When selecting the right 3 phase power distribution box for your application, prioritize models with UL/CSA certification, proper NEMA enclosure ratings (such as NEMA 3R or 4X for outdoor or harsh environments), and sufficient amperage capacity—typically ranging from 100A to 600A . When selecting the right 3 phase power distribution box for your application, prioritize models with UL/CSA certification, proper NEMA enclosure ratings (such as NEMA 3R or 4X for outdoor or harsh environments), and sufficient amperage capacity—typically ranging from 100A to 600A . The outgoing line from the low-voltage end of the transformer is 0. 4kV to the distribution cabinet (primary distribution cabinet), then the outgoing line is led to the distribution box (secondary distribution box) in each building, and finally the outgoing line is led to the distribution cabinet. In many cases, power is supplied by the utility to a building at the utilization voltage. In these cases, the distribution of power within the building is achieved through the use of a simple radial distribution system. In cases where the utility service voltage is at some voltage higher than the. From residential 100-amp panels to massive 600 amp main distribution panels in commercial facilities, this comprehensive guide will help you understand distribution board types, sizing calculations, and installation requirements to make informed decisions about your electrical infrastructure.