Overview
This lecture covers when bonding bushings or enhanced bonding methods are required for electrical raceways and enclosures, according to the National Electrical Code (NEC), focusing on main code sections, specific scenarios, and practical wiring cases.
Key NEC Sections for Bonding
- Main relevant NEC sections: 250.90, 250.92, 250.96, and 250.97.
- Additional sections: 250.68 (grounding electrode conductor in ferric raceways) and hazardous location rules (250.100, 501.30, 502.30, 503.30).
Bonding at Electrical Services (250.92)
- Service raceways with utility conductors require enhanced bonding; standard lock nuts are not sufficient.
- Items needing bonding: raceways, cables, cable trays, and connected enclosures.
- Concentric or eccentric (impaired) knockouts require bonding bushings with jumpers.
- Acceptable methods: bonding bushings, threaded hubs (wrench-tight), bonding-type lock nuts, or listed devices.
- Bonding-type lock nuts cannot be used if concentric/eccentric knockouts remain.
Bonding Over 250V to Ground (250.97)
- Circuits over 250V to ground (e.g., 277/480V): bonding jumpers are required at concentric/eccentric knockouts.
- Exception: no extra bonding required if knockouts are listed for bonding (usually in outlet boxes).
- Under 250V to ground: enhanced bonding generally not required unless connection is inadequate.
General Bonding Principles (250.90, 250.96)
- All metal parts must be bonded to ensure a ground fault will trip a breaker.
- Non-conductive coatings (e.g., paint, Teflon tape) must be removed from bonding surfaces.
- Reducing washers do not require extra bonding if listed and all knockouts are removed.
Grounding Electrode Conductors in Ferric Raceways (250.64(E))
- Ferric (iron/steel) raceways with grounding electrode conductors must be bonded at each end.
- Bonding jumper must be at least the size of the largest enclosed grounding electrode conductor.
Hazardous Locations (501.30, 502.30, 503.30)
- Standard lock nuts do not satisfy bonding requirements in hazardous (classified) areas.
- Enhanced or continuous bonding required from hazardous location to service disconnect or separately derived system.
- Flexible raceways in hazardous locations must have a wire-type equipment bonding jumper.
Practical Bonding Checklist
- No bonding required for non-metallic raceways or enclosures.
- Threaded raceways connected to enclosures (threaded hubs) do not need extra bonding.
- One end of metallic conduit in non-metallic enclosures must be bonded.
- Always verify if a faulted connection will trip the breaker; bond if not certain.
Key Terms & Definitions
- Bonding Bushing — a fitting for creating an assured electrical connection between a raceway and enclosure.
- Enhanced Bonding — added measures (bushing, lock nut, jumper) for reliable ground fault current paths.
- Concentric/Eccentric Knockouts — removable rings in enclosures where remaining rings can impair bonding.
- Ferric Raceway — a steel or iron conduit requiring special bonding for grounding electrode conductors.
Action Items / Next Steps
- Review NEC sections 250.90–250.100, especially 250.92, 250.96, and 250.97.
- Study local code amendments regarding bonding.
- Download and study the bonding bushing flowchart if available.