Building Design and Construction Handbook - Merritt - Ventech!

15.18 SECTION FIFTEEN
Busways must be exposed for heat dissipation. They are arranged with access openings
for plug-in and trolley connections.
For heavy current loads, such as services, several insulated cables may be
mounted in parallel, at least one diameter apart, within a ventilated metal enclosure
with access facilities. Cable bus costs less than bus bars for the same load but
generally takes up more space. Use is limited to dry locations.
15.6.6 Electrical Connections
A variety of devices are commercially available for connecting two or more wires.
One type, a pressure connector, called a wire nut, may be screwed over two or
three wires twisted together. Another type consists of end lugs attached to wires
by squeezing them together under great pressure with a special tool. The lugs have
a flat extension with a bolt hole for connection by bolts to a switch or busway. As
an alternative, two wires may be joined together in a similar manner with a barrelshaped
splice.
All metal connectors should be insulated with either tape or manufactured insulated
covers and should be enclosed in a metal box with cover. Several connections
properly insulated can be enclosed in the same metal box if the box is adequate
in size. The number of spliced conductors in a box is limited by building
codes.
15.6.7 Raceways
A raceway is a general term used to describe the supports or enclosures of wires.
For most power distribution systems in buildings, rigid conduit or tubing is used.
The dimensions of such conduit or tubing and the number of wires of each size
permitted is fixed by tables in the ‘‘National Electrical Code.’’ Three or more conductors
may not occupy more than 40% of the interior area, with some exceptions
for lead-sheathed cable. All metallic raceways must be continuously grounded.
One wide use of rigid steel conduit, galvanized, is for branch circuits buried
in the concrete slabs of multistory buildings.
Electrical metallic tubing is a thin-walled tube that is permitted by codes in
locations where the raceway is not subject to physical damage.
For economy in industrial installations, a continuous, rigid structure may be
designed to carry both power and signal wiring. This structure may be in the form
of a trough, a ladder run, or a channel. It is limited in use to certain cables specifically
approved by Underwriters Laboratories for such use.
Flexible metallic conduit, also known as Greenfield, is a continuous winding
of interlocking metal stripping similar to that used for Type AC metal-clad cables
(BX). These conduits are often used in short lengths at the terminal connection of
a feeder to a motor. For wet locations, a watertight-type (Sealtite) is available.
Surface raceways are usually oval shaped and flat. When painted the same color
as the wall or ceiling, they are less conspicuous than round pipe conduit. Surface
raceways with a larger, rectangular cross section may be used to mount receptacles
or telephone or data outlets, in addition to housing wiring.