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MIL-HDBK-1012/3
1.5.1
Types of Horizontal Pathways.  The following paragraphs
list the various types of horizontal pathways, their advantages
and disadvantages, design criteria and installation parameters as
specified in EIA/TIA 569 and NFPA 70.
1.5.1.1
Underfloor Duct.  Underfloor duct systems are comprised
of a number of horizontal perpendicular raceways embedded in the
concrete floor of a building.  While they may be used to support
both power and telecommunications, the two should not be mixed
within one duct without approved separators and must comply with
local and national electrical codes.  Refer to NFGS-16130,
Underfloor Raceway System for additional information.  These duct
systems may be single, double, or triple level depending on the
requirement and the thickness of the floor as shown in Figures 9,
10, and 11.  Underfloor duct systems may be comprised of enclosed
rectangular ducts or raceways, open trenches with screw-on
covers, or a combination of these.  For the purpose of this
description cellular floors are also included as underfloor
ducts.  Underfloor duct systems use an access hole through the
floor and/or carpet at the junction point to gain access to the
duct for pulling cables.  Access holes are also required for the
placement of inserts at cable service points.
Installed underfloor ducts are categorized as either
feeder ducts or distribution ducts.  Feeder ducts run from the
telecommunications closet to a junction point.  Distribution
ducts join the feeder ducts at right angles and run from the
junction point to the outlet jacks.  There are no inserts in
feeder ducts.
a)
Advantages of underfloor ducts and cellular systems
include:
(1)
Physical protection of distribution cables;
(2)
Out-of-sight cable runs;
(3)  Reduced EMI due to metal shielding (requires
proper grounding and bonding);
(4)
Frequent access at predetermined intervals;
and
(5)
No requirement for plenum cable.
b)  The disadvantages of underfloor ducts and cellular
systems include:
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