by green or green-with-yellow-stripe per 250.119.

250.24 — Grounding at the Service

250.24(C) requires the grounded conductor to be run to each service disconnecting means and bonded to the enclosure through the main bonding jumper. The service equipment is the one place the neutral, the equipment grounding conductor, and the enclosure all tie together.

250.24(A)(5) is the critical companion rule: a neutral-to-case connection shall not be made on the load side of the service disconnecting means. Once you leave the service equipment, the neutral must be isolated from the equipment grounding conductor and all metal enclosures. This is what prevents objectionable neutral current from flowing on metal raceways, cable armor, and water pipes (250.6).

250.28 — Main Bonding Jumper

The main bonding jumper is the connection between the grounded conductor and the equipment grounding conductor at the service equipment (Article 100). 250.28 requires:

• (A) Material: copper, aluminum, or copper-clad aluminum; may be a wire, bus, screw, or similar conductor. Screws must have a green finish visible after installation.
• (B)/(C) Connection: attached per 250.8 methods (listed connectors, exothermic welding, etc.).
• (D) Sizing: a wire-type main bonding jumper is sized per Table 250.102(C)(1) based on the area of the largest ungrounded service-entrance conductor.

Note: In the 2017 NEC the main bonding jumper was sized by Table 250.66; the 2020 NEC relocated this to Table 250.102(C)(1). Both tables share the same row structure but 250.102(C)(1) now also covers supply-side bonding jumpers and the grounded conductor itself. Expect the exam to reference Table 250.102(C)(1).

A manufacturer-furnished main bonding jumper in listed service equipment (panelboard, switchboard) is rated by the listing lab — no field sizing needed. Only field-installed wire jumpers require the table.

Service vs. Subpanel — the Bonding Decision

At the service equipment, the neutral bar and the ground bar are bonded together with the main bonding jumper. The grounded conductor ties the system to earth through the GEC, and the enclosure is bonded to the neutral so a fault from an ungrounded conductor to the case sees a low-impedance return path through the neutral back to the source, tripping the OCPD.

At a subpanel downstream, you must remove the main bonding jumper and install the neutral bar isolated from the enclosure. An EGC is run with the feeder and lands on a separate equipment grounding terminal bar that IS bonded to the subpanel enclosure. The neutral remains insulated. If you bond at the subpanel, feeder neutral current splits between the neutral conductor and the parallel metal path (raceway, EGC, water pipe), creating objectionable current and violating 250.6(A) and 250.24(A)(5).

Diagram: Service Bonding vs. Subpanel Separation

graph TD
  A[Utility Transformer] -->|3-wire 120/240V<br/>2 hots + neutral| B[Service Equipment]
  B -->|Main Bonding Jumper<br/>neutral bonded to enclosure| C[GEC to electrodes]
  B -->|4-wire feeder<br/>2 hots + neutral + EGC| D[Subpanel]
  D -->|Neutral bar ISOLATED<br/>from enclosure| E[Neutral loads]
  D -->|EGC bar BONDED<br/>to enclosure| F[Equipment grounding]
  style B fill:#cfc
  style D fill:#fcc

Worked Example

A service is supplied by 2/0 AWG copper service-entrance conductors. A field-installed wire main bonding jumper is needed in a wireway. From Table 250.102(C)(1), the 2/0 row → 4 AWG copper main bonding jumper. The same 4 AWG is also the minimum grounded (neutral) service conductor size. If this service were 400 kcmil copper, the row "Over 350–600 kcmil" requires 1/0 AWG copper.

Traps

• "You can bond neutral to ground at every panel for redundancy" — false; only at the service (250.24(A)(5)).
• "A green screw in a subpanel is OK if the panel has no separate EGC" — false for new work; Exception 1 of 250.32(B)(1) is existing installations only.
• The grounded conductor is sized per Table 250.102(C)(1), not Table 250.66 — the two tables now differ in scope.
• "Sizing per 250.24(C) uses Table 250.66" — that was the pre-2020 rule; NEC 2020 moved main bonding jumper sizing to Table 250.102(C)(1).