This document provides specifications for various distribution boxes including dimensions, mounting sizes, and number of ways. stallation and use of boxes. The article includes table references that guide the electrician in the selection of the proper box size necessary to safely accommodate ele trical service requirements. The box capacity table shown (page A-5) is reproduced in part from the NEC® as a quick reference and. Home Blog Best Practices Electrical Box Dimensions: Standard Sizes, Types & Selec. Electrical enclosures are boxes that protect your electrical parts from dust, water, and damage. They help keep everything inside safe and working properly.