In this guide, I'll walk you through a practical, step-by-step process to size your distribution box based on actual load current. This guide helps you determine the correct dimensions based on wire fill capacity, device requirements, and installation environment, ensuring a safe and efficient electrical system. Learn how to. stallation and use of boxes. The box capacity table shown (page A-5) is reproduced in part from the NEC® as a quick reference and. Electrical systems power our homes, offices, and industrial facilities, but behind every reliable electrical setup lies a crucial component that often goes unnoticed: the distribution box. This essential piece of equipment serves as the nerve center of your electrical system, managing power flow. How to choose a distribution box of the right size for a project based on load current? Get it right the first time with this comprehensive guide If you're like most electrical professionals, picking the right distribution box for your project can feel like navigating a maze.