9.2 Bonding & the Effective Ground-Fault Current Path

Why Bonding Matters on the TX Journeyman Exam

The PSI Knowledge part routinely asks where bonding is required and which method is listed. The most-missed items are the concentric knockout rule and the over-250 V rule. The exam also probes the conceptual distinction between grounding and bonding — a candidate who confuses them loses two or three questions in a row.

Grounding vs. Bonding — the Conceptual Divide

NEC Article 100 defines the terms. Grounding of a conductive material means connecting it to the earth through a grounding electrode system — the electrode side covered in Chapter 4. Bonding means connecting conductive materials together to establish electrical continuity and the capacity to conduct fault current safely.

• Grounding → earth (high impedance, cannot clear a fault)
• Bonding → source (low impedance, lets the breaker trip)

A ground rod by itself will never trip a 20 A breaker. The earth is a poor conductor. The fault must get back to the system neutral point at the service or separately derived source through a low-impedance metallic path. That path is built from EGCs, bonded raceways, and bonded enclosures.

The Performance Rule — NEC 250.4(A)(5)

The effective ground-fault current path shall be permanent and electrically continuous, shall be capable of safely carrying the maximum fault likely to be imposed on it, and shall have sufficiently low impedance to facilitate the operation of overcurrent protective devices.

This sentence is the answer to almost every bonding question. If a metal raceway or enclosure could become energized, it must be bonded into a path that lets the breaker clear the fault. The path must be:

1. Permanent — not dependent on a movable part or a single setscrew that can loosen.
2. Electrically continuous — every joint is bonded.
3. Capable of carrying the fault — sized and rated for the available fault current.
4. Low impedance — so the OCPD operates within its clearing time.

Bonding at Services — NEC 250.92

250.92(A) requires bonding of all service-entrance conductors' raceways, cable trays, cablebus framework, service cable armor or pipe, and all metallic enclosures containing service-entrance conductors. This is the service bond — on the line side of the main disconnect, where there is no OCPD yet and fault current can be enormous.

250.92(B) lists the permitted methods for service bonding:

The concentric or eccentric knockout rule (250.97 and 250.92 alike): standard locknuts and ordinary bushings are not permitted to be the sole bonding means at a concentric or eccentric (reduced-wall) knockout. The thin annular ring of metal at a concentric KO does not make reliable long-term contact. A listed bonding bushing with a bonding jumper (or a listed grounding-type locknut/wedge) is required.

Bonding Other Than at Services — NEC 250.94 and 250.96

NEC 250.94 addresses bonding for circuits over 250 V to ground and for other-than-service raceways. Where the voltage to ground exceeds 250 V, any metal raceway or metal-sheathed cable enclosing the circuit must be bonded to ensure electrical continuity.

NEC 250.96(A) requires generally that exposed non-current-carrying metal parts of fixed equipment — raceways, fittings, boxes — be bonded together and to the system grounded conductor or EGC. Where necessary to ensure electrical continuity, listed bonding fittings must be used.

NEC 250.96(B) specifically addresses reducible washers and locknuts: where a box or enclosure has knockouts or slotted openings that could impair continuity (such as concentric or eccentric KOs), reducible washers, or listed locknuts that could become loose, bonding means must be used.

Raceways with Circuits Over 250 V to Ground — NEC 250.97

NEC 250.97 requires bonding of metal raceways, cable trays, cablebus, metal-sheathed cable, and similar enclosures for circuits exceeding 250 V to ground. The listed methods are the same as 250.92(B). The exceptions cover:

• Fixture whip extensions on circuits over 250 V where listed flexible conduit is used and the EGC is run inside.
• Metal-sheathed cable listed for the voltage.

Worked Example — Why a Bonding Bushing Is Required at a Service

A 200 A service is run in 2 in. rigid metal conduit (RMC) entering the bottom of a main service panel through a concentric knockout. The electrician lands the RMC with a standard locknut and an ordinary metal bushing inside the panel, then connects the main bonding jumper to the neutral bar only.

Is this compliant? No.

• Step 1 — 250.92(A): the RMC enclosing service-entrance conductors must be bonded.
• Step 2 — 250.92(B): at a concentric knockout, a standard locknut and ordinary bushing are not an approved bonding means. The thin annular ring at the KO does not provide reliable metal-to-metal continuity for service-level fault current.
• Step 3 — Required correction: install a listed bonding bushing with a bonding jumper from the bushing's bonding lug to the neutral bar (or to the main bonding jumper terminal) inside the panel. A listed grounding-type locknut or wedge is also permitted.

The fault path is now: faulted service conductor → RMC → bonding bushing → bonding jumper → neutral bar → main bonding jumper → grounded service conductor (neutral) back to the utility transformer. The service overcurrent device (or the utility-side protection) sees the fault and clears it. Without the bonding bushing, the path depends on a loose concentric ring, the impedance rises, and the breaker may not trip — violating 250.4(A)(5).

Bonding Jumpers — Sizing

• Main bonding jumper at the service — sized by 250.28(D) using Table 250.102(C)(1), not Table 250.122 (Chapter 4 covers this in detail).
• Equipment bonding jumper on the load side — sized by Table 250.122 using the OCPD rating, the same as the EGC.
• Supply-side bonding jumper (line side of the main disconnect) — sized by Table 250.102(C)(1) based on the largest ungrounded conductor.

Keep these three tables straight: Table 250.66 = GEC (earth), Table 250.102(C)(1) = supply-side and main bonding jumpers, Table 250.122 = EGC and load-side bonding jumpers.