National Electrical Code - Article 390: Underfloor Raceways and Article 392: Cable Trays

National Electrical Code (NEC) / NFPA 70: Article 390 - Underfloor Raceways and Article 392 - Cable Trays – Essential Guidance for Electrical Infrastructure

Ensuring the safe, organized, and efficient routing of electrical conductors is paramount in any engineered system. The National Electrical Code (NEC), NFPA 70, addresses these critical aspects through various articles, with Articles 390 (Underfloor Raceways) and 392 (Cable Trays) providing essential guidance for professionals managing complex electrical installations. These articles are foundational for protecting wiring and supporting cable systems in diverse environments, from commercial interiors to industrial facilities.

What is National Electrical Code (NEC) / NFPA 70?

The National Electrical Code (NEC), published by the NFPA, is an industry-leading consensus standard that establishes minimum electrical safety requirements for the nation. Its primary purpose is to safeguard persons and property from the hazards arising from the use of electricity by providing a comprehensive framework for the safe installation of electrical wiring and equipment. The NEC is updated regularly to reflect advancements in electrical technology and safety practices.

Scope & Purpose

Article 390 governs the installation and use of underfloor raceway systems, designed for embedding in concrete floors or installing above concrete slabs to provide a protected pathway for electrical conductors. This article specifies requirements for materials, installation methods, and conductor fill for these systems, primarily in commercial occupancies. Article 392 focuses on cable tray systems, which are structural elements used to support and route electrical cables and raceways. It covers various types of cable trays and their application in industrial and non-industrial settings, detailing their construction, installation, and use for supporting services, feeders, and branch circuits.

Who Must Comply?

• • Primary audience: Electricians, Electrical Engineers, Electrical Inspectors, Construction Managers, and Facility Maintenance Personnel.
• • Industries affected: Commercial Buildings, Industrial Facilities, Healthcare Facilities, Educational Institutions, Data Centers, and Infrastructure Projects.
• • Compliance nature: Mandatory for all new electrical installations covered by the NEC.
• • Enforcement: Typically enforced by local building departments through electrical inspections.

Key Requirements Overview

• • The standard requires underfloor raceways to be laid in straight lines with secure fastenings to maintain alignment during construction, with markers at the ends of runs.
• • Conductors within underfloor raceways must not exceed 75 percent of the raceway's cross-sectional area, and abandoned outlets necessitate the removal of associated conductors.
• • Cable tray systems must be constructed with adequate strength and rigidity, featuring smooth edges to prevent cable damage, and must be corrosion-resistant.
• • Cable trays are permitted to support a wide range of wiring methods, including services, feeders, and branch circuits, with specific provisions for single-conductor installations and equipment grounding conductors.
• • Both underfloor raceways and cable tray systems require proper grounding and bonding to ensure electrical continuity and safety, especially where systems transition or connect to other equipment.

Related Standards & References

These articles work in conjunction with other NEC sections and external standards. Complementary references include:

• • NEC Article 250 (Grounding and Bonding): Essential for ensuring the safety of both underfloor raceways and cable tray systems through proper grounding and