Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A large industrial facility has a 400A, 480Y/277V, 3-phase, 4-wire service. The service entrance conductors are 600 kcmil copper. The service equipment consists of a main circuit breaker and a panelboard. What is the maximum size of the main bonding jumper required for this service?
- A1/0 AWG copper
- B3/0 AWG copper
- C4/0 AWG copper
- D2/0 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: D. 2/0 AWG copper
According to NEC Table 250.66, for service-entrance conductors with an area of 600 kcmil copper, the minimum size for the grounding electrode conductor (and thus the main bonding jumper, as per 250.28(A)) is 2/0 AWG copper. The main bonding jumper size is not permitted to be smaller than the grounding electrode conductor.
Why the other options are wrong
- A. 1/0 AWG copper is too small for 600 kcmil service conductors per NEC Table 250.66.
- B. 3/0 AWG copper is larger than required, leading to unnecessary cost.
- C. 4/0 AWG copper is significantly larger than required, leading to unnecessary cost.
Main Bonding Jumper Sizing
The main bonding jumper (MBJ) connects the equipment grounding conductor(s) and the service raceway or enclosure to the grounded conductor (neutral) at the service. Its size is determined by the size of the largest ungrounded service-entrance conductor, referencing NEC Table 250.66.
- Connects neutral to equipment ground at service.
- Size based on largest ungrounded service conductor (NEC 250.28(A)).
- Minimum size is derived from NEC Table 250.66.
- Ensures effective ground-fault current path.
Memory trick: MBJ's Link: Service conductor size defines the neutral's bridge.