standards and guidelines for communication sites - Radio And ...

RECTIFIER/DC POWER SYSTEMS CHAPTER 6: POWER SOURCES
The power cables that supply DC power to site equipment shall be sized based on their anticipated load
requirements and current carrying capacity (referred to as “ampacity”). Ampacity is determined by the
short-term amperage the conductor can carry before generating sufficient heat to degrade the insulation.
Ampacity is determined by the following factors:
• ambient temperature
• insulation type
• heat dissipating characteristics of the cable transport
A conductor has a higher ampacity in free air that one that is enclosed in conduit because the conduit
retains the heat. In certain electrical codes, a raceway is an enclosed duct that the cables are run through.
As such, the ampacity rating of DC cable in a raceway is lower than that installed on the cable tray.
Independent of short-term ampacity, allowable voltage drop must also be considered when sizing power
cables for DC systems. In many cases, this requires the DC power cables to be larger than the cable
required for an AC system.
6.4.6 FLOOR AND CEILING RUNS, PLENUM GRADE AND RISERS
CABLING
WARNING
Non-plenum rated power cabling shall not be installed within plenums. Failure to use
plenum-rated cables in these areas can result in generation of toxic fumes in the event
of a fire.
NOTE: Feasibility and methods of wiring within plenums and risers shall conform with jurisdictional codes.
The following requirements specify installation practices that help, should a fire occur, minimize smoke
and products of combustion from electrical wiring in areas that handle environmental air. A plenum is
defined as a compartment or chamber to which one or more air ducts are connected and that forms part
of the air distribution system. See NFPA 70-2005, Article 100 for additional information.
• Wiring systems of any type shall not be installed in ducts used to transport dust, loose stock, or
flammable vapors (NFPA 70-2005, Article 300.22(A)).
• Wiring systems of any type shall not be installed in any duct, or shaft containing only such ducts,
used for vapor removal (NFPA 70-2005, Article 300.22(A)).
• Wiring systems may be installed in ducts specifically constructed to transport environmental air
only when such wiring consists exclusively of the following. See NFPA 70-2005, Article 300.22(B)
for additional information:
• Type MI (mineral insulated) cable
• Type MC (metal-clad) cable employing a smooth or corrugated impervious metal sheath
without an overall nonmetallic covering
6-22 68P81089E50-B 9/1/05