MIL-HDBK-1004/5
2.6.2.2
Equipment and Cable Parameters. Rotary equipment and cable
parameters for use by the designer are given in Tables 3, 4, and 5. Some
parameters directly affect voltage drop; other parameters are provided for
information only. Equipment and cable descriptions correspond to those shown
on Figure 1. These values are used to determine the maximum cable lengths (e.
g., medium voltage feeders and low-voltage service circuits and aircraft cable
connections), plus the permissible number of unit loads per feeder cable.
Equipment providing lower voltage-drop parameters is acceptable.
2.6.2.3
Unit Loads. The unit-load basis used herein for voltage-drop
calculations is individual 100-ampere, 0.8-power-factor loads. Two 100-ampere
unit loads can be supplied by a 75-kVA utilization service center.
2.6.3
Maximum Cable Length and Loads. To determine maximum cable length
and loads and the effects of other system parameters, various conditions were
analyzed. The analysis is included in Appendix A. Table 6 shows the maximum
number of unit loads that can be connected to a medium-voltage feeder and meet
minimum voltage levels at the utilization service assembly.
2.6.3.1
Allowable Medium-Voltage Distribution Level. Provide the
medium-voltage distribution level of 4,160 volts. Commercial airports are
using 400-Hz systems with voltages up to 2,400 volts. However, in these cases
the feeder lengths (or distances) are much shorter than the feeder lengths on
the systems used by the naval and Marine Corps Stations. The 2,400-volt
system provides no appreciable cost savings although it requires a reduction
of the maximum feeder length to one-third of that acceptable on a 4,160-volt
system which serves the same load. If feeder lengths are not reduced, then
the 2,400-volt system is capable of serving only one-third of the load that
can be fed by a 4,160-volt system.
2.6.3.2
Maximum Cable Lengths. Normally, do not exceed cable length values
given in Table 7 for medium-voltage cables and in Table 8 for low-voltage
cables. The reason that only four unit loads were permitted in Table 7 is
that the effects of the low-voltage cables were considered. This was not the
case in Table 6. The use of four loads maximum means that the steady-state
load plus the step-load can never exceed 400 amperes as shown in the step-load
capability columns.
2.6.3.3
Exceeding Limiting Cable Lengths.
Justify exceeding the normal
cable length limits only as follows:
a) When the limitation requires another central plant, the
15,000-foot feeder cable length may be increased by 10 percent. Increases over
5 percent must be approved by the Naval Facilities Engineering Command
(NAVFACENGCOM).
b) Due to special site conditions, the aircraft cable length at
such sites may be increased to 70 feet in length, only if approved by
NAVFACENGCOM.
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