A 300-A service with 350 kcmil feeders; what is the minimum MBJ size?

A 300-A service with 350 kcmil feeders; what is the minimum MBJ size?

• A. 1
• B. 3
• C. 2
• D. 4

Answer: (C)

MBJ sizing is based on the current the service and feeders must carry. You choose the smallest MBJ that can safely handle the greater of the service rating or the feeder ampacity. In this case, the service is 300 A and the feeders are 350 kcmil, which have an ampacity in the hundreds of amps range—roughly around 300 A or more. That means you need an MBJ that can carry about 300–400 A, not a smaller one that’s only rated for about 200 A, nor an unnecessarily large one. The minimum size is the MBJ that sits in that mid-range (the rating that covers roughly 200–400 A). Therefore, the correct choice corresponds to the option that matches that mid-range rating.

MBJ sizing is based on the current the service and feeders must carry. You choose the smallest MBJ that can safely handle the greater of the service rating or the feeder ampacity. In this case, the service is 300 A and the feeders are 350 kcmil, which have an ampacity in the hundreds of amps range—roughly around 300 A or more. That means you need an MBJ that can carry about 300–400 A, not a smaller one that’s only rated for about 200 A, nor an unnecessarily large one. The minimum size is the MBJ that sits in that mid-range (the rating that covers roughly 200–400 A). Therefore, the correct choice corresponds to the option that matches that mid-range rating.