Master Electrician Exam (NEC)Grounding and BondingEasy
A master electrician is installing a new service for a commercial building. The service consists of a 480Y/277V, 3-phase, 4-wire system. Which of the following is the primary purpose of the main bonding jumper?
- ATo provide a low-impedance path for fault currents to the earth.
- BTo connect the neutral conductor to the equipment grounding conductor and the service enclosure.
- CTo connect the equipment grounding conductor to the grounding electrode conductor.
- DTo bond all non-current-carrying metal parts of the electrical system together.
Show answer & explanationAnswer & explanation
Correct answer: B. To connect the neutral conductor to the equipment grounding conductor and the service enclosure.
The main bonding jumper's primary role is to connect the neutral conductor (which is grounded at the service) to the equipment grounding conductors and the service enclosure, ensuring a low-impedance path for ground-fault current to return to the source.
Why the other options are wrong
- A. This describes the overall purpose of the grounding electrode system, not specifically the main bonding jumper.
- C. This describes the connection point, but not its primary function in relation to the neutral.
- D. This is a general purpose of bonding, but the main bonding jumper has a specific role at the service entrance.
Main Bonding Jumper (MBJ)
A conductor or strap that connects the neutral conductor to the equipment grounding conductor and the service enclosure at the service entrance.
- Establishes a low-impedance path for fault current.
- Required at the service equipment.
- Bonds the neutral to ground at one point only.
Memory trick: Neutral's main job is to bond the box and ground.