Code Q&A - April 2013
Over the course of my career, I have been asked several code-related questions by both apprentices and other experienced contractors. This column will provide answers to some popular questions.
Can you share any advice on how to install direct-burial cable in trenches? I have been called to review several problems in a residential subdivision that has had the aluminum cables failing from the utility to home service equipment, which was installed by the electrical contractor.
There will always be problems when underground cables are improperly installed. The use of conduits or other approved raceways always makes a better installation, mainly because it protects the conductors from physical and chemical attack, but is not economically feasible to the developer unless there are utility and/or building department requirements.
So when using the direct-burial method, there are requirements in the 2011 NEC-70. Article 300.5, under Ground Installations; Table 300.5. Direct-burial cables or conductors require a trench no less than 600mm (24in.), defined as the shortest distance in millimeters (inches) measured between points on the top surface of any direct-buried conductor, cable, or any other raceway, and the top surface of finish grade.
The first rule is to use cables that are listed and labeled for this use and intended application.
Article 300.5 (D) Protection from Damage. Direct-buried conductors and cables shall be protected from damage in accordance with 300.5(D) (1) through (D)(4).
- Emerging from Grade. Direct-buried conductors and cables emerging from grade shall be protected by enclosures or raceways extending from the minimum cover distance below grade, to a point at least 2.5m (8ft) above finished grade. In no case shall the protection be required to exceed 450mm (18in) below finished grade.
- Conductors entering a building shall be protected to the point of entrance. Be sure to use an appropriate insulated bushing or sleeve and use the proper sealant to resist the introduction of moisture into the structure.
- Since these are direct-buried service conductors, their location must be identified by a warning ribbon that is placed in the trench at least 300mm (12in) above the underground installation, reducing the risk of an accident during excavation near underground service conductors.
Article 300.5(F) Backfill. Backfill containing large rocks, paving material cinders, large or sharply angular substances, or corrosive material shall not be placed in the excavation where materials may damage the integrity of the conductor inner/outer insulation. Where necessary to prevent physical damage, use granular or selected material, suitable running boards, suitable sleeves, or other approved means.
Article 300.5(F) Earth Movement. Where direct-buried conductors, raceways, or cables are subject to movement by settlement or frost, direct-buried conductors shall be arranged to prevent damage by putting an "S" loop to raceway transition. This will allow for earth movement due to frost or settlement as anticipated.
Marc Ramirez has been a member of the NEC Code Making Panel 17 since 1993 and is a member of the IEC National Codes and Standards Committee. He can be reached at (623) 256-9331. Submit your questions to firstname.lastname@example.org.