Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsHard
A utility company is providing a 600A, 480Y/277V, 3-phase, 4-wire service to a new commercial building. The service entrance conductors are two sets of 400 kcmil copper per phase. What is the minimum size copper main bonding jumper required?
- A250 kcmil
- B2/0 AWG
- C3/0 AWG
- D300 kcmil
Show answer & explanationAnswer & explanation
Correct answer: A. 250 kcmil
According to NEC Table 250.102(C)(1), the main bonding jumper is sized based on the equivalent circular mil area of the largest ungrounded service entrance conductors. Two sets of 400 kcmil copper conductors per phase means the equivalent area is 2 * 400 kcmil = 800 kcmil. For service conductors between 600 kcmil and 1100 kcmil copper, the minimum size copper main bonding jumper is 250 kcmil.
Why the other options are wrong
- B. 2/0 AWG is too small for service conductors totaling 800 kcmil.
- C. 3/0 AWG is too small for service conductors totaling 800 kcmil.
- D. 300 kcmil is larger than required, but not the minimum size.
Main Bonding Jumper Sizing (Multiple Conductors)
The process of determining the minimum size of the main bonding jumper when service entrance conductors are installed in parallel, requiring the sum of their circular mil areas.
- Sized according to NEC Table 250.102(C)(1).
- Based on the equivalent circular mil area of the largest ungrounded service entrance conductors.
- Ensures effective ground-fault current path for large services.
Memory trick: Parallel service conductors need their 'total strength' calculated for the bonding jumper.