Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsEasy
A master electrician is installing a new 200A, 120/240V, single-phase service for a dwelling unit. The service entrance conductors are 3/0 AWG copper. What is the minimum size required for the main bonding jumper (MBJ) if it is copper?
- A6 AWG
- B4 AWG
- C2 AWG
- D1/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: B. 4 AWG
According to NEC Table 250.102(C)(1), for service-entrance conductors of 3/0 AWG copper, the minimum size for the main bonding jumper made of copper is 4 AWG.
Why the other options are wrong
- A. 6 AWG copper is for smaller service entrance conductors, not 3/0 AWG.
- C. 2 AWG copper is larger than required for 3/0 AWG service entrance conductors.
- D. 1/0 AWG copper is significantly larger than required for 3/0 AWG service entrance conductors.
Main Bonding Jumper Sizing (NEC 250.102(C)(1))
The main bonding jumper (MBJ) connects the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor of the service, ensuring electrical continuity and fault current path.
- Sized based on the largest ungrounded service-entrance conductor or equivalent area for parallel conductors.
- Referenced in NEC Table 250.102(C)(1).
- Must be copper or aluminum.
Memory trick: Bonding Jumper: Connects the grounds, like a bridge for safety.