This chapter will help you understand the principles of interior wiring. The requirements for installing electrical systems are found in the current edition of the National Electrical Code ® (NEC® ). The requirements are specific and detailed, and they change somewhat as the complexity of the system increases. Therefore, the Code should be checked for proper installation of electrical systems.
An electrical system that is installed in concrete or in direct contact with the earth is considered to be a system below grade.
Below grade conduit layout and direct buried cable or other raceways must be installed to meet the minimum cover requirements of table 5-1. Under buildings, underground cables must be in a raceway that is extended beyond the outside walk of the building. Direct buried cables emerging from the ground will be in protected enclosures or raceways extending from the minimum cover distance, required by table 5-1, below grade to a point at least 8 feet above finished grade. There is no requirement for the protection of direct buried cables in excess of 18 inches below the finished grade.
Conduit in concrete buildings can be installed while the building is being erected. The outlets should be attached to the forms, and the conduits between outlets should be attached to reinforcing steel with metal tie wires so that the concrete can be poured around them. When several conduits pass through a wall, partition, or floor, a plugged sheet-metal tube should be set in the forms to provide a hole for them in the concrete. When a single conduit passes through a wall, partition or floor, a nipple or a plugged sheet- metal tube can be set in the forms.
Ferrous or nonferrous metal raceways, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports, and support hardware may be installed below grade. These materials also may be installed in areas subject to severe corrosive influences when made of material judged suitable for the condition or when provided with corrosion protection approved for the condition.
Underground-feeder cable and branch-circuit cable provide an economical wiring system for wet and corrosive installations. Type UF two-conductor cable resembles Type USE service-entrance cable in general appearance. The insulation is a plastic compound. NEC
® statements with respect to its use are as follows: Underground-feeder and branch-circuit cable may be used underground, including direct burial in the earth, as feeder or branch-circuit cable when provided with overcurrent protection not in excess of the rated current-carrying capacity of the individual conductors. If single-conductor cables are installed, all cables of the feeder circuit, subfeeder circuit, or branch circuit, including the neutral and equipment grounding conductor, if any, will be run together in the same trench or raceway. If the cable is buried directly in the earth, the minimum burial depth permitted is 24 inches if the cable is unprotected and 18 inches when a supplemental covering, such as a 2-inch concrete pad, metal raceway, pipe, or other suitable protection, is provided. Type UF cable may be used for interior wiring in wet, dry, or corrosive locations under the recognized wiring methods of the Code, and when installed as a nonmetallic-sheathed cable, it will conform with the provisions of the Code and be of a multiconductor type. Type UF cable also must be of a multiconductor type if installed in a cable tray.
Type UF cable will not be used (1) as service- entrance cable, (2) in commercial garages, (3) in theaters, (4) in motion-picture studios, (5) in storage- battery rooms, (6) in hoistways, (7) in any hazardous location, (8) embedded in poured cement, concrete, or aggregate except as provided in the Code, and (9) where exposed to direct rays of the sun unless identified as sunlight-resistant.
Ungrounded conductors are available as single or multiconductor cables. These cables are clearly marked to identify them as grounded and groundingContinue Reading