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.
INTERIOR SYSTEMS BELOW
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.
WET AND CORROSIVE INSTALLATIONS
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 grounding