The NFPA has published an overview of Article 300 of the National Electrical Code (NEC), which outlines general requirements for wiring methods and materials.
The guidance, authored by NFPA Senior Electrical Specialist Dean Austin, explains how Article 300 serves as a catchall section for wiring methods not covered in specific NEC articles.
The document provides insights into underground installations, minimum cover requirements, wet location considerations, earth movement effects, and proper backfilling techniques.
NFPA’s guidance focuses on Section 300.5 of the NEC, which covers underground installations.
It details the minimum burial depth for various wiring methods using Table 300.5(A), which specifies cover requirements for direct burial cables, conductors, and raceways in different locations.
Austin provided an example of a 120/240-volt, 80-amp feeder installation to a pole barn, emphasizing that if a schedule 40 PVC conduit is used, it must be buried at least 18 inches below finished grade.
He also noted that when measuring depth, the outside diameter of the conduit must be considered to ensure compliance with NEC requirements.
According to Section 300.5(B), all underground wiring is classified as being in a wet location.
This designation means that conductors must be rated for wet conditions, such as THWN or THW, which include a “W” in their type lettering.
Austin pointed to Article 310 of the NEC, which provides a table of conductor types suitable for wet locations.
He advised that in addition to selecting the correct conductor type, installers should consider temperature ratings, which may differ between wet and dry locations, when sizing conductors for feeders and branch circuits.
The NFPA’s guidance also highlights the impact of frost heave and earth movement on underground conduit installations.
Section 300.5(J) requires expansion fittings to be used in regions where frost heave occurs to prevent damage to PVC conduits and connected equipment.
Austin explained that proper placement of securing and supporting methods, as outlined in Article 352, is necessary to allow for conduit movement.
He noted that securing the conduit above the expansion fitting helps prevent damage to the emergency disconnect or other termination points.
Section 300.5(F) addresses proper backfilling of trenches used for underground wiring.
The guidance warns against using backfill materials that contain rocks, sharp objects, or corrosive substances that could damage conduits.
Austin recommended using sand or other approved materials to cover the PVC conduit before backfilling with the original excavated material.
He emphasized that trench installations must be inspected and approved by the local authority having jurisdiction (AHJ) before backfilling can proceed.
The NFPA has published guidance on NEC Article 300, detailing general requirements for wiring methods and materials.
The guidance, written by NFPA Senior Electrical Specialist Dean Austin, explains key provisions related to underground wiring, wet location classifications, conduit movement due to frost heave, and trench backfilling requirements.
Section 300.5 provides burial depth requirements for underground wiring, with Table 300.5(A) listing cover requirements for various installations.
Wiring in wet locations must use conductors rated for moisture exposure, such as THWN or THW.
To accommodate frost heave, Section 300.5(J) requires expansion fittings in conduit installations where movement is expected.
Section 300.5(F) outlines backfilling requirements to prevent conduit damage.
Installations must be inspected by the AHJ before finalizing trench work.
The NFPA recommends using NFPA LiNK for searching NEC terms related to wiring installations.
