Master Electrician Exam (NEC)General Electrical KnowledgeHard
A master electrician is installing a new 200A service for a residential dwelling. The service entrance conductors are 2/0 AWG copper. What is the minimum size required for the main bonding jumper (MBJ) in this service, per NEC Article 250?
- A4 AWG copper
- B2/0 AWG copper
- C2 AWG copper
- D1/0 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: C. 2 AWG copper
NEC 250.28(D)(1) references Table 250.66 for sizing the main bonding jumper. Table 250.66 requires the main bonding jumper to be sized based on the largest ungrounded service-entrance conductor. For 2/0 AWG copper service conductors, Table 250.66 specifies a minimum 2 AWG copper for the main bonding jumper.
Why the other options are wrong
- A. 4 AWG copper is for service conductors up to 1/0 AWG copper.
- B. 2/0 AWG copper would be the size of the service conductor, but the MBJ is typically smaller unless the service conductors are extremely large.
- D. 1/0 AWG copper is for service conductors up to 3/0 AWG copper.
Main Bonding Jumper Sizing
The main bonding jumper (MBJ) connects the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor (neutral) at the service. Its size is critical for providing a low-impedance fault path.
- Sized according to NEC Table 250.66, based on the largest ungrounded service-entrance conductor.
- Ensures the service disconnect enclosure is properly bonded to the grounded conductor.
- Required at the service equipment to establish a single point of connection between the grounded and equipment grounding systems.
Memory trick: MBJ Size: Service Conductors Set the Bar.