Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A commercial building has a 208Y/120V, 3-phase, 4-wire service. The calculated load for the service is 300A. The service entrance conductors are 350 kcmil copper. What is the minimum size copper main bonding jumper required?
- A2/0 AWG
- B1/0 AWG
- C2 AWG
- D3/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: A. 2/0 AWG
According to NEC Table 250.102(C)(1), for service entrance conductors up to 350 kcmil copper, the minimum size copper main bonding jumper is 2/0 AWG. The 300A calculated load is irrelevant for main bonding jumper sizing, which is based on the service entrance conductor size.
Why the other options are wrong
- B. 1/0 AWG is too small for service conductors up to 350 kcmil.
- C. 2 AWG is too small for service conductors up to 350 kcmil.
- D. 3/0 AWG is larger than required, but not the minimum size.
Main Bonding Jumper Sizing
The process of determining the minimum size of the conductor that connects the grounded service conductor to the equipment grounding conductor(s) and to the service enclosure.
- Sized according to NEC Table 250.102(C)(1).
- Based on the size of the largest ungrounded service entrance conductor.
- Ensures continuity of the ground-fault current path.
Memory trick: Bonding jumpers are the 'glue' for grounding; size them to the biggest wire.