NEC Requirements for Outdoor Distribution Boxes: Complete specification guide for outdoor electrical distribution boxes covering NEC Article 312 requirements, NEMA ratings, sizing calculations, and selection criteria for commercial and residential applications. Selecting the appropriate junction box size prevents overcrowding, overheating, and potential hazards. Learn how to. Electrical enclosure sizes are not universal, but most manufacturers follow common size families. This guide explains typical wall-mount and floor-standing dimensions, how to read catalog sizes, and how to choose the right enclosure size for your layout. 💡 Specification Insight: NEC 312. IS 13703 (Part-1&2)-1993 / IEC 60263/1-1986:. Proper sizing of pull boxes is essential to ensure safe, code-compliant, and maintainable electrical installations. Internal usable space is what your components actually occupy after deducting wall.