MIL-HDBK-1004/2A
c) Provide isolation to prevent transmission of fires or explosions
to adjacent vaults.
d) Provide all vaults with drainage. When normal drainage is not
possible, provide a sump pit to permit the use of a portable pump.
3.6
Cable Ampacities. Design the current-carrying capacities for
underground cables, either direct burial or in ducts, in accordance with NFPA
70 adjusted to fit the specific application. The NFPA 70 tables list
individual ampacities for various sizes and number of conductors, with other
assumed conditions, which may or may not apply to the design under
consideration. Parameters given are as follows:
a)
An ambient earth temperature of 68 degrees F (20 degrees C).
b) An arrangement with cables spaced either 7.5 inches (190.5 mm)
or 24 inches (609.6 mm) center-to-center.
c)
A 100 percent load factor.
d)
A thermal resistance (RHO) of 90.
e) A conductor temperature of 194 degrees F (90 degrees C) or 167
degrees F (75 degrees C) dependent upon voltage.
Adjustment factors are given only for different ambient earth temperatures.
No corrections are included for different load factors, thermal resistances,
conductor spacings, or cable temperatures. A load factor of 50 percent and an
RHO of 60 are not unusual which could increase the listed ampacity by as much
as 15 to 50 percent. Detailed calculation methods along with ampacities for
other conditions are contained in IEEE/ICEA P-46-426, Power Cable Ampacities.
3.7
Safety Considerations. Effectively ground all electrical equipment
and hardware installed in vaults and manholes by the use of ground rods.
Ground exterior shields of cables as covered previously. Ground metallic
sheaths of cables at each splice and each termination. Bonding together of
metallic sheaths in a manhole maintains the sheaths at a common potential near
ground and reduces personnel danger and arcing when a cable fault occurs.
Where it is the policy of the facility, a ground conductor may be required to
be installed with each primary feeder and the ground conductor should be
interconnected at manholes and vaults to their grounding systems.
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