Calculate service entrance sizing, panel loads, demand factors, and ensure NEC Article 220 compliance. Always verify calculations with a qualified electrical engineer and local authority having. Determines the total number of branch circuits, wire sizes, breaker ratings, and GFCI/AFCI protection requirements for residential electrical systems. While many families are familiar with these boxes, there is often a lack of understanding regarding their specifications and proper. Pro Insight: A well-planned distribution box feels like a silent partner—you only notice it when something's wrong. Our goal? Make sure you never notice it. Before we dive into calculations, let's get familiar with a few essentials: 1.