CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
An electrician installs EMT into a service panel through a concentric knockout, securing it with a single locknut and bushing. A licensing inspector requires a bonding jumper be added. Why is this necessary?
- ABonding jumpers are required on every raceway regardless of knockout type
- BThe panel is undersized for the number of circuits installed
- CEMT is never permitted to terminate directly into service equipment
- DConcentric knockouts can leave paint or a poor electrical connection, preventing reliable bonding through the ring alone
Show answer & explanationAnswer & explanation
Correct answer: D. Concentric knockouts can leave paint or a poor electrical connection, preventing reliable bonding through the ring alone
Where concentric or eccentric knockouts are punched, the reduced material in the rings may not provide a reliable low-impedance path to ground. NEC 250.92(B) requires bonding jumpers or bonding-type locknuts/bushings in these cases to ensure an effective ground-fault current path at service equipment.
Why the other options are wrong
- A. Bonding jumpers are only specifically required for concentric/eccentric knockouts or where reliance on locknuts is insufficient, not universally for all knockouts.
- B. Panel sizing is unrelated to the bonding jumper requirement described.
- C. EMT is a recognized raceway type for service equipment when properly bonded.
Concentric Knockout Bonding
NEC 250.92(B) requires bonding jumpers at service equipment where concentric or eccentric knockouts may compromise the reliability of the grounding path.
- Concentric rings reduce metal thickness, risking poor continuity.
- Bonding jumpers or listed bonding bushings restore a reliable path.
- Applies specifically at service equipment enclosures.
Memory trick: 'Rings can break the ring of safety' — bond around weak knockouts.