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CABLE
CLASSIFICATION:
House electrical circuits are wired with metal armored (BX) cable,
nonmetallic sheathed (ROMEX) cable, or with insulated wires running
through metal or plastic pipe called conduit.
Metal-Armored
Cable-
Armor-wrapped cable is commonly called BX, a trade name. It has
an outer layer of flexible galvanized steel that contains two or
three wires. Each wire is individually wrapped with paper. If the
cable does not have a ground wire, the metal sheathing serves as
a ground whenever it comes in contact with a metal junction, switch,
and outlet box.
Use
of BX cable sometimes is restricted by electrical codes. Check the
local codes where the material is sold. BX also is restricted to
use indoors in dry locations. It is sometimes specified for use
where power wires need extra sheathing protection from nails used
in carpentry or decorating.
Nonmetallic
Cable-
You probably will buy and work with nonmetallic plastic-sheathed
cable more than any other conductor or wire. It is often called
by a trade name, Romex, which has almost become synonymous with
any non metallic electrical cable. Local codes may allow nonmetallic
cable only in certain locations, or may specify that you use another
type, such as metallic armored cable, or wires running in conduit.
The outer
sheath of nonmetallic cable is usually a moisture -resistant, flame-retardant
material. Inside, there are two or three insulated power wires,
and perhaps a grounding wire. For most residential wiring, two types
are often used.
- Type
NM- This is based only in dry locations. Each wire, with the
possible exception of the ground wire, is wrapped in its own plastic
sheath. The three wires are then wrapped in a paper insulator,
and the wrapping is covered with plastic.
The wire in Type NM is either AWG No. 12 or AWG No. 14 for house
circuits. Larger sizes such as No. 10 or more are used for heavy
appliances. NM is available in two or three conductors, plus the
ground.
- Type
UF- For use in wet locations, including underground, UF cable
is an alternative to conduit. The cable has individual wires embedded
in water-resistant solid plastic. The cable is available in AWG
No. 12 and No. 14, with the ground.
Conduit-
According to code, conduit can be galvanized steel pipe or plastic
pipe. Metal conduit comes in three types: rigid (often preferred
for outdoor use), intermediate, and electrical metal tubing (EMT)
- a newer type popular for house wiring. Standard conduit diameters
are 1/2 3/4" 1" and 1-1/4". There are fittings to join conduit for
straight runs and at 45-degree angles. The material is bent with
a tool called a hickey.
In some
areas, by code, short lengths of BX cable may be enclosed in a conduit.
UNDERSTANDING
THE MARKINGS:
Markings on the insulation, plastic sheathing, and on nonmetallic
cable explain what is inside and identify the type of insulation
covering. Consider the following designation:
12/2 With Ground, Type NMC, 600(UL)
The first
number tells you the size of the wires inside the insulation or
cable, in this case No. 12 gauge. The second number tells you that
there are two conductors (wires) in the cable. There also is an
equipment grounding wire, as indicated. The type of cable is given;
the number following indicates the maximum voltage allowed through
the cable. Finally, the UL notation assures you that the cable has
been rated as safe for the uses for which it was designed.
To connect
3-way switches- switches that control a light from two points -
use No. 12/3 with ground cable. No. 12/3 has a black, white, red,
and ground wire. The red wire is called a "traveler" wire and runs
the power between both switches.
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