Journeyman Electrician Exam (NEC)Grounding and BondingHard
A journeyman is installing a new 400-ampere, 120/208-volt service for a commercial tenant. The service-entrance conductors are 500 kcmil copper. The main bonding jumper is made of copper. What is the minimum size copper main bonding jumper required for this service, according to NEC® 250.102(C)(1)?
- A2/0 AWG
- B3/0 AWG
- C300 kcmil
- D250 kcmil
Show answer & explanationAnswer & explanation
Correct answer: D. 250 kcmil
According to NEC® Table 250.102(C)(1), for ungrounded service-entrance conductors with a size of 500 kcmil copper, the minimum size copper main bonding jumper required is 250 kcmil.
Why the other options are wrong
- A. 2/0 AWG is for service conductors up to 400 kcmil.
- B. 3/0 AWG is for service conductors up to 500 kcmil in some older tables or for specific applications, but not for MBJ with 500 kcmil service conductors.
- C. 300 kcmil is larger than the minimum required for 500 kcmil service conductors.
Main Bonding Jumper Sizing (NEC® 250.102(C)(1))
The main bonding jumper (MBJ) connects the equipment grounding conductor to the grounded service conductor and service enclosure. Its minimum size is determined by the largest ungrounded service-entrance conductors, using NEC® Table 250.102(C)(1).
- Connects ground and neutral at the service.
- Size derived from the largest ungrounded service-entrance conductors.
- Reference NEC® Table 250.102(C)(1).
Memory trick: MBJ's size mirrors the biggest service wire's might.