Referring to the above scenario, what is the determining factor for total amperage capacity?

• A. The cable on the line side of the distro box
• B. The distro box rating
• C. The load side cable
• D. The length of the conductors

Answer: (A)

The most important factor is the line-side feeder entering the distro box. The box can only carry as much current as the feed conductor can safely handle. If the line-side cable is rated for a certain amperage, that sets the maximum flow through the box, even if the downstream wiring or the box itself could handle more. The load-side conductors and the box rating might be capable of higher current, but you won’t exceed what the incoming feed allows. Length of conductors mainly affects voltage drop, not the inherent ampacity, so it doesn’t determine total amperage capacity in the same way.