CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
During a service equipment installation, a raceway knockout is larger than the connector, requiring the use of reducing washers. Per NEC 250.92(B), what additional measure is required to maintain a reliable equipment grounding path at service equipment?
- AReducing washers are prohibited in all electrical installations
- BAn additional ground rod must be driven at that enclosure
- CDouble locknuts alone are sufficient without further bonding
- DA bonding jumper must be installed to bypass the reducing washers
Show answer & explanationAnswer & explanation
Correct answer: D. A bonding jumper must be installed to bypass the reducing washers
NEC 250.92(B) does not recognize reducing washers, oversized knockouts with locknuts alone, or standard locknuts/bushings as reliable bonding methods at service equipment. When reducing washers are used, a bonding jumper must be installed to ensure a low-impedance path for fault current, since normal threaded connections cannot be relied upon there.
Why the other options are wrong
- A. Reducing washers are permitted in general wiring but require supplemental bonding at service equipment.
- B. An additional ground rod does not address the equipment grounding path continuity issue.
- C. Locknuts alone are not a listed bonding method at service equipment per 250.92(B).
Bonding at Service Equipment (250.92B)
At service equipment, certain connection methods (like reducing washers or oversized knockouts) are not considered reliable for bonding and require a supplemental bonding jumper.
- 250.92(B) lists acceptable bonding methods at service equipment
- Reducing washers alone are not an acceptable bonding method there
- A bonding jumper must bypass unreliable connections to maintain fault current path
Memory trick: Washers wobble, so wire a jumper to guarantee the ground path.